Keywords

1 Introduction

Widespread insecticide resistance has been a major problem in a sustainable and cost-effective biointensive integrated pest management (BIPM) strategy. In addition, the increasing public concern over pesticide safety and possible damage to the environment has resulted in increasing attention being given to natural products for the control of agricultural pests (Rajendran and Sriranjini 2008; Zapata and Smagghe 2010). The coevolution of plants along with insects has compelled the use of natural chemical defenses for the management of insect pests. Several studies have focused on the potential use of botanical applications in biological control of different insect pests, since some are selective, biodegrade to nontoxic products, and have few effects on nontarget organisms and the environment (Singh and Upadhyay 1993; Isman 2000; Kim et al. 2010). Apart from all the advantages and safety aspects of botanicals as compared to synthetics, it will also give farmers the necessary psychological satisfaction. At the same time, it will be helpful in reducing the present excessive use of synthetic insecticides, which are not compatible with many biological and microbial components of an IPM package (Rattan 2010). In the past few decades, several studies have focused on the potential use of the essential oil applications in biological control of different economically important insect pests.

The essential oils may be more rapidly degraded in the environment than synthetic compounds, and some have increased specificity that favors beneficial insects, and the pesticides of plant origin are gaining increased attention and interest among those concerned with environment-friendly, safe, and integrated crop management approaches. In addition, they are playing a vital role in organic food production globally (Pillmoor et al. 1993; Rattan 2010). Their action against stored product insects has been extensively studied (Negahban et al. 2007a; Sahaf et al. 2008a, b, c; Rastegar et al. 2008; Saeidi and Moharramipour 2008; Sahaf and Moharramipour 2007; Arabi et al. 2008a, b; Ghasemi et al. 2009). Moreover, these natural derivatives are considered to be an alternative means of controlling pestiferous larvae of Lepidoptera (Jamal et al. 2011; Hasheminia et al. 2011; Yi et al. 2007; Vanichpakorn et al. 2010; Lee et al. 2001a, b). That is to say that recent research has demonstrated their larvicidal and antifeeding effects (Negahban and Moharramipour 2007a; Negahban and Moharramipour 2008; Sahaf and Moharramipour 2008), their capacity to delay development and adult emergence and cause egg mortality, their deterrent effects on oviposition (Negahban and Moharramipour 2007b; Shakarami et al. 2004; Sahaf et al. 2008b), and their arrestant and repellent action (Negahban et al. 2007b; Moharramipour et al. 2008; Sahaf et al. 2008a). In view of the various activities of the essential oils against agriculture product pests as reported by various workers, plant extracts contain compounds that show ovicidal, repellent, and antifeedant properties and can combine with other methods such as gamma radiation (Ahmadi et al. 2008a,b).

The insecticidal constituents of many plant extracts and the essential oils are monoterpenoids. Camphor, camphene, 1,8-cineol and α-pinene, linalool, methyl acetate, limonene, menthone, geraniol, citral, citronellal, thymol, carvacrol, eugenol, geraniol, and trans-anethole are well-known examples of pesticide compounds (Phillips et al. 2010; Negahban et al. 2007a; Isman and Machial 2006; Isman 2000). Additionally, monoterpenoid compounds have been considered as potential pest control agents because they are acutely toxic to insects and possess repellent (Mediouni-Ben Jemaa and Tersim 2011; Kim et al. 2010) and antifeedant properties (Sbeghen-Loss et al. 2011; Shukla et al. 2012) and ovicidal, larvicidal, pupicidal, and adulticidal activities (Yang et al. 2004; Waliwitiya et al. 2009; Murugan et al. 2012).

Finally, mostly the work has been carried on studying the effects of the essential oils, their lethal doses, and the time to achieve lethal effects, but their formulations especially as a form of micro- or nanoencapsules are in general not fully elucidated. Nanotechnology of the essential oils may act through the control release of active ingredients and overcome the restrictions of plant essential oils usage in storage and farms. Nanoencapsulation is a process in which tiny particles or droplets are surrounded by a coating to give small capsules many useful properties. In a relatively simplistic form, a nanocapsule is a small sphere with a uniform wall around it. The material inside the nanocapsule is referred to as the core, internal phase, or fill, whereas the wall is sometimes called a shell, coating, or membrane. Most nanocapsules have dimensions measured in nanometers. Nanocapsules have a polymeric shell; the active substances are usually dissolved in the inner core but may also be adsorbed at their surface (Khoee and Yaghoobian 2009).

Nanoencapsulation technique can serve as new model formulations for the development of the essential oils and their compound derivatives with enhanced activity or environmental friendliness. However, their functions and clarity on the specificity of the metabolites responsible for proclaimed insecticidal activity is lacking. Worthwhile for us, the plant origin nanoinsecticides could be exploited for the development of novel formulations with highly precise target for sustainable insect pest management in agriculture. In this chapter, we review the essential oil sources of insecticidal activity, their constituents, and mode of action and discuss the few botanical materials with potential action and their development of formulation with encapsulation for producing nanoinsecticidal products.

2 Potential and Improvement of Plant Essential Oils as New Insecticides

The use of the essential oils extracted from aromatic plants to control pests has been investigated and is well documented (Isman 2006). As a result, most essential oils come from highly aromatic species such as those in the Asteraceae, Myrtaceae, Apiaceae, Verbenaceae, and Lamiaceae plant families. Many aromatic plant species are indigenous to Iran (Naghibi et al. 2005). Iran is situated in arid and semiarid areas and has many endemic aromatic plants from different families. It therefore seems very worthwhile to mount a comprehensive screening program to determine the insecticidal efficacy of such plants. Additional research from our laboratory shows that several essential oils possess insecticidal activity effects (Table 7.1). The genus Artemisia is a member of the large and evolutionary advanced plant family (Asteraceae). More than 300 different species comprise this diverse genus which is mainly found in arid and semiarid areas of Europe, America, and North Africa as well as in Asia (Heywood and Humphries 1977). Many Artemisia species are used medicinally and hence to be of more commercial values. It has been reported that Artemisia herba-alba Asso. (Asteraceae) inhibits the asexual reproduction of Aspergillus niger Tiegh (Eurotiales: Trichocomaceae), Penicillium italicum Wehmer (Eurotiales: Trichocomaceae), and Zygorhychus sp. Vuillemin (Tantaoui-Elaraki et al. 1993). Artemisia scoparia Waldst et Kit (Asteraceae) is used as a choleretic, anti-inflammatory, and diuretic agent in the treatment of hepatitis (Hikino 1985).

