Keywords

1.1 Introduction

The evolutionary interactions between plants and insect pests are asymmetric: the biochemical and structural assortment of the angiosperms offers a profusion of niches for the evolutionary radiation (cladogenesis) of insects, while insects do not affect plant evolution or may create anagenic adaptations within the plants. So, plants have not developed resistance to insects, but the insects have, or were, evolved ever since their emergence, especially in food selection. Host plant selection is usually a behavioural procedure which is managed basically by chemoreception system. Consequently, the emergence of specific insect/host plant relations mostly is the consequence of evolutionary conversations in the insects’ chemosensory processes. The adaptations to the food preference of the new host plant may be a minor procedure. The ‘plant realm’ and also the ‘class Insecta’ are addressed as the two prevailing gatherings of living organic entities, as far as the bounty of species just as in the measure of biomass (Schoonhoven et al. 2005). Thus, a lion’s share of 300,000 plant species needs bee pollinators for multiplication (Tanda 2019a, b, c, 2020, 2021a, b, c, d, e, f, g). Vivid, scented blossoms and flower nectarines were created by plants for alluring the animal pollinators. The blossom life structures guaranteed that while taking care of, the pollinators additionally got the pollen dust (Kearns et al. 1998). Therefore, to forestall over-abuse, plants have likewise developed a structural and biochemical hindrances for assurance against insects and different herbivores. While a portion of these boundaries are combined by plants paying little heed to the presence of constitutive protection, numerous others are created uniquely because of induced defences in the plant system. Just those insect species, which can beat these impediments at least one plant animal categories by shirking, detoxification, and so forth, can get to that plant species as food. Insects which harm the monetarily significant plants have been named as insect pests. The significant mutualistic and opposing communications among plants and insects are presented hereunder.

1.2 Insect Pollinators and Angiosperm Mutualism

The most clear and broad material illustration of mutualism is between bee-pollinated blossoming plants and their pollinators (Tanda 2019a, b, c, 2020, 2021a, b, c, d, e, f, g). Almost 80% of all blooming plants are bisexual and bear blossoms with stamen and pistils in a same bloom. This advances self-fertilization and subsequently inbreeding occurs. The plants avoid self-fertilization either by separating the sexes in time and space (differences in the timing of maturation) or by self-incompatibility. Both mechanisms promote cross-pollination, which is assisted by various agencies, e.g. wind, water, animals, etc. More than three-fourths of all blooming plants are completely or mostly pollinated by insects (Faegri and Pijl 1971; Tanda 2019a, b, c). The monetary estimation of bug pollinators is colossal. Most of the important plants such as oilseeds, pulses, fruits, vegetables, nuts, spices, and ornamentals (Hill 1997; Atwal 2000) show improved yields with animal pollination (Klein et al. 2007). It has been estimated that animal pollination has an economic value of €153 billion annually, which is nearly one-tenth of the global agricultural production (Galai et al. 2009). Some of the widely accepted estimates of the number of angiosperms pollinated by animals vary from 67 to 96% of all angiosperm species (Axelrod 1960; Nabhan and Buchmann 1997). Ollerton et al. (2011) observed that these estimates are not based on firm data. They compiled data on published and unpublished community-level surveys of plant-pollinator interactions and concluded that proportion of animal-pollinated species was 78% in temperate-zone communities and 94% in tropical communities, with a global mean of 87.5% of all flowering plants. The pollinators benefit from rewards in the form of nectar and pollen. Both are nutrient-rich foods with nectar containing 50% sugars and pollen 15–60% proteins and other essential elements (Proctor et al. 1996; Roulston et al. 2000). Together, they provide nourishment for the bees, which are the most important among insect pollinators (Schoonhoven et al. 2005). The entomophilous flowering plants and the pollinating insects constitute an example par excellence of mutualism. However, the degree of mutualism varies among various plant-pollinator combinations (Schoonhoven et al. 2005). In some cases, there is an obligate mutualism, and a species of plant can only be pollinated by a single species of pollinator, which depends on it for food (Tanda 2019a, b, c, 2020, 2021a, b, c, d, e, f). Figs (Ficus spp.) are reliant upon the fig wasps for pollination (Wiebes 1979). Different species of fig are cross-pollinated by a particular species of wasp, as the pollination in Ficus carica Linnaeus is carried out by the fig wasp, Blastophaga psenes (Linnaeus) (Ramirez 1970). Similarly an obligate relationship is found in yucca moths (Prodoxidae) and yucca plants (Agavaceae). The yucca moths are the alone pollinators of yucca plants and lay eggs in the locule of the ovary so that the hatching caterpillars can start feeding on the seeds (Pellmyr and Krenn 2002). Darwin described that the Christmas orchid, Angraecum sesquipedale Thouars, had a long green nectary and forecasted that there must be a gigantic moth species with long proboscis to suck the nectar from the long flower nectary. Later on, Rothshild and Jordan observed the Morgan’s sphinx moth, Xanthopan morganii Walker, with an enlarged proboscis as the only pollinator of A. sesquipedale, native to Madagascar (Kritsky 2001). This type of mutualistic relationship in plant pollinators is not very common. Burkle and Alarcon (2011) reported that plant-pollinator associations are largely distributed with a high degree of annual turnover of pollinator populations and the significance of an insect pollinator may differ for pollination services for the same plant under dynamic climatic conditions.

Doubtlessly, various insect pollinators have served the development of angiosperms differently, and fossil reports describe that pollination mechanism was found around 250 Myr ago (Labandeira 2013). The old flowering plants might be cross-pollinated by the wind birds or other animals. Due to the benefits of insect pollinators, its significance enhanced in the future (Cox 1991; Crepet et al. 1991). Entomophilic plants possess various flower sizes, shapes, colours, and fragrances which may have been resolved by the needs of flower pollinator species. The pollen grains may have a shape or sticky materials which help to adhere to the body of the insect . The hairy body parts of the insects also assist in the spread of the pollen. Foxglove, (Digitalis purpurea Linnaeus), flowers cross-pollinated by the bumblebees look like a bell; however, the flowers of Calopheria spp. fertilized by the butterfly have tubular corolla, adapted for the enlarged proboscis (Schoonhoven et al. 2005). Additionally, the later consists of maximum amino acids than blossoms nourished on by flies (Baker and Baker 1986). Few brightly coloured flowers develop sterile ‘reward anthers’ to allure pollinators (Nepi et al. 2003). The flowers of orchid Mirror of Venus, Ophrys speculum Link, mimic the virgin female wasps of their pollinator, Dasyscolia ciliata (Fabricius), by discharging female sex pheromone to attract the male wasps. The male wasps attempt to mate with the blooms and operate as pollination agent (Ayasse et al. 2003). At present, hymenopterans are the predominating class working in plant pollination; however, other insect groups are too crucial in the history of pollination. Basically the beetles and flies pollinate the basal flowers (Thien et al. 2000). Honeybees have adapted themselves to a flower nutrition (Atwal 2000) and like pollen grains in spite of impermeable cuticle (Velthius 1992). Honeybees show flower constancy by foraging the blooms of a single plant . It enhances the effectiveness of a pollinator and assists in plant reproductive isolation. The insects’ ability to recall amalgamations of flower odours and colours contributes a crucial role in flower fidelity. Honeybees have been observed to have the capacity to differentiate 700 various floral fragrances (Schoonhoven et al. 2005).

1.3 Antagonistic Plant-Pest Relationships

Insects are the most assorted and massively effective living beings on this globe and attack plants for food. Species in Lepidoptera, Orthoptera, Phasmida, or predominantly Hemiptera and Thysanoptera are herbivorous; however, Coleoptera, Hymenoptera, and Diptera are partially herbivorous but mostly carnivorous (Schoonhoven et al. 2005). Several insects live on all parts of plants; however, solid feeders are defoliators or borers, and others suck the sap (aphids, jassids), lessen plant vigour, and act as vectors such as whitefly. Mostly insects are specific in their food plant choice. Monophagous insects live on a single or a few related plant species, but oligophagous insects feed on a number of plants. Polyphagous species feed on various plants belonging to many families for survival (Panda and Khush 1995). Studies on herbivorous insects have demonstrated that one-tenth of these insects are capable of living on plants of more than three families. Each insect host range is determined by structural, biochemical , and ecological elements. Excluding Orthoptera, all other herbivores are highly consisted of specific species living on specialized species of plant (Schoonhoven et al. 2005). Bruce (2015) described that the herbivores get converted to specialize over time; however, few of polyphages carry on as crop pests. Insects can identify and respond to host signals for nourishing and egg-laying. In spite of this, antagonistic connections between plants and phytophagous insects continue to work, as herbivory has been seen to enhance the plant development and strength in few instances (Owen 1980; Vail 1994; Sadras and Felton 2010). Production minimizes due to insects; however, there are instances of enhanced yield reported in insect-attacked in comparison to insect-unattacked crop plants (Harris 1974). The automatic reply to damage may in few plants more than counterbalance the damage done. It is based on how plants answer to damage by insects or other herbivores. Sesame tissues cultured alone or with okra suppressed egg hatch and penetration of roots by juveniles, delayed adult development, and encouraged development of males in Meloidogyne incognita. Gall formation was inhibited on excised roots of okra by co-culturing with sesame. Sesame callus reduced penetration, discouraged nematode build-up in okra, and caused an increase in numbers of males showing antagonism of sesame to root-knot nematode on okra (Tanda and Atwal 1988; Tanda et al. 1988, 1989).

1.3.1 Plant Defence Mechanisms

Plants are motionless and have to protect themselves against herbivores. Many plants in natural environments exhibit small or no evident attack despite of large populations of plant feeders. Insects feed about 10% of all plant biomass annually (Barbosa and Schulz 1987; Arora and Sandhu 2017). Plants have developed a large range of structural and biochemical attributes to save from herbivores. Contrastingly, insect injury is more in cropping area as many of these attributes have been strayed while breeding more palatable and tasty plants and outyielding the crop genotypes used traditionally. There is a demand to investigate such plant defence mechanisms to manipulate them in agro-industry (see Chap. 10).

1.3.2 Plant Structural Defence Mechanisms

1.3.2.1 Epicuticular Waxes

The epicuticle protects the plant surface by waxes against desiccation, herbivore , and disease attack. Thickness, structure, and wax coating number may be different in plants giving rise to variations in the total plant dry weight. These wax layers work as resistance to the insect pest attack (Jeffree 1986), and the mechanoreceptor and chemoreceptor present on the tarsi of insects and mouth parts get negative tactile and chemical stimuli. In Brassicaceae, leaf epicuticular wax ensues in non-preference for ingesting by the flea beetle, Phyllotreta cruciferae (Goeze) (Bodnaryk 1992). However, wax coating may also have adverse influence by liking few insects. Plants with glossy leaf surfaces have also been reported to be resistant or non-preference to insect pests in many cases (Eigenbrode and Espelie 1995). Indirectly crystals of wax and waxy flowers may also damage the sticking, mobility, and efficiency of predators eventuating in higher herbivore abundance (Eigenbrode et al. 1999).

1.3.2.2 Hairy Structures

In plant mostly, the epidermal surface is protected with hairlike structures, which vary in form, size, position, and their role (Werker 2000). Generally, the hairs on the aerial parts of a plant are called as trichomes; however, the pubescence is mentioned when plant surface is protected by the collective trichomes. These trichomes vary in size from a few microns to several centimetres, and the form differs largely in various plants. They are glandular and non-glandular (Payne 1978). Non-glandular trichomes may work as fence for the attack of insects on the surface of plants or stop the herbivores’ feeding on the plant tissues, thus preventing the plants from any damage (Ram et al. 2004). Glandular trichome structures are developed to produce a number of chemical substances (Fahn 2000), which perform as crucial chemical barricades against insect pests and diseases (Glas et al. 2012). In black bean, Phaseolus vulgaris Linnaeus, curved trichomes were observed to stick the aphid, Aphis craccivora Koch (Johanson 1953), and the leafhopper, Empoasca fabae (Harris), resulting in damage and death (Pillemer and Tingey 1978). In few instances, the density of trichomes has been found to be persuading interestingly, in response to insect nourishing. When plants were attacked by the cabbage white butterfly, Pieris rapae (Linnaeus), and the cabbage looper, Trichoplusia ni (Hubner), on young black mustard, Brassica nigra (Linnaeus) W. D. J. Koch, plants led to enhanced trichome density on new foliage (Traw and Dawson 2002). Few insect pests have also been observed to have evolved morphological and biochemical modifications to counteract the effect of hairy growth. These trichomes may also have role in plant resistance indirectly, by restricting the searching efficiency of predators of herbivores. On glabrous varieties, than on hairy leaf surfaces, the parasitic wasp, Encarsia formosa Gahan, is greatly more effective in searching the whitefly nymphs (van Lenteren et al. 1995).

1.3.2.3 Leaf Surface Rigidity

In a lowland tropical forest, plant toughness was reported as the best forecaster of interspecific differences in herbivory ranking (Coley 1983). By overthrow of cellulose, lignin, suberin, and callose with sclerenchymatous fibres, plant cell walls become stronger and resistant to piercing and sucking by insect mouth parts and ovipositors of adult females and chewing by insects using mandibles. Solid-stemmed cultivars of wheat with toughness were found to be resistant to stem sawfly, Cephus cinctus Norton (Platt and Farstad 1946). Rind hardness was a significant element in sugarcane, in minimizing the internode borer Diatraea saccharalis (Fabricius) attack (Martin et al. 1975). In alfalfa, seed losses due to the seed chalcid Bruchophagus roddi (Gussakovsky) were lower in genotypes possessing more lignified pod walls (Springer et al. 1990).

1.3.2.4 Design and Plant Size

The preference of a plant cultivar to be a host for insect pests may differ with plant design, architecture, and size. The spacing of plant canopy, variations in stem, leaf and bud shapes and their sizes, and angles of branches may impact insect liking and survivance. From monocots through herbs and bushes and trees, the increase in size and design of plants is related with the enhancement of diversity of the connected insect life (Lawton 1983). The indirect impacts of plant structures on herbivory are also arbitrated through their effect on the parasites and predators. As compared to normal leaf varieties, okra-leaved cultivars in cotton are less attacked by bollworms, whitefly, and boll weevil (Ram et al. 2004). Varieties with little cotyledons and unifoliated leaves in soybean were tolerant to the legume seedling fly, Ophiomyia phaseoli (Tryon), and these are the sites where the female insects oviposits (Talekar and Tengkano 1993).

1.3.3 Biochemical Productions as Barriers

Plants have developed a number of chemical structures to avoid attack by insects and other herbivores. As few chemicals are associated with basic metabolism, several other compounds have been reported to deter, repel, kill, or stop insects and other herbivores from feeding on some plants as their food (Chapman 1974; Harborne 1993; Mithofer and Boland 2012). As plant feeding insects have evolved the capacity to manipulate their hosts, the plants have acknowledged by developing defending biochemical secretions to prevent herbivore damage (Johnson 2011). These biochemicals secreted by plants may be grouped into holistic nutrition and allelochemicals.

1.3.3.1 Holistic Nutrition

Crop plant suitability as a host for one or more insect pests is reliant on its capacity to offer holistic nutrition for growth and multiplication of insects. Plants generally provide nutrients at suboptimal ratios, from an insect ’s outlook, which are amalgamated with indigestible structural mixtures of cellulose and lignins, and a diversity of allelochemicals (Schoonhoven et al. 2005). The biochemicals apply a large number of behavioural, physiological, and growth-impeding impacts, some of which may even result in to insect killing. Many insects have the same needs for food, comprising of carbohydrates, amino acids, fatty acids, sterols, and some micronutrients; however, plants are often nutritionally inferior in itself. The key classes of basic plant amino acids, carbohydrates, and lipids intricated in the physiological plant procedures act as important nutrients for herbivores. So, alterations in basic plant metabolism and nutrients highly impact the living and reproduction of plant feeders (Berenbaum 1995). Mainly, nitrogen is crucial as insects are incompetent to utilize plants organic and inorganic nitrogen as it is suboptimal for the insect needs (Schoonhoven et al. 2005). This may work as a main obstacle for the full utilization of plants by a large number of insects. Appealingly, the herbivores consist of about 50% of the total arthropods in less than one-third of insect fauna, showing that once the nitrogen insufficiency is broken, these insects are capable to reach a sufficient nutrition supplies (Strong et al. 1984).

1.3.3.2 Crop Plant Nutrient Deficiency: A Resistance Mechanism Against Insects

Crop host plant , having insufficient one or more important elements of nutrition needed by the herbivores, may demonstrate insect tolerance through antibiotic and antixenotic impacts on the insect development and similarly may also form disparity of accessible essential nutrients (Arora and Dhaliwal 2004).

Cotton Genotypes

Few cotton cultivars with built-in protection depending on essential nutrients have been developed for the leafhopper, Amrasca biguttula (Ishida); whitefly, Bemisia tabaci (Gennadius); stem weevil, Pempherulus affinis (Faust); and thrips complex (Uthamasamy 1996). The whitefly B. tabaci, with genetically resistant genotypes, exhibited more amounts of K, P, and Mg and small of N and Fe in comparison to susceptible cultivars. However, sugars, proteins, Ca, and Cu did not exhibit important correlation with whitefly population multiplication. Another report mentioned that total contents of sugar in some cotton genotypes were positively associated with whitefly attack at the vegetative stages but negatively connected with it after crop flowering period (Rao et al. 1990). For the leafhopper, A. biguttula, more prone cultivars, Acala 4–42, had large contents of reducing sugars (2.55%), proteins (18.49%), and free amino acids (10.15 mg/g) in comparison to highly tolerant BJR 741 holding 1.63% reducing sugar, 13.45% proteins, and 6 mg/g free amino acids (Singh and Agarwal 1988).

Rice Genotypes

Resistant rice cultivars to thrips, Stenchaetothrips biformis (Bagnall), contained notably low reducing sugars and free amino acids as compared to the prone cultivars (Thayumanavan et al. 1990). Contents of asparagine in small amounts in rice cultivar ‘Mudgo’ were contemplated to be the fundamental reason of tolerance to brown plant hopper, Nilaparvata lugens (Stal). Brown plant hopper confined with Mudgo variety showed underdeveloped ovaries with few eggs; however, those kept with susceptible cultivars developed normal ovaries with maximum eggs (Sogawa and Pathak 1970). The gall midge Orseolia oryzae (Wood-Mason)-resistant cultivars PTB 18, PTB 21, and Leuang 152 had greater amounts of free amino acids and low sugars in their shoot apices than non-tolerant cultivars Jaya and IR8. Stems of resistant (TKM6) and moderately resistant (Ratna) cultivars had low amino acids and sugars than susceptible varieties (IR8) against the stem borer, Scirpophaga incertulas (Walker), (Vidyachandra et al. 1981).

Legume Contents

Auclair (1963) demonstrated the significance of amino acid contents in the pea plant on the susceptibility to aphid , Acyrthosiphon pisum (Harris). He reported that the amounts of amino acids in the susceptible cultivars were significantly greater than those in the resistant varieties. High proportions of non-reducing sugars and little percentage of starch in the seeds of chickpea GL 645 might be accountable for the less attack of the pod borer H. armigera in the cultivar under trial in comparison to the infestor (Chhabra et al. 1990). In pigeon pea, genomics against pod borers, small amino acid, protein and sugar amounts, and high phenol contents developed resistance. Sugar amounts were more in seeds (3.64–4.82%) and in the pod coat (3.66–4.92%) of susceptible genotypes (ICPLI, ICPLS7, and UP AS20). Total sugar proportions in the resistant varieties varied between 2.86 (ICPLS3024) and 3.51% (HS9–2) in the seeds and 2.91 (ICPLS3024) and 3.44% (HS9–2) in the pod coat. The amino acid concentrations were small in the pod coating (1.40–1.52 mg/g) and seed (1.39–1.55 mg/g) of resistant pigeon pea genotypes assessed in comparison to the susceptible varieties (1.89–2.57 mg/g in pod coat, 2.04–2.62 mg/g in seed). Extremely significant positive correlation found between amino acid present and infestation of borers helped the potential contribution of amino acids in providing resistance to the pod borers (Sahoo and Patnaik 2003).

1.3.3.3 Phytochemicals

The allelochemicals or phytochemicals secreted by plants are mostly minor metabolites which do not contribute largely in the basic plant pathways of metabolism. As the basic metabolic pathways are the same in almost all angiosperms, these secondary materials differ largely in various crop plants (Schoonhoven et al. 2005). Fraenkel (1959) established that these metabolites serve to repel many herbivores. It has been reported that the plant develop a number of minor substances, and more than 200,000 of these have been recognized (Dixon and Strack 2003). Further, the allelochemicals have been categorized into two classes such as allomones, which help the host plant , and kairomones, which assist the herbivores. In different types of insect-plant associations, the action of allelochemicals can decide the ranking of a plant either as a kairomone host and non-host or as allomone-resistant host plant and allomone-susceptible host (Panda and Khush 1995). Allomone plants are contemplated as a big element accountable for plant defence mechanism against herbivores, and these have been utilized to enhance levels of resistance in many field crops (Green and Hedin 1986). The different secondary plant metabolites used in plant defence mechanisms against insects have been described shortly (Rosenthal and Berenbaum 1991; Arora and Dhaliwal 2004; Schoonhoven et al. 2005; Arora and Sandhu 2017).