Table 7.1 Examples of essential oils and insects which are known to be affected by these oils
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Artemisia is a genus that grows in many areas of Iran. There are several reports showing that species of this genus are highly toxic to stored product insects such as Callosobruchus maculatus (F.) (Coleoptera: Bruchidae), Sitophilus oryzae (L.) (Coleoptera: Curculionidae), and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) (Negahban et al. 2006b, 2007a). The effects of Artemisia annua L. (Asteraceae) crude leaf extracts on the toxicity, development, feeding efficiency, and chemical activities of small cabbage Pieris rapae L. (Lepidoptera: Pieridae) have been evaluated (Hasheminia et al. 2011). Artemisia vulgaris L. (Asteraceae) has been reported to be repellent and toxic to T. castaneum (Wang et al. 2006). The fumigant toxicity of the essential oils extracted from two plants of the Iran flora, Ruta graveolens L. (Rutaceae) belonging to Rutaceae and Ferula gummosa Boiss from Apiaceae, was investigated on C. maculatus (Hosseinpour et al. 2011). Also the toxicity of 42 essential oils extracted from Myrtaceae has been tested against S. oryzae and T. castaneum (Lee et al. 2004). Negahban and Moharramipour (2007c) reported the fumigant toxicity of three Myrtaceae species oil, Eucalyptus intertexta R.T. Baker (Myrtaceae), Eucalyptus sargentii Maiden (Myrtaceae), and Eucalyptus camaldulensis Dehnh (Myrtaceae), against three major stored-product beetles, C. maculatus, S. oryzae, and T. castaneum.

Lamiaceae family is best known for their essential oils common to many members of the family. These plants have been used by human since prehistoric times and are one of the major sources of culinary, vegetable, and medicinal plants all over the world (Naghibi et al. 2005). From this family, the genus Thymus (Lamiaceae) consists of 14 species in Iranian flora, four of which including Thymus persicus (Ronniger ex Rech. f.) are indigenous to Iran (Rechinger 1982; Nickavar et al. 2005). Thymus species have strong antifungal, antibacterial, and insecticidal activity (Stahl-Biskup and Saez 2002; Rasooli et al. 2006). These plant species and their extracts are known to have various effects on insect pests, including stored-product insects. Several studies have assessed the ability of the Thymus essential oils and their constituents as fumigants and repellents against a number of insect pests. The insecticidal and repellent activities of Thymus vulgaris L. have been reported against T. castaneum (Clemente et al. 2003), S. oryzae (Lee et al. 2001a), Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae) (Passino et al. 2004), Lasioderma serricorne (F.) (Coleoptera: Anobiidae) (Hori 2003), Spodoptera litura (F.) (Lepidoptera: Noctuidae) (Hummelbrunner and Isman 2001) and mushroom sciarids (Lycoriella mali Fitch, Sciaridae) (Choi et al. 2006). Other plant essential oils with biological activity include Thymus mandschuricus Ronniger against S. oryzae (Kim et al. 2003) and Thymus mastichina (L.) against T. castaneum (Pascual-Villalobos and Robledo 1998). Moreover, Thymus serpyllum L. which is rich in the phenols thymol and carvacrol has fumigant effect against the bean weevil Acanthoscelides obtectus (Say) (Coleoptera: Bruchidae) (Regnault-Roger et al. 1993; Regnault-Roger and Hamraoui 1995). The fumigant effect of the essential oil extracted from aerial part of the T. persicus has been investigated against the red flour beetle, T. castaneum, and rice weevil S. oryzae (Taghizadeh Saroukolai et al. 2010). Salvia mirzayanii (Rech. F. and Esfand) (Lamiaceae) is a wild-growing flowering plant belonging to the family Lamiaceae and is found in the southern area of Iran (Yamini et al. 2008). Many studies indicated antioxidant, antimicrobial, and antiviral activities of some Salvia species (Sivropoulou et al. 1997; Javidnia et al. 2002). Soleimannejad et al. (2011) has reported the high-toxicity effects of S. mirzayanii essential oil on nutritional indices of the Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae) adults.

Perovskia abrotanoides Karel (Lamiaceae), the Caspian Russian sage, is a perennial silvery white spicy foliage plant with long-lasting dark blue flowers. Despite its common name, P. abrotanoides is native to Iran, Afghanistan, Pakistan, Tajikistan, Turkmenistan, and Tibet China province (Xizang region) (Rechinger 1982; USDA 2007). The efficiency of the essential oil from P. abrotanoides has been reported as a fumigant in the management of S. oryzae and T. castaneum (Arabi et al. 2008a). Also, the oviposition deterrence and repellent activity of Thymus kotschyanus Boiss and Hohen and Mentha longifolia L. (Lamiaceae) was reported on C. maculatus (Akrami et al. 2011). Ajwain, Carum copticum C. B. Clarke (Apiaceae), is a medicinal plant, and its oil is used as pharmaceutical and in flavoring. It is an annual plant which grows in the Iran, Pakistan, and Egypt with white flowers and small brownish fruits (Zargari 1991). Chaste tree, Vitex pseudo-negundo Hand I. MZT (Verbenaceae), naturally grows around seasonal rivers in Iran. Medicinal properties of this species caused to introduce V. pseudo-negundo as a familiar drug (Filekesh et al. 2005). The insecticidal activity of C. copticum (Sahaf et al. 2007) and V. pseudo-negundo (Sahaf et al. 2008a, b) has been determined against T. castaneum, S. oryzae, and C. maculatus (eggs, larvae and adults), as an important storage pest of legume seeds in Iran (Sahaf and Moharramipour 2008).