Unusual Amino Acids

In many unrelated higher and some lower plants, nonprotein or unusual amino acids are ubiquitous. About 600 amino acids have been described from different legumes. Nonprotein amino acids may provide defence against natural enemies and diseases due to their constructional analogy to the usually important amino acids in nutrition. The biological impacts on herbivores are partially due to the correlated molecules which get involved wrongly into the insect protein synthesis system or through stopping of biosynthetic tracks (Rosenthal 1991; Huang et al. 2011; Yan et al. 2015). In creating insect development disturbance, canavanine, azetidine-2-carboxylic acid, 2,4-diaminobutyric acid, mimosine, 3-hydroxyproline, 5-hydroxynorvaline, β-cyanoalanine, and pipecolic acid are important (Parmar and Walia 2001; Yan et al. 2015). Root exudates and extracts from in vitro grown seedlings of sesame showed an inhibitory effect on egg hatch and juvenile penetration by root-knot nematode, Meloidogyne incognita. Analysis of root exudates of sesame showed seven free amino acids, i.e. aspartic acid, glutamic acid, valine, proline, serine, glycine, and leucine, whereas the exudates of okra had ten free amino acids, viz. glycine, serine, leucine, isoleucine, alanine, arginine, glutamic acid, glutamine, lysine, and cystine. Three sugars, viz. fructose, glucose, and sucrose, were found in sesame root exudates and extracts. Commercial amino acids both singly and in combination inhibited egg-hatching (Tanda et al. 1989).

Organic Compounds

Terpenoids are the biggest and greatest diverse group of organic compounds observed in crop plants. They show gigantic chemical variations and complexness; however, all are established by blend of five-carbon isopentane, and many of them are lipophilic compounds (Ruzicka 1953). Terpenoids attain their highest structural and functional variety in the plant flora. About 30,000 terpenoids are found in plant systems, and a large number of them act as protection against insect pests and diseases or as allures for crop pollinators and fruit scattering organisms. Gershenzon and Croteau (1991) reported that the terpenoids are composed of two or more than five carbon units in their forms: monoterpenoids (2×C5), sesquiterpenoids (3×C5), diterpenoids (4×C5), triterpenoids (6×C5), tetraterpenoids (8×C5), and polyterpenoids [(C5) n where n > 8].

Monoterpenoids have been established to act as toxins and restraints for feeding and laying eggs against many herbivores. Among monoterpenoids, the important example of insect toxin is pyrethrum, working as a botanical insecticide, reported in the leaves and flowers of Chrysanthemum spp. In pyrethrum, the active ingredient is a combination of monoterpene esters commonly called as pyrethroids (Casida 1973).

Cotton and other plants belonging to Malvaceae have pigmented spherical glands found in their foliage, blossoms, and branches of plants. The pigments of these glands, in addition to anthocyanin, possess more amounts of a variety of monoterpenoids and sesquiterpenoids particularly gossypol. Gossypol is a phenolic compound, sesquiterpene dimer with two aldehyde remainders. Gossypol is poisonous to a number of insect pests, resulting in big decline in the survivance, multiplication, and development of numerous major lepidopterous and coleopterous herbivores. The contagion of gossypol to insects is assumed to arise from its irrevocable to proteins in the gastrointestinal area, bringing about a decrease in the digestion of proteins. In the gastrointestinal region, the proteins may be the eaten dietary proteins or the digestive enzymes developed by the herbivore (Meisner et al. 1977). A key secondary metabolite of the common dandelion, Taraxacum officinale G. H. Weber ex Wiggers, the sesquiterpene lactone, beta-d-glucopyranosyl ester (TA-G), saves the plant against its main native root feeders, the common European cockchafer, Melolontha melolontha Linnaeus, by discouraging larval infestation (Huber et al. 2016).

In terpenoids, triterpenoids (C30) are the biggest with six C5 isoprene units. The three main classes of triterpenes are the cucurbitacins, limonoids, and saponins which have important contributions in plant-insect herbivore interplays. In the Cucurbitaceae, cucurbitacins are a class of approximately 20 very bitter and toxic tetracyclic triterpenes, cramped chiefly to the host plants. These substances act as poisons and restraints for feeding against a large number of herbivores (Tallamy et al. 1997). Few specific insects attacking cucurbits are capable to absorb or prevent these poisons and even utilize cucurbitacins as host identifying signals (Abe and Matsuda 2000).

With a fundamental structure of 26 carbon atoms, the limonoids are a big class of very oxygenated compounds and are reported in Rutaceae, Meliaceae, and Cneoraceae. These limonoids are very strong feeding deterrents against many herbivores. More than 100 triterpenoids have been detected from the neem (Azadirachta indica A. Juss.) seeds, and a many of them are working as deterrents and antifeedants against crop insect pests. Azadirachtin is the chief among these triterpenoids, which is effectual at doses as minimum as 50 parts per billion. Over 400 insects have been found to be susceptible to neem compositions at different concentrations. With antifeedant actions, neem is demonstrated to influence the living, growing, multiplication, vigour, and egg-laying capacity of herbivores (Schumutterer 1995; Dhaliwal and Arora 2001).

In many crop plants, saponins are most common and made up of a sugar part (glycoside) associated with a hydrophobic aglycone, which may be a triterpene or a steroid, both of which develop from the C30 precursor, squalene. In soybeans, beans, peas, tea, spinach, sugar beet, and quinoa, triterpenoid saponins have been isolated. In oats, capsicum, peppers, aubergine, tomato seed, allium, and asparagus, steroidal saponins are detected (Francis et al. 2002). Saponins apply a powerful insecticidal reaction against many insect groups resulting in enhanced kill, decreased food intake, weight loss, growth lagging, and moulting faults (Geyter et al. 2007).

Natural Substances

The alkaloids are a diverse group of natural substances that found in all groups of living life; however, they are typical in plant systems. They mostly comprise primary products that have one or more nitrogen atoms, mostly in amalgamation as component of a cyclic system. Many of them are products of usual amino acids, for instance, as lysine, tyrosine, tryptophan, histidine, and ornithine (Facchini 2001). They occur in about 20% of the angiosperms. Mostly, each species carrying alkaloid exhibits its own distinctive, alkaloid shape explained genetically. Many alkaloids have been described to be poisonous or repellent to herbivores. Due to their nature containing nitrogen, several alkaloids impede with the major elements of acetylcholine transference in the nervous system. Nicotine and nornicotine obtained from tobacco were important as botanical insecticides before the invention of organic insecticides made synthetically (Dhaliwal and Arora 2001). Pyrrolizidines, quinolizidines, indole alkaloids, benzylisoquinolines, steroid alkaloids, and methylxanthines are many classes of alkaloids at dietary concentrations over 0.1% and act as insect ’s deterrents and to other herbivores (Schoonhoven et al. 2005).

Glucosinolates

About 100 sulphur or nitrogen carrying unique minor compounds such as glucosinolates comprise a little class of which work as harbingers of oils in mustard. Together with the family Brassicaceae, glucosinolates are found generally in the Brassicales order. Glucosinolates seem to work as successful chemical protections against a number of non-adapted herbivores (Fahey et al. 2001). Heynhold genome, at a minimum 52 genes, is intricated in glucosinolate biosynthesis in the thale cress Arabidopsis thaliana (Linnaeus) (Arabidopsis Genome initiative 2000; Halkier and Gershenzon 2006). When insects infest crop plants, glucosinolates are broken down by myrosinase enzyme into many metabolites acting as deterrents against insects (Hopkins et al. 2009). On the flip side, a small group of Brassica feeders are capable to use glucosinolates in searching and identifying the host. Glucosinolates and their evaporative compounds formed by hydrolysis are also utilized as signals by predators of Brassica feeding herbivores (Louda and Mole 1991).

Juvenoids and Ecdysteroids

For the growth, development, multiplication, and survival of herbivores, the endocrine system is crucial. Though several insect hormones are demonstrated, the juvenile hormone (JH) and the ecdysone or moulting hormone (MH), two strong hormones are established to contribute in these procedures. Juvenoids and ecdysteroids are match of these hormones. It is assumed that plants may have evolved juvenoids and ecdysteroids as fine defence mechanisms against herbivores. Crops possessing more ecdysteroid amounts, i.e. >1000 ppm, are prevented by insect pests. There are few main juvenoids derived from plants such as farnesol, sesamin, juvabione, sterculic acid, bakuchiol, and thujic acid which are familiar to disorder metamorphosis, moulting, and multiplication in herbivores.

Crop Proteinase Inhibitors

In some crop plants, protease inhibitors (PIs) comprise a plentiful and significant group of substances which have a defending mechanism against insect pests (Dunaevsky et al. 2005). New reports utilizing microarrays and proteomic proposals have disclosed that the plant defence mechanisms relying on proteins contribute more significantly against insects than perceived before (Felton 2005; Zhu-Salzman et al. 2008). Arginases, polyphenol oxidases, and peroxidases, the defence proteins, may have properties against microbes, and others, for example, chitinases, cysteine proteases, lectins, and leucine amino peptidases, may also be poisonous (Zhu-Salzman et al. 2008). Nevertheless, the anti-insect action of plant proteins is easily disabled by proteases, and proteolysis-susceptible proteins can be avoided with PIs (Mithofer and Boland 2012). Serine, cysteine, and aspartate proteinases and metallocarboxypeptidases stop the actions of different enzymes in herbivores mostly insect peptidases which are intricated in the development and multiplication of insects. The PIs also minimize the food digesting capability of the herbivores, thus resulting in to the scarcity of amino acids, the major components of food finally decelerating the growth and affecting the starvation. Numerous PIs have been found in plant system (De Leo et al. 2002), which are successful against a number of lepidopteran and hemipteran herbivores (War and Sharma 2014). PIs were positively assessed for their trypsin and H. armigera gut proteinase inhibitory action in various parts of the tomato plants (Damle et al. 2005).