Repellency effect, oviposition deterrency, and ovicidal activity of several essential oils of medicinal plant species growing in Iran have been investigated by Rafiei Karahroodi et al. (2008; 2009; 2011). In recent years, some commercial plant extracts have been introduced to varroa control. Acaricidal activity of the essential oils of T. kotschyanus, Ferula assa-foetida L. (Apiaceae), and E. camaldulensis has been demonstrated against Varroa destructor Anderson (Mesostigmata: Varroidae) (Ghasemi et al. 2011). In addition, combined application of the essential oils and gamma radiation is an ecologically safe method which could be used in the management of stored-product pests (Ahmadi et al. 2008a, b). Under in vitro experimental conditions, the essential oil had no significant direct effect on frequency of micronucleus induced in T. castaneum ovaries. However, using gamma radiation alone and in combination with Rosmarinus officinalis L. (Lamiaceae) has significantly increased the induced micronucleus. They indicated that gamma radiation can induce significant cytogenetic effects and in combination with R. officinalis could show synergistic effect (Ahmadi et al. 2009b).

In spite of these hopeful properties, problems related to their volatility, poor water solubility, and oxidation potentiality have to be resolved before use as an alternative pest control system, in the hope that using encapsulated formulations of the essential oil components seems to be the best choice (Clancy et al. 1992; Moretti et al. 2002). Controlled release by nanoencapsulated formulations allows the essential oil to be used more effectively over a given time interval, suitability to mode of application, and minimization of environmental damage. Our studies indicated a postingestive toxicity of the essential oil from Artemisia sieberi Besser (Asteraceae) using the nanoencapsulated formulation as potential insecticide for the control of Plutella xylostella (L.), (Lepidoptera: Plutellidae), and T. castaneum (Negahban et al. 2011b; 2013a). Also, ovicidal activity of nanoencapsulated essential oil of C. copticum on diamondback moth P. xylostella was evaluated by Jamal et al. (2012). Consequently, our results showed higher repellency rates in nanocapsule than in pure essential oil due to controlled-release formulations allowing smaller quantities of the essential oil to be used more effectively over a given time interval. The reasons for nanocapsulating the essential oil have been to improve its stability to reduce side effects or to reduce dosing frequency and total dosing amount, to obtain better repellent activity, and for sustained (long-lasting) release. Therefore, the nanocapsulation of the essential oil might provide a new method for the management of pests (Negahban et al. 2013a, b). Also, persistence or half-life time of the nanoencapsulated essential oil of A. sieberi was significantly longer than A. sieberi oil against stored product insect, such as T. castaneum. These results elucidate the suitability of nanoencapsulated essential oil as an insect control agent in organic food protection (Negahban et al. 2010). Despite fundamental differences in biology, physiology, and feeding behavior of tested insects, the general response of insect’s toxicity, repellency, and antifeedant activities of nanoencapsuled essential oil was similarly effective in many aspects compared to pure essential oil. Overall, it seems that the findings of research could be promising to make practical use of plant essential oils, as the new technology in nanoencapsuled essential oil through the control release of active ingredients overcome the restrictions of plant essential oils usage in storage and farms. Nevertheless, more studies are necessary to economize these novel technologies in natural environments such as warehouses, greenhouses, and farms.

3 Essential Oils: Extraction Methods, Constituents, and Their Efficacy

Methods used for extracting the essential oil include traditional press extraction (Chen et al. 2008), steam distillation extraction (Mostafa et al. 2010; Xavier et al. 2011), organic solvent extraction (Sarikurkcu et al. 2009), subsequent ultrasound-assisted solvent extraction (Sereshti et al. 2011), supercritical CO2 extraction (Akgun et al. 2009; Zizovic et al. 2007), microwave-assisted steam distillation (Chemat et al. 2006; Sahraoui et al. 2008), and so on. On the other hand, some known disadvantage factors, such as lower extraction rate, loss of active composition, residual of organic solvents, and higher production cost, limit their application in industrial process. Compared with others, hydrodistillation is the most commonly used method due to its advantages: easy to operate, lower cost, and good quality. However, because of involvement of higher temperatures and long time in the extraction process, some thermal-sensitive compounds in the essential oil are destroyed (Liu et al. 2009). Thus, reduction of distillation time in hydrodistillation process will provide a promising extraction method for the essential oil. If the distillation time could be reduced significantly under the premise of full extraction, the advantages of hydrodistillation are retained, while the quality of the resulting oil can be improved. Extracting the essential oil by hydrodistillation can be described as several steps. First, the essential oil molecules exist initially in plant cells in contact with water molecules, then the total pressure of mixture will reach the amount of partial pressure of each component at same temperature, finally the essential oil is brought out by water vapor. Ultrasonic processing technology is the most effective method for mixing liquid material currently. It is reasonable to believe that ultrasonic processing has positive effects on reducing distillation time through promoting contact of the two molecules. Additionally, it has been reported that the extraction rate of the essential oil can be enhanced by adding an appropriate amount of inorganic salts during the extraction process (Ji et al. 2008).

The essential oils are complex mixtures of 20–60 organic compounds and hundreds of different constituents (Bakkali et al. 2008; Rajendran and Sriranjini 2008). Depending on its chemistry, an essential oil can have widely different therapeutic and insecticidal actions. The ingredients found in the essential oils are organic due to their molecular structure which is based on carbon atoms held together by hydrogen atoms. Oxygen atoms and sometimes nitrogen and sulfur atoms are also present. Chemical structures of some of the essential oil constituents possess potent biological activity (Fig. 7.1) and are responsible for the bitter taste and toxic properties. Essential oil constituents are primarily lipophilic compounds that act as toxins, feeding deterrents, and oviposition deterrents to a wide variety of insect pests. Insecticidal properties of several monoterpenoids have been reported on housefly, red flour beetle, and southern corn rootworm (Rice and Coats 1994; Lee et al. 2004; Wang et al. 2006, Kim et al. 2010). As mentioned above, the essential oils are complex mixtures of natural organic compounds which are predominantly composed of terpenes (hydrocarbons), such as myrcene, pinene, terpinene, limonene, phellandrene, etc., and terpenoids (oxygen containing hydrocarbons), such as acyclic monoterpene alcohols (geraniol, linalool), monocyclic alcohols (menthol, 4-carvomenthenol, terpineol, carveol, borneol), aliphatic aldehydes (citral, citronellal, perillaldehyde), aromatic phenols (carvacrol, thymol, safrole, eugenol), bicyclic alcohol (verbenol), monocyclic ketones (menthone, pulegone, carvone), bicyclic monoterpenic ketones (thujone, verbenone, fenchone), acids (citronellic acid, cinnamic acid), and esters (linalyl acetate). Some essential oils may also contain oxides (1,8- cineole), sulfur-containing constituents, methyl anthranilate, and coumarins. Zingiberene, curcumene, farnesol, sesquiphellandrene, termerone, and nerolidol are examples of sesquiterpenes (C15) isolated from essential oils. Mono and sesquiterpenoidal essential oil constituents are formed by the condensation of isopentenyl pyrophosphate units. Diterpenes usually do not occur in the essential oils but are sometimes encountered as by-products (Koul et al. 2008). The eight main chemical components found in the essential oils are as follows.

Fig. 7.1
figure 1

Chemical structure of some plant essential oil components with insecticidal activities

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4 Monoterpene Alcohols

They contain 10 carbon atoms often arranged in a ring or in acyclic form. They are colorless and highly volatile. They can deteriorate very quickly and therefore need to be kept at cool temperatures and to be antiviral, antibacterial, and antifungal. Being typically volatile and rather lipophilic compounds, they can rapidly penetrate into insects and interfere with their physiological functions (Lee et al. 2002) and have insecticidal properties. Eugenol is reported as toxic to S. litura, Sitophilus granarius (L.) (Coleoptera: Curculionidae), Musca domestica L. (Diptera: Muscidae), and Diabrotica virgifera Lee Conte (Coleoptera: Chrysomelidae) (Hummelbrunner and Isman 2001; Obeng-Ofori et al. 1997; Lee et al. 1997). Eugenol is also active against Drosophila melanogaster Meigen (Diptera: Drosophilidae), Aedes aegypti (L.) (Diptera: Culicidae), and American cockroach, Periplaneta americana (L.) (Blattodea: Blattidae) (Bhatnagar et al. 1993, Ngoh et al. 1998). Cornelius et al. (1997) evaluated toxicity of monoterpenoids against Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae) (a subterranean termite) of which eugenol was found most effective as termiticide. Moreover, Meepagala et al. (2006) found that apiol isolated from Ligusticum hultenii Fernald (Apiaceae) exhibited high termiticidal activity and similar effect was shown by vulgarone B, isolated from Artemisia douglasiana Besser (Asteraceae). According to Raina et al. (2007) and Chauhan and Raina (2006) d-limonene, E, Z- nepetalactone, and Z, E-nepetalactone caused mortality to formosan subterranean termite, C. formosanus, and there was significant reduction in feeding. It was also effective as a fumigant and as feeding deterrent. Similarly, thymol has shown to have direct contact toxicity to larvae of Agriotes obscurus (L.) (Coleoptera: Elateridae) (Waliwitiya et al. 2000). Varying concentrations of the essential oil of red thyme having thymol are toxic to A. aegypti and Aedes albimanus Wiedemann (Barnard 1999). Thymol was also found to repel mosquitoes (Kalemba et al. 1991, Chokechaijaroenporn et al. 1994). According to Taghizadeh Saroukolai et al. (2010), the insecticidal activity of T. persicus could be related to these constituents.

Citronellal is toxic to S. litura, M. domestica (Hummelbrunner and Isman 2001; Lee et al. 1997), and C. maculatus (Don-Pedro 1996). There are several reports on insecticidal activity of camphor (Negahban et al. 2007a) in some species of Artemisia, and 1,8-cineole was the most toxic fumigant found in eucalyptus, rosemary, and Perovskia essential oils (Lee et al. 2003; Negahban and Moharramipour 2007c, Lee et al. 2001a; Negahban et al. 2007a; Arabi et al. 2008a). Coats et al. (1991) found the fumigant toxicity effect of myrcene and a-terpineol against S. oryzae after 24 h. α-Pinene is reported to be toxic to T. confusum (Ojimelukwe and Alder 1999). Hierro et al. (2004) have reported the toxicity activity of geraniol, citronellol, citral, carvacrol, and cuminaldehyde against Anisakis simplex Rudolphi (Ascaridida: Anisakidae). Menthone, trans-anethole, and cinnamaldehyde are well-known anti-insect compounds (Marcus and Lichtenstein 1979, Harwood et al. 1990, Lee et al. 1997, Franzios et al. 1997, Huang and Ho 1998; Hummelbrunner and Isman 2001; Chang and Ahn 2001; Chang and Cheng 2002). d- Limonene, linalool, myrcene, and terpineol significantly increased the nymphal duration in German cockroach, Blattella germanica (L.) (Blattodea: Blattellidae), when fed through artificial diet (Karr and Coats 1992). Also, d-limonene is toxic to M. domestica, D. virgifera, S. litura (Lee et al. 1997; Hummelbrunner and Isman 2001), and some stored grain pests and cockroaches (Don-Pedro 1996, Coats et al. 1991). Similarly, limonene found in the essential oil of various citrus leaves and fruit peels has exhibited significant insect control properties (Karr and Coats 1988). Acaricidal activity of thymol and 1,8-cineole from the essential oils of T. kotschyanus, F. assa-foetida, and E. camaldulensis against V. destructor was evaluated (Ghasemi et al. 2011). Essential oils rich in 1,8-cineole are also effective against house dust mites (Miresmailli et al. 2006). Among pure constituents, citronellal, eugenol, menthol, pulegone, and thymol are moderately active against various mites (Calderone and Spivak 1995; Perrucci 1995; Ellis and Baxendale 1997).