Phytohaemagglutinins

Lectins or phytohaemagglutinins are proteins with a capability to reversibly attach to the carbohydrate moieties of complicated carbohydrates without changing the covalent shape of any of the identified glycosyl myths. Lectins are found globally all over the plant kingdom, where they comprise 6–11% of the total plant proteins. Mostly the legume seeds cotyledons are abundant in lectins. Lectins are connected with the defence mechanisms of crops against insect pests and diseases (Liener 1991). Arisaema helleborifolium Schott lectin showed anti-insect action towards the melon fruit fly second instar larvae, Bactrocera cucurbitae (Coquillett) (Kaur et al. 2006).

Plant Phenolics

In plants phenolics are very common and are fragrant compounds with one or more hydroxyl groups (Harborne 1994). Hydroxybenzoic acids like vanillic acid, hydroxycinnamic acids like caffeic acid, and coumarins are comparatively simple phenolics (Schoonhoven et al. 2005). Coumarins comprise a 5,6-benz-2-pyrone skeleton and may be differently hydroxylated, alkylated, alkoxylated, or acylated. Coumarins can discourage eating and impede with growth of herbivores. Coumarin, bergamottin, is capable of killing eggs of Colorado potato beetle, Leptinotarsa decemlineata (Say), though mammein is insecticidal to the beetles attacking mustard. Coumarins seem to work as kairomones for some insects that are particularly eating on coumarin carrying plants (Berenbaum 1991b). Flavonoids are established mostly in all higher plants in the phenolics, and many plants exhibit their own distinguishing flavonoid contour. To make a water-soluble glycoside, flavonoids have a basic C6-C3-C6 structure, which is related to a sugar moiety. Flavonoids derived from plants are catechin, a botanical insecticide rotenone, and phaseolin, all of them operate as impediments against insect pests (Schoonhoven et al. 2005).

Tannins are polyphenolic combinations generally available in higher plants. The phenolic hydroxyl groups of tannins tie to nearly all soluble proteins, developing insoluble copolymers. Proteins joined to tannins are indigestible and thus reduce the nutritious worth of plant parts (Schoonhoven et al. 2005).

Latex in Laticifers

Latex is found in special tissues known as laticifers, having chemically unspecified milky suspensions or emulsions in aqueous fluids (Agrawal and Konno 2009), and as a defence mechanism. Tiny insects get ensnared in latex physically or their mouthparts may stick together, and chemical constituents in latex including proteins and toxins adversely affect the insect growth (Dussourd 1995). Injuring of laticifers by insects leads to leakage at injury site (Mithofer and Boland 2012). In the milkweed, Hoodia gordonii (Masson) Sweet ex Decne, feeding by larvae and oviposition by T. ni adults, was discouraged when latex was mixed in artificial diet or applied on the leaves of the host plants (Chow et al. 2005).

1.3.3.4 Allelochemicals as Host Plant Defences

Allelochemicals in Maize

Maize is damaged by a variety of insect pests, and anti-herbivore defences in maize comprise small molecules known as benzoxazinoids (Frey et al. 2009), chlorogenic acid (Cortes-Cruz et al. 2003), and maysin (Rector et al. 2003) besides defence-linked proteins (Chuang et al. 2014). Xie et al. (1992) examined for hydroxamic acid many lines of maize resistant to western corn rootworm, Diabrotica virgifera Le Conte. Root extracts in all tests were having four main hydroxamic acids such as 2,4-dihydroxy-7methoxy-l,4-benzoxazin-3-(4H)-one (DIMBOA), 2,4-dihydroxy7,8dimethoxy-l,4-benzoxazin-3(4H)-one (DIM2BOA), 2-hydroxy,7-methoxy,1,4-benzoxazin-3(4H)-one (HMBOA), and 6-methoxy-benzoxazolinone (MBOA). These hydroxamic acids retarded the growth and development and loss in weight and also reduced the head capsule width of rootworm larvae. Wiseman et al. (1992) described a greatly significant negative link between weight variations in corn earworm, Helicoverpa zea (Boddie), and in the fall armyworm, Spodoptera frugiperda J.E. Smith, larvae and maysin contents in the silks of many corn lines.

Allelochemicals in Cotton

In cotton, the allelochemicals such as gossypol, gossypurin, heliocides, hemigossypolone, tannins, anthocyanins, flavonoids, and phenolics have been found to affect negatively on infesting herbivores. Gossypol was described to show resistance to cotton bollworm Heliothis zea (Bottger et al. 1964). Generally, many cotton varieties grown commercially have a gossypol concentration of about 0.5% in squares. With more gossypol cotton varieties (No. 16482, 6501, and Termez-14) had harmful effects such as prolonging incubation period, higher kill in young larvae, and reducing the weight of larvae in comparison to the cultivars having less gossypol contents (Vilkova et al. 1988). They also reported that antibiotic effect of high level of gossypol contents decreased the fecundity (more than 50%) of H. armigera. Gossypol is also described to influence the boll nutritional value adversely by developing complex compounds joining with amino acids, proteins, and enzymes. Mohan et al. (1994) reported that genotypes with maximum gossypol glands on the surface of the ovary in Asiatic cotton Gossypium arboreum Linnaeus decreased the attack of bollworm complexes in H. armigera, Earias vittella (Fabricius), and Pectinophora gossypiella (Saunders). When healthy cotton plants were analysed, for the cotton stem weevil, P. affinis, the amount of tannins was minimum in susceptible MCU5 and more in the resistant successions. The contents grew in the gall region when the plants were attacked, and the concentration was high in resistant accessions in comparison to the susceptible genotype MCU5. No difference was reported in the total phenolic amounts in the resistant and susceptible accessions of healthy stems. The amount of total phenolics, however, enhanced in the gall regions significantly when attacked, even the concentration rose more in resistant accessions. It can be established that more development in tannin and phenolic amounts might offer a defensive tool against the stem weevil infestations (Uthamasamy 1996).

Allelochemicals in Vegetable Crops

In Solanum species against the Colorado potato beetle (CPB), L. decemlineata, and the potato leafhopper, E. fabae, glycoalkaloids in potatoes perform as natural resistance mechanisms. Many wild Solanum species have exhibited a positive relationship between total leaf glycoalkaloid concentration and resistance to Leptinotarsa species. Completely stopping eating, leptin is a very successful antifeedant, whereas tomatine and demissine are halfway in action, followed by the effectiveness of solanine and chaconine (Tingey 1984). Resistance of tetraploid potato (Solanum tuberosum L.) selection ND 2858-1 in the field and its backcross progeny against the Colorado potato beetle is developed by antibiosis mechanism. In detached leaf tests on resistant cultivars, neonates of CPB evolved slowly, and weight gain in larvae after 4 days was discouraged by 75% in proportionate to larval growth and weight gain on susceptible accessions. Foliar glycoalkaloid assays showed low levels of leptins I and II in resistant genotypes (Lorenzen et al. 2001). Against the tomato fruit borer H. zea, the wild species of tomato, Lycopersicon hirsutum and L. hirsutum f. glabratum, exhibited the phenomenon of antibiosis. For the antibiosis process, the chemicals involved were l-tomatine, 2-tridecanone, phenolics, and elements including iron zinc (Ferry and Cuthbert Jr 1975; Dimock and Kennedy 1983; Kashyap 1983). The toxicity tested of allelochemical 2-tridecanone was maximum against H. zea, Manduca sexta Linnaeus, and L. decemlineata. Maximum phenolic amounts have been reported to conclude resistance to the H. armigera species (Banerjee and Kalloo 1989), whereas high contents of tomatine are harmful to the greenhouse whitefly, Trialeurodes vaporariorum (Westwood) (Steehius and van Gelder 1985). In tomato, the protease inhibitor and chlorogenic acid were involved in resistance against aphid (Felton et al. 1989). In glandular trichomes of Lycopersicon hirsutum f. typicum accession (LA) 1777, the sesquiterpene carboxylic acids (SCA), (+) E-α-santalen-12-oic, (−)-E-endo-α-bergamoten-12-oic, and (+)-E endo-β-berqamoten-12-ion acids were developed which is more resistant to herbivores generally attacking tomato, L. esculentum, grown commercially. The larvae of tomato fruitworm, H. zea, and the beet armyworm, Spodoptera exigua (Hubner), showed reduction in eating, growth, and survival in the presence of such chemical compounds. In diet found lethal to the larvae, at concentrations as low as 2 mg SCA/g of diet and a concentration of 60 mg SCA/g, the sublethal effects were reported (Frelichowski Jr and Juvik 2001).

Root exudates and extracts from in vitro grown seedlings of sesame showed an inhibitory effect on egg hatch and juvenile penetration by root-knot nematode, Meloidogyne incognita. Analysis of root exudates of sesame showed seven free amino acids, i.e. aspartic acid, glutamic acid, valine, proline, serine, glycine, and leucine, whereas the exudates of okra had ten free amino acids, viz. glycine, serine, leucine, isoleucine, alanine, arginine, glutamic acid, glutamine, lysine, and cystine (Tanda et al. 1989).

1.3.4 Various Plant Defence Mechanisms

Plant defence mechanisms may be categorized into basic, which are found in the host plants disregarding the occurrence of insect or non-insect pests, and incited, which are developed in response to different abiotic and biotic stressors.

1.3.4.1 Basic Plant Defence Mechanisms

Plants have developed an abundance of structural and chemical defence mechanisms that are incorporated into their tissues disregarding the presence or absence of insects. These basic plant defence mechanisms can repel, deter, inebriate, derange, or disrupt the feeding on plant tissues, growth, and development of herbivores (Arora and Dhaliwal 2004; Ram et al. 2004; Mithofer and Boland 2012). These phytotechnological protection systems comprise of the following:

  1. (a)

    The texture and composition of the plant covers (Johnson 1975)

  2. (b)

    Existence of anatomical structures, for example, thin veins, thorns, silica, trichomes, or resins (Hanover 1975)

  3. (c)

    Lack of essential nutrients (House 1961)

  4. (d)

    Existence of substances similar to hormones which inhibit the growth of insects (Williams 1970)

  5. (e)

    Inappropriate pH or osmotic pressure (Beck 1965)

  6. (f)

    Accretion of secondary metabolites (Chapman 1974)

There are a large number of secondary metabolites comprising amino acids to alkaloids, terpenes, phenolics, steroid, cyanogen, and glycosides in mustard oil (Mithofer and Boland 2012). Additionally, plant systems may also transform nitrogen into compounds which are not accessible to herbivores (White 1978). The benefits of similar basic defences to insects are that these are developed during the time of high metabolic activities and can be used over an increased time period. These physiological devices act against a large number of generalist insects; however, regular subjection to such chemicals develops powerful critical pressure on the plant feeders, which may lead in the development of specialist herbivores.

1.3.4.2 Incited Plant Defence Mechanisms

To deter feeding by insect pests and stop colonization, incited plant defence mechanism is operated in the existence of insects and allows the plant (Sadras and Felton 2010). The insect feeding processes switch on many protection signals, resulting in to acceptable defence reactions (Wu and Baldwin 2010; Hogenhout and Bos 2011; Bruce 2015). Various plant species have also been described to acknowledge to insect females for depositing eggs in the same way (Hilker and Meiners 2006). Volatile organic compounds (VOCs) liberated by plants have been reported to allure predators of herbivores (Tamiru et al. 2011; Fatouros et al. 2012) or incite direct protections so that the rate of insect development is decreased on plants nursing eggs (Gieselhardt et al. 2013). Crop plants react to elicitors produced from mouth secretions of herbivores, mechanical injury, and exogenous inducer application. The mouth secretions/regurgitants of herbivores consist of many plant defence extrinsic molecules of elicitors; the crucial ones are conjugates of fatty acids (FACs). FACs are comprised of two related groups or moieties such as a fatty acid or an amino acid. It has been found that the fatty acid and amino acid develop from the plant and the insect , respectively, and are made in the midgut of herbivores. FACs not only act as main elicitors for plants to show the unique insect-plant interplay to discern insect infestation but also are intricated in insect nitrogen metabolism process. The first FAC separated from mouth secretion of the beet armyworm S. exigua larvae was N-(17-hydroxylinolenoyl)-l-glutamine (volicitin), and it excites maize plant tissues to develop volatiles, which allure natural enemies of the insect pest (Alborn et al. 1997). In tobacco plants, regurgitation of the tobacco hornworm, M. sexta, has N-linolenoyl-glu, a potential elicitor of volatile emissions. Additionally, few FACs stimulate mitogen-activated protein kinase (MAPK) pathway, developing many plant defence chemicals playing a part in signalling transduction regarding different stresses such as drought, diseases, and insect infestations.

MAPK signal pathway in eukaryotes and its important contribution in plant gesturing particularly for pathogen stresses are well demonstrated. The chief role of MAPK in governing plant transcriptomes has been described (Wu and Baldwin 2010). In tobacco plants, few FACs stimulate accretion of 7-epi-jasmonic acid, which operates as insect defence genes. Moreover, FACs also encourage nicotine and proteinase inhibitors (PI) in the coyote tobacco, Nicotiana attenuata (Torr. ex S. Watson) (Wu and Baldwin 2010; War and Sharma 2014). The plant plasma membrane is open to the environment and stimulates a large number of events following identification of pest injury. Alterations in cell membrane potential (Vm) actuated by insects are followed by fast electrical cues, which are functioning systematically in nature. Calcium ions (Ca+2) act as a second messenger signal pathways in many crop plants. After the insect attack, the signal may be seen a few seconds as a single transient oscillation or duplicate spikes with particular subcellular localization lag time, amplitude, and frequency. The Ca+2 cues stimulate calmodulin and other calcium-sensing proteins. This encourages a cascade of downstream impacts, like changed protein phosphorylation and gene expression structures (Furstenberg-Hagg et al. 2013). Herbivory results in the gathering of plant hormones; the main are salicylic acid (SA), jasmonic acid (JA), and ethylene. They arbitrate different signal transduction pathways found in plant defence devices against different biotic and abiotic strains. The important transduction pathways associated with plant protection against insects are phenylpropanoid and octadecanoid pathways arbitrated by SA and JA, respectively. All these pathways result in the synthesis and gathering of toxins at the site of feeding or in other plant parts, which are then carried to the location of feeding. Besides, antioxidative enzymes engaged in plant defence get gathered in plant tissues damaging site (Wu and Baldwin 2010). Yan et al. (2015) described the amassing of nonprotein amino acid 5-hydroxynorvaline in leaves of maize inbred line B73 following herbivory by the corn leaf aphid Rhopalosiphum maidis (Fitch) and the beet armyworm S. exigua and in response to application with methyl jasmonate, salicylic acid, and abscisic acid. Basic and actuated defences can either be direct or indirect. Direct defences spot the herbivores, while indirect defences work through the engagement of predators of herbivores in the support of plants. Some volatile organic compounds (VOCs), having terpenoids, fatty acid derivatives, and a few aromatic compounds, act as attractants to the natural enemies of insect pests (Mithofer and Boland 2012).

1.3.5 Insect Defences Encountering Plant Defence Mechanisms

Through an abundance of structural and chemical defence mechanisms, plants avoid themselves from insect attack. These defences may have applied heavy selection pressure on the herbivores leading to the development of adaptations in insects. The insect modifications to plant defences can be physical, behavioural, or biochemical and consist of different devices, for example, penetration barricades, toxic excretions, sequestrations, temporary attaching with carrier proteins and stocking of toxins in adipose plant tissues, enzymatic detoxifications, and feeding site mutations. It is crucial to know about these insect modifications to plant defences to reduce their impacts on the steadiness of resistance in plants to insects. The major counteracting defence insect strategies to plant defences (War and Sharma 2014; Bruce 2015) are briefly presented below.

1.3.5.1 Insect Processes to Plant Defence Mechanisms

Several herbivores have evolved special tools to get the better of the slippery waxy cover which gives a big hurdle to the walking and motion of insects on plant surfaces. For good bonding to the slippery cover, the minute setae on tarsal pulvilli of few chrysomelids expel an adhesive substance (Gorb and Gorb 2002). Leafhoppers of Empoasca species can utilize their tarsal pulvilli as suction cupping device (Lee et al. 1986), though several lepidopteran caterpillars have adhesive silken thread as a rope ladder to the leaf cover to act as a foot clasping instrument (Eigenbrode 2004). To control the complication of trichomes on the plant layers, the aphid Myzocallis schreiberi Hille Ris Lambers and Stroyan possesses a special device in the shape of claws and flexible empodia that function to have a fine grasp on the short woolly trichomes on the host plant surface, the Holm oak, Quercus ilex Linnaeus (Kennedy 1986). The hardness of leaves has been reported to minimize the attack of plant feeders. As a modification to the plant toughness, in caterpillars of Pseudaletia unipuncta Haworth, the head and chewing musculature are double the size when eat on tough grasses as compared to soft artificial nourishment; nevertheless, body mass is alike (Bernays 1986). Water lily beetles Galerucella nymphaeae (Linnaeus) consuming the tough water lily have excessively larger mandibles than conspecifics eating on the great water dock grin, Rumex hydrolapathum Huds., second host with mushy leaves (Pappers et al. 2001).

1.3.5.2 Insect Modifications Against Protease Preclusions

In a few host plants when the insect attack, they make protease inhibitors for protection. Insect attack on N. attenuata immediately produces and gathers trypsin PIs; M. sexta and S. exigua larvae accomplished better on trypsin PI-lacking plants in comparison to alike plants making PIs (Zavala et al. 2004; Steppuhn and Baldwin 2007). Still, several insects have modified to host plant PIs, which enhances the attack to the host crop plants. This defensive response to PIs by herbivores is a key barricade to the exploitation and use of PIs for a firm plant protection permitting the devices by which insects prevent the PI-based plant defence mechanism. In crop insect pests, two kinds of resistance or adaptation strategies to protease inhibitors have been established. One of them is based on the different proteases which are contrary to PIs (Parde et al. 2010). These unfeeling proteases can be found constitutively in the plant system and/or are actuated when the other proteases are forbidden to recompense their damages (Jongsma et al. 1995; Parde et al. 2012). S. exigua has been described to modify to potato proteinase inhibitor II by the gut proteinase actions, which is not developed by the PIs. Additionally, when tested on the soybean proteinase inhibitor (SPI) diet, insensitivity to the inhibitor for larval proteases was observed (Brioschi et al. 2007). Trypsin insensitivity to host plant PIs has been delineated from Agrotis ipsilon (Hufnagel), T. ni, and H. zea (Volpicella et al. 2003). Other resistance mechanisms to PIs in herbivores require the synthesis of particular proteases, which are capable to break down the protease inhibitors so as to decrease their inhibitory action. Proteolytic inactivation modification is a crucial mechanism evolved by herbivores to resist the proteolytic inhibition by PIs. When feeding on artificial diet with soybean PIs, a trypsin-like enzyme is manufactured newly by S. frugiperda (J.E. Smith) larvae (Brioschi et al. 2007). The larvae of diamondback moth, Plutella xylostella Linnaeus, have been observed to be insensitive to mustard trypsin inhibitor 2 (MTI2). Such insensitiveness has been ascribed to the deterioration of MTI2 by the pest , thus preventing the impact of the PI (Yang et al. 2009).

1.3.5.3 Conversions to the Mustard Oil Bomb

The ‘mustard oil bomb’, also called as the glucosinolate-myrosinase system, found in Brassicales (Brassicaceae, Capparidaceae, Tropaeolaceae), composes the most efficient and well-researched plant defence process against herbivores. Glucosinolates are categorized and are secured from thioglucosidase—myrosinase—their hydrolysing enzyme under usual environmental conditions. Although the glucosinolates are found in several plant cell systems, the myrosinase is only centralized in dispersed plant tissue cells. The myrosinase and glucosinolate meet together developing the unstable aglycones on tissue feeding, which automatically adjust into different active substances, mostly nitriles and isothiocyanates (Li et al. 2000; Hopkins et al. 2009). It has been described that more glucosinolate- and myrosinase-rich lines of Brassica juncea (Linnaeus) Czern. are more resistant to larvae of Spodoptera eridania (Cramer) than those with little contents of these inhibiting chemicals (Li et al. 2000). The larvae of T.ni prevented A. thaliana ecotypes that developed isothiocyanates on hydrolysis of glucosinolate and rather attacked on ecotypes that caused nitriles (Lambrix et al. 2001). Additionally, some parasitoids utilize glucosinolates that are produced by feeding herbivores to trace their host insects. In these instances, glucosinolates have a double function for the damaged plant , in direct as well as in indirect protection (Hopkins et al. 2009). Even few insects utilize glucosinolates for their own shield. Myzus persicae (Sulzer), Athalia rosae (Linnaeus), and P. rapae isolate glucosinolates into their haemolymph and body cells (Muller and Brakefield 2003; Kazana et al. 2007; Bridges et al. 2002). Upon the attack of natural enemies, the haemolymph releases glucosinolates that discourage the ants and the predatory wasps (Muller and Brakefield 2003). Few aphids particularly Brevicoryne brassicae (Linnaeus) and Lipaphis erysimi (Kaltenbach) seclude glucosinolates from the sap of phloem (Kazana et al. 2007; Bridges et al. 2002). Moreover, caterpillars of P. rapae clean the glucosinolates from plants by altering them contrary to toxic products to inert metabolites using a process of nitrile-specifier protein (NSP). The NSP procedure in the gut of P. rapae regulates the glucosinolate hydrolysis resulting into nitrile formation rather than toxic isothiocyanates (Wittstock et al. 2004).