5 Sesquiterpene Alcohols

Which hydrocarbons comprise of 15 carbon atoms. These terpenes are not as volatile as monoterpenes and having a calming effect as well as being anti-inflammatory and anti-infectious. Varying properties include anti-inflammatory, antiviral, and anticarcinogenic. New insecticidal sesquiterpene extracts of the root bark of Celastrus angulatus M. have been reported (Wei et al. 2011).

6 Other Compounds

Aldehydes: antimicrobial, anti-inflammatory, disinfectant, and sedative

Esters: antispasmodic, anti-inflammatory, antifungal, calming, and sedative

Ethers: antispasmodic and analgesic

Ketones: anticatarrhal, regenerative, and analgesic

Oxides: expectorant and stimulant

Phenols: Strongly antimicrobial, stimulants to the immune and nervous system, and irritating to mucous membranes.

The essential oils work as the chemical defense mechanism of the plant. They ward off insects and play a role in initiating the regeneration process for the plant. They have a chemical structure that is similar to that found in human and animal cells and tissues. This makes the essential oils compatible with human and animal protein and enables them to be readily identified and accepted by the body. Diffused essential oils can increase atmospheric oxygen and provide negative ions that inhibit bacterial growth and can break down and render potentially harmful chemicals nontoxic. Some benefits of pure therapeutic grade essential oils are as follows: regenerating, oxygenating, and immune defense properties of plants, also containing oxygen molecules, which help to transport nutrients to the starving human cells. Because a nutritional deficiency is an oxygen deficiency, disease begins when the cells lack the oxygen for proper nutrient assimilation. By providing the needed oxygen, the essential oils also work to stimulate the immune system, very powerful. Antioxidants create an unfriendly environment for free radicals. They prevent all mutations, work as free radical scavengers, prevent fungus, and prevent oxidation in the cells, removing metallic particles and toxins from the air.

Plant essential oils are produced commercially from several botanical sources. Examples include 1,8-cineole, eugenol, thymol, menthol, asarones, carvacrol, and linalool from many plant species. A number of source plants have been traditionally used for protection of stored commodities (Koul et al. 2008), especially in the Mediterranean region and in Southern Asia, but interest in the oils was renewed with emerging demonstration of their fumigant and contact insecticidal activities to a wide range of pests in the 1990s (Isman 2000). The rapid action against some pests is indicative of a neurotoxic mode of action, and there is evidence for interference with the neuromodulator octopamine (Kostyukovsky et al. 2002) by some oils and with GABA-gated chloride channels by others (Priestley et al. 2003). Recent evidence for an octopaminergic mode of action for certain monoterpenoids (Bischof and Enan 2004; Kostyukovsky et al. 2002), combined with their relative chemical simplicity, may yet find these natural products useful as lead structures for the discovery of new neurotoxic insecticides with good mammalian selectivity. The knowledge on the mode of action of available insecticides will help to determine the chemical properties of novel compounds that may be ideally suited for modification of insect behavior at doses that can be used safely and economically in agriculture (Haynes 1988). The plant origin insecticides could be exploited for the development of novel molecules with highly precise target for sustainable insect pest management in agriculture (Rattan 2010). Given these encouraging results, further experiments are in progress to assess the suitability of natural active principle formulations for application as a new tool in integrated control of different pest.

7 Fumigant Properties of Essential Oils

More current research shows that the essential oils and their constituents may have the potential as alternative compounds to currently used fumigants (Nattudurai et al. 2012; Kim et al. 2010; Suthisut et al. 2011). Major constituents from aromatic plants, mainly monoterpenes, are of special interest to industrial markets because of other potent biological activities in addition to their toxicity to insects (Kubo et al. 1994; Isman 2000; Weinzierl 2000). Several studies have been undertaken in our laboratory to explore the potential of the essential oils and their constituents as insect fumigants. Table 7.2 provides examples of bioassay test to determine the LC50 values of insecticidal activity of a few of the better-known essential oils. Their action against stored product insects has been extensively studied. Moreover, these natural derivatives are considered to be an alternative means of controlling harmful larvae of field crop insect pests. The optimum use of the essential oils with maintaining their active ingredients by developing applicable methods is the essential way to control insects. For taking into account the limitations and the physicochemical characteristics of the essential oils, nanoencapsulated formulations of the essential oil components seem to be the best choice. To get the best out of our data, high fumigant toxicity and persistence of nanoencapsulated A. sieberi essential oil have been demonstrated as a new formulation against C. maculatus and T. castaneum (Negahban et al. 2011a; 2010). Also, preparation and characterization of nanoparticles containing Cuminum cyminum L. (Apiaceae) oil as a potential agriculture insecticidal application have been evaluated against insect pests (Zandi et al. 2010).

Table 7.2 Fumigant toxicity of some selected plant essential oils on insects and mites
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8 Antifeedant Properties of the Essential Oils

Antifeedant chemicals may be defined as being either repellent without making direct contact to insects or deterrent from feeding once contact has been made with insects (Koul et al. 2008).