1.3.5.4 Modifications Against Tannins

With the protein amino groups, tannins make hydrogen or covalent bonds, which result in the precipitation of proteins and the digestive enzymes of insect pests. Additionally, the chelation process of metal ions in the insect body by tannins minimizes their accessibility to the insects, thus influencing their development and multiplication. Tannins have also been described to stop feeding on plants and develop midgut lesions and pharmacological toxicity in insects (Bernays and Chamberlain 1980). Nevertheless, insects have evolved many adaptations to prevent the tannin poisoning. The important modified procedures insects utilize to keep away from the toxicity of tannins is the pH of the gut in alkaline form, assimilation of tannin using peritrophic membrane, polymerization, and discharging of the polyphenols after concentrating them (War and Sharma 2014). The surfactants created by lipid digestion in the gut lumen avoid protein precipitation (Martin et al. 1987). The degree of oxygen in the foregut also contributes in the tannin toxicity. When the pH is more, oxygen levels are short; it minimizes the tannin autoxidation process, resulting in reduced toxicity. In lowering the toxicity of tannin, the antioxidative structure of insects also contributes significantly. Ascorbate lowers the tannin oxidation and result in reactive oxygen species (ROS) in the insect stomach (Krishnan and Sehnal 2006). Grasshoppers acquire a powerful midgut antioxidative defence mechanism, which empowers them to resist tannins. This antioxidative defence system mostly contains glutathione, α-tocopherol, and ascorbate. In S. gregaria, the resistance to tannins and its relation with peritrophic membrane have been ascribed to the ultrafiltration of tannins. In few insects such as Melanoplus sanguinipes (Fabricius), tannic acid does not tie with the peritrophic membrane. Additionally, peritrophic membrane saves the insect epithelium against lesions and any harm by ROS by assimilating extremely reactive ferrous ions (Barbehenn 2003).

1.3.5.5 Phytochemicals and Their Enzymatic Detoxification

To conquer plant chemical protections, enzymatic detoxification of toxic chemicals arbitrates the modification of herbivores to plant allelochemicals and thus assists the insects. Herbivores respond completely to the toxic allelochemicals, when offered with the natural host plant diet or merged in the artificial diet, by enhancing the metabolic processes that lead to the making of detoxifying enzymes, for example, monooxygenases and glutathione-S-transferases (GST) (Nitao 1989; Wadleigh and Yu 1988). The procedures of detoxification that work in insects rely on the chemistry of plant , and its degrees are mostly affected by the concentration of allelochemicals in the host plant (War and Sharma 2014). Insects use different enzymes for the detoxification of insecticides and allelochemicals of plants, and few strategies are universal (Francis et al. 2005; Scott et al. 2010). The most important is the process of polysubstrate monooxygenases which is also known as mixed-function oxidases. The constituent at the end of this structure is cytochrome P450, which is known as it assimilates maximum light around 450 nm when combined with carbon monoxide. Cytochrome P450 amalgamates even with the toxic substrate and with molecular oxygen, accelerating the oxidation process of the substrate. Cytochrome can merge with various lipophilic substrates and occurs as many isozymes that differ in their substrate explicitness (Feyereisen 2006). The P450s are considered as one of the main operators in insect-plant coexistence, as these are utilized by the host plants to release toxins and by the herbivores for plant chemical detoxification (Schuler 1996). Drosophila mettleri Heed living in desert area feed on cactus having toxic allelochemicals contain adaptable quantities of P450 associated with the metabolism of such toxins (Danielson et al. 1997). The metabolism of isothiocyanates, for example, 2-phyenylethylisothiocyanate, indole-3-carbinol, and indole-3-acetonitrile, in S. frugiperda midgut microsomes is Cyt P450-reliant (Yu 2000). Conversion of lepidopterans to minor metabolites of plants, for example, furanocoumarins, has been ascribed to P450s. Black swallowtail, Papilio polyxenes Fabricius, living on plants having furanocoumarins in diet allows up to 0.1% xanthotoxin (Berenbaum 1991a), which is cleaned by P450 monooxygenases (Bull et al. 1986). An apparent concept of participation of P450 in detoxification of allelochemicals in plant system occurred after CYP6B1 sequencing from P. polyxenes, which instructs for P450s. Coding for P450s, expression of CYP6B161 and CYP6B162 is actuated in cell lines of lepidopterans, showing the participation of P450s in metabolism of linear furanocoumarins, for instance, xanthotoxin and bergapten (Ma et al. 1994). Several P450s found in phytochemical detoxification have been derived from insects, such as from parsnip webworm, Depressaria pastinacella Duponchel (Cianfrogna et al. 2002), M. sexta (Stevens et al. 2000), and Helicoverpa species. Moreover, the transformation of dihydrocamalexic acid to camalexin, which are the main Arabidopsis phytoalexins, is accelerated by cytochrome P450 PAD3 (Schuhegger et al. 2006). Resistance to glucosinolates in aphid is ascribed to the CYP81F2, which is a downriver bit of the indolic glucosinolate pathway mechanism (Pfalz et al. 2009). P450s have also been delineated from several other herbivores where they work to metabolize the phytochemicals. For instance, in Musca domestica Linnaeus, CYP6A1 detoxifies the terpenoids (Andersen et al. 1997). In H. armigera, P450 monooxygenase CYP6AE14 metabolizes gossypol (Mao et al. 2007); in Anopheles gambiae Giles, CYP6Z1 detoxifies xanthotoxin and bergapten, furanochromones, and natural myristicin, safrole, and isosafrole (Chiu et al. 2008). However, CYP6Z2 detoxifies xanthotoxin, lignin, piceatannol, and resveratrol (McLaughlin et al. 2008); and in Diploptera punctata Eschscholtz, CYP4C7 metabolizes sesquiterpenoids (Sutherland et al. 1998). In bark beetles, Ips pini Wood and Bright and Ips paraconfusus Lanier metabolize the monoterpenes, sesquiterpenes, and diterpenoid resin acids by using P450s (Seybold et al. 2006). Another enzyme system, the glutathione-S-transferase (GST) is occurring in insect resistance to host plants by metabolism of xenobiotics and catalyzation of the conjugation of electrophilic molecules using thiol category of lowered glutathione, leading in fast defecation and deterioration (Francis et al. 2005). This enzyme class has been incriminated in detoxification of insecticides that are neurotoxic and influence the development and multiplication of insects. Among them are spinosad, diazinon, DDT, nitenpyram, lufenuron, and dicyclanil (Sintim et al. 2009). Many reports have suggested the significance of GST in insect modification to phyto-glucosinolates, and minor plant metabolites added in the artificial diet of S. frugiperda, S. litura, T. ni, M. persicae, Aulacorthum solani (Kaltenbach), and A. pisum (Enayati et al. 2005). In M. persicae, more production of GST has been assigned to insect modification to glucosinolates and isothiocyanates in Brassicaceae, though there is no straight conflict of isothiocyanates, as aphids prick with their stylets right into the phloem tissues (Francis et al. 2005; Kim et al. 2008).

1.3.5.6 Counter-Defence Mechanisms in Herbivoral Gut Symbiontic Systems

In reply to the insect attack, the evocation of plant defences has been reported to be regulated by chat between jasmonic acid (JA) and salicylic acid (SA) pathways of signalling. Insects have various microbes in their gut area, and such symbionts can adapt to plant-insect interplays (Hogenhout et al. 2009). In tomato, Chung et al. (2013) described that the grubs of Colorado potato beetle, L. decemlineata, utilized digestive track bacteria in their mouth secretions to control anti-herbivore defences. The antibiotic-untreated larvae reduced the development of JA and JA-responsive anti-herbivore defence mechanisms but enhanced SA gathering and SA-reactive gene expression. The reduction at the cellular level or plant defences led in increased larval development. In a study, the gut bacteria in three genera Stenotrophomonas, Pseudomonas, and Enterobacter were incriminated for defence subduing. Hammer and Bowers (2015) suggested the ‘gut microbial facilitation hypothesis’, saying that differences in insects in their ability to feed chemically protected plants can be due to the differences in their related microbial groups. Such reports have worn help from molecular research on gut bacteria. In Japanese common stink bug, Megacopta punctatissima (Montandon), the gut bacteria are competent of decarboxylating oxalate, a minor metabolite common in plants (Nikoh et al. 2011). Bacteria nursing in the gut of the mountain pine beetles is related with the terpene metabolism (Adams et al. 2013) and are efficient in detoxification of terpenes in vitro (Boone et al. 2013). From the midguts of gypsy moth larvae, the Acinetobacter species are able to detoxify the dietary phenolic glycosides (Mason et al. 2014). With the existence of gut bacteria largely in the insect mouth secretions, these may be linked with seizing of plant defence reactions in other instances of insect-plant interplay.

1.4 Insect Responses to Artificially Induced Plant Defence Mechanisms

With a contagious long history in the USA, the Hessian fly (HF), Mayetiola destructor (Say) (Cecidomyiidae: Diptera), is a major pest of wheat and spread in North Africa, Europe, West and Central Asia, North America, and New Zealand (Buntin and Chapin 1990). It has been effectively controlled using insect-resistant varieties bearing HF-specific R-gene(s). Nevertheless, in 6–8 years, virulent biotypes of HF are competent of defeating its resistance (Chen et al. 2009; Stuart et al. 2012). After egg-hatching, the neonate HF larvae creep on the upper surface of leaves and go to the seedling, where it continues feeding in susceptible cultivars but flops to do so in resistant genotypes. HF carrying virulent biotypes on a susceptible variety lead to a suitable interaction obliging pest elite; however, a virulent biotype in 3–5 days on the resistant variety causes incompatible interplay and death of insect (Subramanyam et al. 2015).