8.1 Repellent Properties

One other great thing about the essential oils is that they have been tested as potential sources of insect repellents. Similarly, our laboratory bioassays have been conducted to determine the activity of some natural essential oils against some stored product and field crop pests. Repellency of M. longifolia and T. kotschyanus on C. maculatus was recorded at 90 % and 73.33 % At 800 ppm, respectively (Akrami et al. 2011). Also, at 1.5 ppm, the essential oil of A. sieberi was significantly more repellent to T. castaneum (65.90 %) than S. oryzae (59.70 %) and C. maculatus (55.80 %) (Negahban et al. 2007b), while A. scoparia strongly repelled T. castaneum (63.80 %) and S. oryzae (62.01 %) than C. maculatus (48.57 %) (Negahban et al. 2006a). Equally at 3 ppm, S. mirzayanii was significantly more repellent to T. confusum (86.66 %) than C. maculatus (70 %) (Nikooei and Moharramipour 2010). The strongest repellency has been shown in Anethum graveolens L. (Apiaceae) (100 %), T. vulgaris (100 %), and R. officinalis (93.33 %) and the weakest repellency in Hyssopus officinalis L. (Lamiaceae) (7.69 %) and Petroselinum sativum Hoffm. ex Gaudin (Apiaceae) (9.48 %) against P. interpunctella (Rafiei Karahroodi et al. 2009). Repellency of C. reticulata was significantly higher than Citrus limon (L.) Burm.f. (Rutaceae) and Citrus aurantium L. (Rutaceae). The adult insects were exposed to the concentration of 1, 3, 5, and 7 ppm of citrus peel essential oils to estimate repellent activities. Repellent values for Citrus reticulata at the abovementioned concentrations were estimated to be 26.66, 33.33, 33.66, and 40 %, respectively. C. reticulata essential oil was significantly more repellent to C. maculatus at 7 ppm (Saeidi et al. 2011).

Analysis of the data by Taghizadeh Saroukolai et al. (2009) has shown that the essential oil of P. acaulis strongly repelled adult insects and was significantly differed between insect species. The repellency of oil tested at the highest concentration (2 μl/ml acetone) was 83.6, 71.6, and 63.6 % on S. oryzae, C. maculatus, and T. castaneum, respectively. However, reverse observations have been made by several other studies concerning the strong repellent effects on T. castaneum rather than S. oryzae and C. maculatus. Therefore, further study is necessary to elucidate the mode of action of these essential oils. To the best knowledge of Negahban et al. (2013b), at 1.9 ppm, the nanocapsule of Artemisia oil was shown here to possess more repellent activity (80 %) to P. xylostella compared to Artemisia pure oil (62 %). The results showed higher repellent rates in nanocapsule than in the essential oil. The reasons for nanocapsulating the essential oil have been to improve its stability to reduce side effects or to reduce dosing frequency and total dosing amount, to obtain better repellent activity, and for sustained (long-lasting) release. These results showed that medicinal plants could be used as repellent of insects. These essential oils can be used for protecting agricultural products from pest injury.

Recent research has focused on insecticidal property of essential oil plants in biological control of insects. Nerio et al. (2010) reported that the increase in repellence activity for the essential oils is highly dependent on the product composition. Formulations based on creams, polymer mixtures, or microcapsules for controlled release resulted in an increase of repellency duration (Nentwig 2003; Chang et al. 2006). For example, Zanthoxylum limonella oil was successfully microencapsulated in glutaraldehyde cross-linked gelatin (a polymer), in order to improve mosquito-repellent properties (Maji et al. 2007). Consequently, controlled release by nanoencapsulated essential oil seems to get the best out of the formulation for increasing the efficiency and providing a new method for the pest management.

8.2 Deterrent Properties

One of the significant aspects to the application of plant essential oils is that they have been considered as an important feeding deterrence in integrated pest management (IPM) programs. Needless to say, these are environmentally less harmful than synthetic pesticides and act in many insects in different ways. Several experiments have been designed to measure the nutritional indices such as relative growth rate (RGR), relative consumption rate (RCR), efficiency of conversion of ingested food (ECI), and feeding deterrence index (FDI). Bioefficacy of A. sieberi and A. scoparia exhibited antifeeding activity against T. castaneum, while A. sieberi oil was highly effective compared to A. scoparia and significantly decreased the RGR and RCR. Moreover, A. sieberi oil was more effective on FDI than A. scoparia (Negahban and Moharramipour 2007b). In another study (Sahaf and Moharramipour 2009), the efficacy of C. copticum and V. pseudo-negundo essential oils has also been determined against T. castaneum related to increase FDI.

Generally, antifeedant activity of C. copticum was more effective than V. pseudo-negundo. Soleimannejad et al. (2011) studied the effects of Salvia mirzayanii essential oil on nutritional indices of the T. confusum adults. S. mirzayanii showed a strong feeding deterrence action against adults of T. confusum. As there was an increase in the concentration of the essential oil, the growth rate (RGR), relative consumption rate (RCR), and efficiency of conversion of ingested food (ECI) were reduced significantly. However, there was a significant increase in feeding deterrence (FDI) action as increase in concentration of the essential oil. Therefore, the essential oil by fumigation could affect nutritional indices of the food by pre-ingestive and postingestive behavior. Reduction in growth and feeding rate may affect ECI and FDI, respectively. ECI indicates a sign of postingestive physiological effect. As in our experiments, food did not contaminated with the essential oil directly; therefore, it seems that indirect toxic properties of fumigation may have reduced growth and consequently ECI. Nevertheless, FDI shows pre-ingestive behavior of the insect. Consequently, the reduction in growth rate could be due to the feeding behavior of insect. It is possible that the reduction in feeding and growth rate of insects was mainly due to the feeding deterrent action. As in this study, strong feeding deterrent properties of the essential oils were observed. So, in addition to toxic effect of the essential oils, they have potential to affect growth and consumption rate of the insect.

9 Mechanism of Action of Essential Oils

As a matter of fact, the mode of action and site of effect for insecticidal activities from the essential oil is worthwhile for various authors (Enan 2001; Kostyukovsky et al. 2002; Priestley et al. 2003). Mostly the work has been carried on studying the effects of the essential oils, their lethal doses, and time to achieve lethal effects, but mode of action is in general not fully elucidated (Rattan 2010). Since large numbers of chemical defenses are present in the nature, very little is known about their mode of action at molecular level. The most striking thing to mention here is that the essential oils and their constituents disrupt the endocrinologic balance of insects (Rattan 2010). Toxicity from the essential oils in insects and other arthropods points to a neurotoxic mode of action; most prominent symptoms are hyperactivity followed by hyperexcitation leading to rapid knockdown and immobilization (Enan 2001).

Several essential oils from aromatic plants, monoterpenes, and natural products have been shown as inhibitors of acetylcholinesterase (AChE) against different insect species (Kostyukovsky et al. 2002; Shaaya and Rafaeli 2007; Rajendran and Sriranjini 2008). Recently, it was found that azadirachtin (a tetraterpenoid) significantly inhibits the activity of AChE in Nilaparvata lugens Stal (Nathan et al. 2004). Furthermore, octopamine is a target for the essential oils activity in insects. The acute and sublethal behavioral effects of the essential oil compounds on insects are consistent with an octopaminergic target site in insects, which acts by blocking octopamine receptors (Enan 2001, 2005). Another possible target suggested for the essential oils is the interference with GABA-gated chloride channels in insects (Priestley et al. 2003). A case in point is that the thujone has been classified as a neurotoxic insecticide, which acts on GABA receptors (Hold et al. 2000; Ratra and Casida 2001). Koul et al. (2008) reported that the rapid action against some pests is indicative of a neurotoxic mode of action, and there is evidence for interference with the neuromodulator octopamine (Kostyukovsky et al. 2002) by some oils and with GABA-gated chloride channels by others (Priestley et al. 2003). On the whole, however, their functions on the specificity of the metabolites responsible for proclaimed insecticidal activity are lacking. Nevertheless the plant-based insecticides could be exploited for the development of novel molecules with highly precise target. For moving on the way to green chemistry processes and continuing need for developing new crop protection tools with novel modes of action makes discovery and commercialization of natural products as green pesticides with more empirical evaluation of active components for sustainable insect pest management in agriculture

10 Problems and Prospects

As a matter of fact, the essential oils and their components have certain advantages since they have been used in traditional medicine, in pharmaceutical preparations, and as natural flavorings. In addition to insecticidal activity, some of the essential oils have the advantage of showing fumigant, repellent, and contact and antifeedant action against agricultural product pests. Nonetheless, fumigants from plant sources lack the most important property of an ideal fumigant, i.e., sufficient vapor pressure for diffusion and penetration into commodities to kill the pests. Evidently, compounds of plant origin can be used only for small-scale applications or for space treatments. They require some carrier gases (e.g., CO2) for even distribution and penetration into the commodities or they can act as adjuvant for conventional fumigants. Sometimes it has been claimed that monoterpenoids have comparable fumigant action to that of methyl bromide (Rajendran and Sriranjini 2008). However, this has not been established in large-scale trials with mixed-age cultures of target insects. The stability of the essential oils and their components is very important for toxic action. Negahban et al. (2006a) noted that the fumigant action of A. sieberi leaves decreased after processing over a period.

Very few studies have been conducted on the stability of promising the essential oils and their constituents. High sorption is one of the important limiting factors for the application of natural compounds in large-scale commodity fumigations, and it might lead to more residues; treated commodities will require longer aeration period. The most striking thing to mention here is that the nanoencapsulation process is a really helpful method for entrapping the essential oils of a very different chemical composition. In order to take full advantage of the essential oils, nanoencapsulated formulation reduces loss of the active principles, leading to high-loaded nanoparticles that offer protection against environmental agents; it also offers the possibility of controlled oil release. These effects appear maximized by nanocapsule adhesion to the hair structures typically present in some of the main defoliator families. Negahban et al. (2011a, b) reported that the fumigant toxicity of nanoencapsules was significantly higher than non-formulated oil at sublethal doses after 7 days exposure. When the nanoencapsulated oil is sprayed on the insect body, because of suspension capability in water, it completely wets all organs of the insect which is covered with fuzz.

Low viscosity of suspension resulted in uniform coatings, and the presence of surfactant in nanoformulation suspension causes a decrease in surface tension which increases stability of minute droplets of the essential oil in the suspension. Lai et al. (2006) have investigated the formulation of emulsion of Artemisia arborescens L. (Asteraceae) essential oil with solid lipid and its toxic effect on Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). They showed that the type and amount of the essential oil did not alter fast during the period after being sprayed. The evaporation rate of the essential oil in the formulation is extremely low in comparison with control samples. Nanoformulation can be an up-to-date asset for pest control in agriculture. Moretti et al. (2002) reported the highest larval mortality occurred by microparticle adhesion to the hair structures of insects. Besides, the mortality rate was significantly high in experiment in closed condition in all fumigant, contact, and feeding nanoencapsulated treatments. This could be as a result of high volatility of the toxic mono- and sesquiterpene compounds which likely delivered fumigant toxicity by vapor action via the respiratory system, leading to more influence of complex and higher amount of active principles (Phillips et al. 2010, Kim et al. 2010). It can be concluded that mortality caused by nanoformulations is mainly wrought by contaminating the diet with the active principles released from the capsules and is a consequence of ingestion (Passino et al. 2004).

In general, the results show that the toxicity of various formulations besides chemical components depends on different factors such as application method as fumigant or contact toxicity or feeding repellent effects. Nanoencapsulation protects the quality and durability of active ingredients and thus increases the contact toxicity of the essential oil (Ziaee et al. 2014a, b). Moretti et al. (2002) and Passino et al. (2004) reported that insects’ mortality due to nanoencapsulation is higher than non-formulated oil as the release rate of active ingredients can be controlled by loading them into nanocapsules over time. Therefore, according to researches, the appropriate application of nanoformulations can increase the insecticidal potential of the essential oils. Nevertheless, more studies are necessary to economize these novel technologies in the IPM.

The results of repellency tests indicate that although the non-formulated oil was more effective than nanoformulation in short time period (maximum 6 h after exposure), it is a strong repellent after 24-h exposure. In addition to feeding deterrency, the possibility of postingestive toxicity of nanoformulations is clearly increased.