About 35 resistance genes (H1–H3, h4, H5–H34, and Hdic) from wheat and associated crop plants have been distinguished and incorporated in wheat varieties grown commercially (Chen et al. 2006; Stuart et al. 2012). For study of gene-for-gene (GNG) interaction, the HF wheat system is regarded as a model process between host and the insect (Hatchett and Gallun 1970; Subramanyam et al. 2015). Plants react to the injury of HF larvae by gathering of reactive oxygen species (Liu et al. 2010) in resistant varieties having R genes and the making of inhibitor enzymes (Wu et al. 2008), lectins (Williams et al. 2002; Subramanyam et al. 2008), and other minor metabolites (Liu et al. 2007). Contrary to this, the adaptable interactions are distinguished by enhanced nutrient accessibility at the place of injury along with the gathering of nitrogen-rich molecules (Liu et al. 2007; Williams et al. 2011). Reports mention that the HF is competent to control resistance via recessive mutations in similar avirulence (HFAvr) genes (Aggrawal et al. 2014). The HFAvr genes code for proteins known as effectors that are inserted with the saliva into the plant tissues during the injury (Hogenhout et al. 2009). Plants bearing R genes can identify such secretions and excite the defensive routes (Chisholm et al. 2006). In virulent HF biotypes, the Avr proteins are adapted to either prevent discovery by the plant or fail to activate the defending mechanism (Chen et al. 2016).

Darwin in his magnum opus On the Origin of Species in 1859 mentioned that the ‘Coadaptations of organic beings to each other…’. Organisms interact with each other with either the similar or another type of animal. Coevolution relates to genetic swap in two interacting animal species. Ehrlich and Raven (1964) reported the first interaction between Monarch butterfly and milkweed (a host plant ). Neither any plant is susceptible to all the herbivores nor is any insect a pest of all plants it experience in environment. Additionally, less than one-third of all insects such as exclusively Lepidoptera, Orthoptera, and Phasmida; predominantly Hemiptera and Thysanoptera; or partially Coleoptera, Diptera, and Hymenoptera are plant pests; however, these belong to half of all herbivores. So mostly all crop plants have evolved having impressive structural and biochemical defensive mechanisms against the insect pests. However, those insects which are competent to break these barricades in one or more plants can reach these plants for feeding (Arora 2012). Insect pests continue evolving adaptations for detoxification or breaching such defensive systems. The results of a coevolutionary system since 400 My are the extant phytophages and their host plants (Labandeira 2013). For discerning their ecosystem, determining the sensory input, and reacting to it appropriately, insects have developed a sensitive biological system (Martin et al. 2011). Lucrative host locating and liking are basically governed by chemical signals. The insect reactions rely on host and environmental conditions (Riffell et al. 2009; Webster et al. 2010). Plants have also developed many structural and chemical defence mechanisms against insects. The insects one after another have developed to prevent these barricades, and many ideas have been put forward.

1.4.1 Coevolution

Theory of coevolution was detailed by Ehrlich and Raven (1964) and backed by Berenbaum (1983) later on. Many plants produce a prototypical phytochemicals according to this theory that is balmily poisonous to phytophages and active in the plant autecologically or physiologically. Few insect species attack on plants with mild phytochemicals, thus reducing plant strength. Due to plant mutations, their recombinants produce novel, more noxious phytochemicals to occur in the plant system. Similar phytochemicals can occur independently in distantly associated plants. Insect attack is decreased because of toxic or repellent characteristics of the novel phytochemicals, so crop plants with higher potent defences are liked by the herbivores. Phyllobrotica species infest monogamously on Scutellaria species as reported by the cladograms (Farrell and Mitter 1990). Evidence is also available at the level of populations. Analysis reports of various populations of wild parsnip, Pastinaca sativa Linnaeus, and its pest the parsnip webworm, Depressaria pastinacella Duponchel, described trait matching between furanocoumarin-based chemical defence mechanisms in plants and cytochrome P450 monooxygenase-based insects’ detoxification profiles (Berenbaum and Zangerl 1998; Zangerl and Berenbaum 2003).

Coevolution is well defined in the brassicaceous plants and the pierid butterflies. The glucosinolate-myrosinase system developed in Brassicales 90 Myr before shows a major finding in anti-herbivore defence devices by plants. However, the Pierinae butterflies which used Fabales as host showed a metabolizing process as nitrile-specifier protein (NSP) and colonized the Brassicales. This resulted in increasing the species diversification rates in Pierinae as compared with that of their sister clade Coliadinae, whose members did not colonize Brassicales, thus lending strong support to the coevolutionary theory (Wheat et al. 2007; Edger et al. 2015).

1.4.2 Sequential Evolution Theory

The evolution of herbivores that comes after the evolution of plants, without affecting plant evolution significantly, is proposed by the theory of sequential evolution (Jermy 1976, 1984). So far reciprocal selective interactions between plants and insects have not been demonstrated. Insects select their hosts generally on the basis of chemical signals. Any alterations in chemical composition of plants or their chemosensory perception by herbivores may result in the development of new associations of insects and host plants. More evidence in support of the theory was presented by Labandeira (1998) and Janz et al. (2006), who showed that species richness in butterfly family Nymphalidae was strongly correlated with diversity of host use.

1.4.3 Diffuse Coevolution Theory

Diffuse coevolution or community coevolution theory suggests that in lieu of the pairwise reciprocal evolutionary interactions, coevolution must be regarded in a community context and not simply as a reciprocal interaction of two different species. Plant may be influenced by herbivore abundance, diseases, competing conspecifics, and plants of different species along with different host plants and insect pests and organisms at higher tropic levels (Fox 1988).

1.4.4 Geographic Mosaic Theory of Coevolution

This theory expresses that the coevolutionary system works at the level of populations rather than at a species level. Thompson (1994, 1999, 2005) stated that interspecific interactions frequently vary in biodiverse system. Additionally, populations vary in the extent to exhibit extreme specialization to one or more species. Gene flow among species, genetic drifting, novel trait selections, and extinction of some species reform the geographic mosaic of coevolution as the adaptations and specialization designs evolved locally, distributed to other population, or are finished. The result is a dynamic geographic pattern of coevolution between any two or more species. Across the Japanese islands, the coevolutionary relationship between the obligate seed predator; the camellia weevil, Camellia japonica Linnaeus; and its host plant , the Japanese camellia, Camellia japonica Linnaeus, serves as an interesting instance of geographic mosaic (Toju and Sota 2006; Toju et al. 2011). The thickness of camellia pericarp through which the female weevils inserted its ovipositors to deposit eggs into seeds linked with the length of the rostrum in female insects. In addition, the pericarp was significantly wider on islands abundant of weevils than on islands lacking weevils, and this characteristic was genetic.

1.5 Applied Aspects of Insect-Plant Correlations

For sustainable agriculture, the complex comprehension of insect-plant correlations has great applied importance. Growers desire to reduce crop damages brought about by insect pests and enhance crop yield. The fundamental strategies of insect-plant interplays are the main to obtain these aims as below:

1.5.1 Reproducing Resistant Cultivars Against Insect Pests

The most eco-friendly are the insect-resistant varieties which deliver economically practicable and ecologically durable choices for insect pest control . For more than a century, the research on breeding of insect-resistant crops has been carried out and bloomed as a new area of studies with the innovative research of Prof. R H Painter at Kansas State University, Manhattan, Kansas, USA, in the first half of the twentieth century (Painter 1951). An excellent early victory in using host plant resistance in pest control was achieved by grafting the European grapevines onto the resistant North American rootstocks (Painter 1951) against the grape phylloxera Daktulosphaira vitifoliae (Fitch) in France. In India, the research of Hussain and Lal (1940) found hairy cotton varieties resistant to jassid and resistant to cultivars, for instance, Punjab 4F, LSS, and 289 F/43, grown on large agriculture areas where jassid was a major pest by 1943. Reproducing stress-resistant plants has obtained great significance over the past 70 years with the participation of national and international agricultural research institutes along with seed producers from private sector. Dozens of insect-resistant varieties have been evolved globally and are cultivated largely for crop production sustainability (Panda and Khush 1995). Economically, insect-resistant genomes used in cropping recently rescue us more than US$2 billion every year (Smith and Clement 2012). Isolation and cloning of genomes for wanted chemical traits have accelerated the breeding of insect-resistant varieties. An advance comprehension of plant resistance reactions to herbivory is also required for further manipulation of generated resistance and plant-emitted volatile substances for breeding of insect pest-resistant cultivars (Sandhu and Arora 2013). Utilization of insect-resistant genomes from unconnected microbes and their chartering into elite germplasm is another fruit-bearing proposal which has established largely in applied research. From Bacillus thuringiensis , 20 Bt genes revealing resistance to lepidopteran and coleopteran pests have been merged into cotton, corn, potato, soybean, and other crops (Shera and Arora 2015).

1.5.2 Insect Pest Management Using Cultural Practices

For suppressing insect pest attack, cropping pattern and insect control measures such as cultural practices play a crucial role. The knowledge of host plant-insect pest association is beneficial to alter the plant ecosystem against the insect pest or in favour of the parasites and predators. Early crop sowing, for example, in Northern India, has been observed to decrease the gall midge and leaf folder attack in rice, shoot fly and headbug losses in sorghum and millets, white grub injury in groundnut, and aphid destruction in crucifers (Dhaliwal and Arora 2006). Enhanced intra-field diversity via interculturing, sowing of trap crops, or hedging rows led to minimum losses by many insect pests. Tomato interculturing in cabbage has been described to decrease the destruction of diamondback moth. Intercropping with sesame resulted in decreased penetration of okra roots by Meloidogyne incognita second-stage juveniles (J2) and delayed nematode maturation; it favoured development of M. incognita males and increased yields of okra and chickpea in field tests (Tanda and Atwal 1988). The largest effect of intercropping sesame with okra was when they were 15–30 cm apart. In pot tests, most J2 penetrated okra roots in sandy loam soil and fewest in clay soil. Trap crop of African marigold decreases the damage of fruit borer H. armigera in tomato (Srinivasan 1994). Napier grass and Napier millet work as trap crops for minimizing the attack of stem borer C. partellus in maize and sorghum (Khan 1999; Dhaliwal and Arora 2006). The natural enemies of herbivores may achieve larger population densities in polycultures as compared to monocultures, because polycultures often provide extra food sources, for example, honeydew, nectar and, pollen, and more refuges where insects can protect in the shade (Coll 1998). Among 130 predators observed in polycultures, more than 50% achieved higher population densities, as compared to monocultures, whereas less than 10% of them attained poor population densities when surveyed (Andow 1991).