11 Conclusion

Plant essential oils and their individual metabolites have demonstrated optimal potential for insecticidal activity against several species, not only insects, but other kinds of arthropods. However, its high volatility decreases the times of protection. What it does is that the major inconvenience of the use of this oil and, in general, of the essential oils is their chemical instability in the presence of air, light, moisture, and high temperatures that can determine the rapid evaporation and degradation of some active components. It was concluded that essential oils could be encapsulated successfully. All studied formulations demonstrated a high physical stability and a good capability to reduce the essential oil evaporation. The best results were obtained with nanoencapsulated formulation, which did not vary in size even after the spraying procedure. A central feature is that nanocapsules can be generally applied over large areas with conventional spray equipment, and numerous variables can be manipulated to control the release characteristics, e.g., capsule wall thickness, capsule size, capsule wall composition, and internal composition. Given these encouraging results, further experiments are in progress to assess the suitability of natural active principle formulations for application as a new tool in integrated control of different pests. Therefore, encapsulating the essential oils has a considerable potential as commercial insecticide products. Incorporation of the essential oils in controlled release with nanocapsule formulations could solve their problems and offer several advantages. Following an extensive literature review of the existing nanocapsulation technology, we concluded that several nanocapsulation applications would bloom if a low-cost continuous polymerization process for high-performance nanocapsules became available.

The essential oils have the ability to control a wide range of insects. Unfortunately, this broad-spectrum nature can adversely affect many nontarget beneficial species such as bees and parasitoid wasps along with the pest. Effectiveness of the essential oils can vary depending on where the chemicals are being applied. For instance, they may kill a large percentage of the target pest because it is a controlled environment, but in a real-life situation, the number may be much smaller. Evaluating uses of new formulations in similar situations as that of natural conditions may help in estimating the kind of effect it will have.

The essential oils are considered as a shorter-lived, fast-acting, and more acutely toxic material. But encapsulation technologies may modify them to longer-lasting, slow-acting, and less toxic materials that may be better for chronic pest problems. As essential oils are reported to be often the most phytotoxic, this property requires serious attention when formulating products. Other constraints such as lack of data for the essential oils on sorption, tainting, and residues in food commodities are included. Therefore, implementation of essential oil-based control systems will require more knowledge about their side effects in pest management systems. Because the knowledge of essential oils in this area is very limited, considerably more research will be required to develop and implement these new growing technologies. Acquiring this knowledge base will necessitate coordinated efforts among many scientific disciplines.

The main objective in future work is to achieve reactive polymerization of nanocapsule compositions to be used in controlled-release applications. These new nanocapsule formulations will achieve a high selectivity, specificity, and accuracy in delivering the optimum dose of the active ingredient to the desired site at the appropriate time. It will also seek to obtain a maximum activity on the target while producing minimal effect on the nontarget materials. A secondary goal to the main objective is to extend this research by improving the understating of the released mechanisms. Therefore, it is time to focus the attention of the researchers toward the development and application of known essential oils and their constituents by advanced formulation technologies.

Plant essential oils and their individual metabolites have demonstrated optimal potential for insecticidal activity against several species, not only insects, but other kinds of arthropods. However, its high volatility decreases the times of protection. There are likely several reasons for encapsulating method that are good potential carrier to bring into being the biopesticides of the essential oil in agriculture:

  1. 1.

    What it does is that the major inconvenience of the use of this oil and, in general, of the essential oils is their chemical instability in the presence of air, light, moisture, and high temperatures that can determine the rapid evaporation and degradation of some active components. It was concluded that the essential oils could be encapsulated successfully. All studied formulations demonstrated a high physical stability and a good capability to reduce the essential oil evaporation. The best results were obtained with nanoencapsulated formulation, which did not vary in size even after the spraying procedure. A central feature is that nanocapsules can be generally applied over large areas with conventional spray equipment, and numerous variables can be manipulated to control the release characteristics, e.g., capsule wall thickness, capsule size, capsule wall composition, and internal composition. Given these encouraging results, further experiments are in progress to assess the suitability of natural active principle formulations for application as a new tool in integrated control of different pests. Therefore, encapsulating the essential oils has a considerable potential as commercial insecticide products. Incorporation of the essential oils in controlled release with nanocapsule formulations could solve their problems and offer several advantages. Following an extensive literature review of existing nanocapsulation technology, we concluded that several nanocapsulation applications would bloom if a low-cost continuous polymerization process for high-performance nanocapsules became available.

  2. 2.

    The essential oils have ability to control a wide range of insects. Unfortunately, this broad-spectrum nature can adversely affect many nontarget beneficial species such as bees and parasitoid wasps along with the pest. Effectiveness of the essential oils can vary depending on where the chemicals are being applied. For instance, they may kill a large percentage of the target pest because it is a controlled environment, but in a real-life situation, the number may be much smaller. Evaluating uses of new formulations in similar situations as that of natural conditions may help in estimating the kind of effect it will have.

  3. 3.

    The essential oils are considered as a shorter-lived, fast-acting, and more acutely toxic material. But encapsulation technologies may modify them to longer-lasting, slow-acting, and less toxic materials that may be better for chronic pest problems.

  4. 4.

    As essential oils are reported to be often the most phytotoxic, this property requires serious attention when formulating products.

  5. 5.

    Other constraints such as lack of data for the essential oils on sorption, tainting, and residues in food commodities are included. Therefore, implementation of the essential oil-based control systems will require more knowledge about their side effects in pest management systems. Because the knowledge of essential oils in this area is very limited, considerably more research will be required to develop and implement these new growing technologies. Acquiring this knowledge base will necessitate coordinated efforts among many scientific disciplines.

12 Future Focus of the Work

The main objective in future work is to achieve reactive polymerization of nanocapsule compositions to be used in controlled-release applications. These new nanocapsule formulations will achieve a high selectivity, specificity, and accuracy in delivering the optimum dose of the active ingredient to the desired site at the appropriate time. It will also seek to obtain a maximum activity on the target while producing minimal effect on the nontarget materials. A secondary goal to the main objective is to extend this research by improving the understating of the released mechanisms. Therefore, it is time to focus the attention of the researchers toward the development and application of known essential oils and their constituents by advanced formulation technologies.