1.5.3 Botanical Insecticides

Several crop plants have evolved tracks to diverse groups of chemicals to avoid their utilization by insect pests since many centuries. These biochemicals put forth behavioural, physiological, and biochemical impacts on insects, and some of them may even kill the sensitive insects. Bioinsecticides have been used since ancient times by humans. In various parts of the globe for centuries, many plants and their products such as neem, pyrethrum, Tephrosia, tobacco, derris, Ryania, and sabadilla have been used to save the farm crops, grains, and other commodities from the devastations of insects and other types of pests (Dhaliwal and Arora 2001). Plant chemicals have also acted as prototypes for the evolution and synthesis of Nobel categories of insecticides. Pyrethrum, isolated from the dried flowers of Chrysanthemum cinerariaefolium Linnaeus, has been utilized as an important insecticide since prehistoric times. It is a powerful mephitic against insects and relatively harmless to mammals; however, it is very photolabile (Casida 1973). Consequently, the chemical structure of pyrethrum was explicated to synthesize its analogues with better photostability. During the 1980s, several chemicals such as fenvalerate, deltamethrin, fluvalinate, and cyfluthrin rose as famous insecticides (Dhaliwal and Arora 2006). Likewise, similar synthetic analogues of nicotine, an important bioinsecticide named as neonicotinoids, was developed from tobacco and now largely utilized against a variety of sucking insect pests and mites (Simon-Delso et al. 2015). Accordingly, bioinsecticides have not only demonstrated beneficial directly in pest management but have also worked as a model for the modern groups of synthetic insecticides. As plants possess tens of thousands of similar biochemicals, the compass of their utility in insect pest control is almost unlimited.

1.5.4 Insect Pest Biocontrol

For enhancing the effectiveness of natural biocontrol and integrated pest management technology (IPMT) as we proposed, the significance of research on tritrophic and multitrophic interplays can scarcely be aggravated. Volatile substances released by plants are demonstrated to allure predators of herbivores (Weseloh 1981). Ramachandran et al. (1991) described that the parasitoid Microplitis demolitor Wilkinson was captivated by the volatile 3-octanone liberated by soybean, a host plant of the soybean looper, P. includens. Parasitoid was distinctly more apprehended by the volatile guaiacol, which was located in its hosts’ frass only; however, similar interactions may not benefit the predators. Hare (1992) observed a gamut of interactions between the predators and resistant plants such as synergistic to additive to none apparent and disruptive or antagonistic effects. A meta-analysis of 27 studies on interaction of resistant plant varieties and biocontrol of insect pests was carried out by Dhaliwal et al. (2004). About 29.6% showed antagonistic effect, whereas 25.9 and 33.3% were the cases of synergism and additive relationship, respectively. As understanding of multitrophic interactions enlarges, scientists and IPMT research workers should use it for insect pest control strategies (Verkerk 2004).

1.5.5 Use of Insect Behaviour in Control

In response to olfactory, visual, tactile, acoustic, and gustatory-sensory reaction from the host plant and ecosystem, insect behaviour is educed. For feeding and oviposition on host plants, cues are used by insects which can assist in the exploitation of similar behaviour, resulting in decreased crop losses (Foster and Harris 1997). In pest control, alluring and killing technique is the most famous behavioural manipulation manipulated. The attack of Japanese beetle Popillia japonica Newman is effectively controlled by a mixture of phenethyl propionate, eugenol, and geraniol, a female sex pheromone, and a food lure (Ladd et al. 1981). Foods baits have also been reported beneficial for monitoring and managing of tephritids. In the USA, protein hydrolysate-baited traps having insecticides have been proved effective against the Mediterranean fruit fly, Ceratitis capitata Wiedemann (Chambers 1978). Attraction and annihilation, which are innovative techniques, have resulted successfully against the apple maggot fly, Rhagoletis pomonella (Walsh). Using olfactory and visual stimuli, the female flies search apple plants and acceptable oviposition locations. Wooden spheres in red colour and layered with a sticky material at one trap tree−1 provided a better control of fruits from R. pomonella (Aluja and Prokopy 1993; Foster and Harris 1997).

1.5.6 Insect Control by Push-Pull Mechanism

The push-pull IPMT or stimulo-deterrent proposal in pest control is a new manipulation of the behavioural technology for the use of pulling and pushing constituents in sequence to repel the insect away from the principal host plant and towards the crop used for trapping, from where they may be detached later on (Khan et al. 1997; Cook et al. 2007). This approach has been effectively practicable for the control of stem borers such as C. partellus, Eldana saccharina Walker, Busseola fusca Fuller, and Sesamia inferens Hampson attacking maize and sorghum in Eastern and Southern Africa. The egg-laying female borers are repulsed from the main crop by repellent non-host intercrops, especially molasses grass, silver leaf desmodium, or green leaf desmodium (push), and like to deposit eggs on alluring trap crop plants, basically Napier grass or Sudan grass (pull). Interculturing of molasses grass with maize enhanced parasitization by Cotesia sesamiae Cameron in addition to decreasing the damages of stem borer (Khan et al. 2011). Push-pull plans have also been efficiently used against Helicoverpa in cotton, L. decemlineata in potato, rapeseed pollen beetle Brassicogethes aeneus (Fabricius) in oilseed rape, onion maggot Anthomyia antiqua (Meigen) in onions, striped pea leaf weevil Sitona lineatus (Linnaeus) in beans, western flower thrips, Frankliniella occidentalis (Pergande) in chrysanthemum, and bark beetles (Scolitidae) in conifers, along with many veterinary and medical insect pests (Cook et al. 2007).

1.5.7 Insect Biotype Control

Insect biotypes are insects that can live on and damage cultivars that have resistant genes. The cultivation of insect-resistant varieties regularly applies selection pressure on the aimed pest , which reacts by evolving new physiological and behavioural tools for feeding and multiplication on the resistant varieties. In breeding schemes for cultivar resistance, biotype selection is one of the key pressures experienced. This idea concerns gene-for-gene connection between the host plant-resistant gene and insect pest virulent gene. Aphids have 18 species out of 39, in which 2 or more biotypes have been observed (Sandhu and Arora 2013). Brown plant hopper, Nilaparvata lugens Stal, on rice (Brar et al. 2015) and Hessian fly, M. destructor, on wheat are the key pests in which biotype evolution has resulted in to collapse of resistance in the field (Aggrawal et al. 2014; Subramanyam et al. 2015). Insect resistance can be enhanced by sequential release of varieties, gene pyramiding/stacking, and gene rotation (Sandhu and Arora 2013). A better advance knowledge of insect-plant interactions is important for effective control of insect biotypes for higher resistant genotype stability.

1.5.8 Biocontrol of Weeds

The damages created by weeds are more than those done by insect pests to field crops, and the herbicide utilization surpasses that of insecticides in plant protection programs, so there is an immediate demand to reinforce biological management of weeds. Exotic weeds may be effectively controlled by monophagous or oligophagous insects from the origin of plant place. Prominent successful weed control instances are of shellmound prickly pear, Opuntia stricta (Haworth) Haworth, in Australia using small Argentinian moth, Cactoblastis cactorum Berg (Dodd 1940), and of giant Salvinia, Salvinia molesta D. S. Mitchell, in Papua New Guinea by the release of weevil Cyrtobagous salviniae Calder & Sands imported from Brazil (Room 1990). Extensive schemes on biocontrol of weeds in Hawaii using herbivorous insects and pathogens have been carried out, leading in effective management of 7 out of 21 aimed weeds and significant partial management of another 3 species (Gardner et al. 1995; McFadyen 2003). For the management of native weeds, native insects have also been artificially reared and released. Native coccids, Austrotachardia sp. and Tachardia sp., are released for the suppression of Cassinia sp., native woody shrubs in Australia (Holtkamp and Campbell 1995). For controlling the parasitic weeds, Orobanche spp. in the southern USSR, the stem-boring agromyzid Phytomyza orobanchia Kaltenbach has been conserved and released (Kroschel and Klein 1999).

1.5.9 Insect Pollinator Protection for Crop Production

Crop insect pollinators are crucial for effective pollination and reproduction by a large variety of angiosperms (Tanda 2019a, b, c, 2020, 2021a, b, c, d, e, f). Even self-pollinating cotton crop species may exhibit yield boosting close to an efficient pollinator and its ecosystem (Tanda 1983, 1984, 2020, 2021a, b, c, d, e, f). Coffee shrubs present great yield enhancement in areas with strong native or introduced bee pollinator abundance (Roubik 2002; Tanda 2021a, b). Several investigations on plant-pollinator processes have targeted on a single plant species and mostly one or a few closely related groups. However, new reports have demonstrated that pollinator networks are comparatively vague, due to spatiotemporal differences in foraging by pollinators (Herrera 1996; Waser 1998; Burkle and Alarcon 2011). It is crucial to comprehend the fundamentals of spatial and temporal difference in plant-pollinator interplay to reply questions in group structure and its functioning. It will also be beneficial in designing optimal pollinator’s protection measures (Burkle and Alarcon 2011). Environment and habitat alteration may disorder the coevality between the flower development period of plants and the active time of pollinators. Lack of nectar and pollen in crucial time may result in a decrease in pollinator’s abundance (Hoover et al. 2012; Sharma et al. 2014). An accurate knowledge of the flowering crop plant and insect pollinator interactions may be beneficial in avoiding pollinator reduction and sustainable agricultural production.

1.6 Conclusions

In the agricultural ecosystem, both insects and crop plants are ruling life forms and are engaged in complex interrelationships. For effective reproduction, a large number of flowering plants need the services of pollinators. The flower shape, size, colour, and scent all serve to allure bee pollinators, which generally feed on nectar and pollen developed by these crop plants. Additionally, about 50% of all insect populations are herbivorous and rely on green plants for food, shelter, and egg deposition. Accordingly, the plants have developed a surprising diversity of structural and biochemical barricades to save themselves from insect pests and plant diseases. The insects which are capable to manage these barriers via avoidance, detoxification, and sequestration can obtain sufficient food supplies with very small competition from other insect species. Since 400 million years ago, reciprocal modification and counterconversion between plants and insects have, thus, been the main procedure driving a steady enhancement in biodiversity of both these life forms. The investigations of such interrelationships between insects and flowering plants are of great practical significance for future sustainable agricultural productivity. Genomics, proteomics, and RNAi, advanced bio-techniques of molecular biology, provide exciting breaks for future exploration and accurate comprehension of insect-plant relationships. These biotechnological studies are important for the protection of ecosystem biodiversity and evolving insect-resistant cultivars for sustainable insect pests and weed control system.