Keywords

3.1 Introduction

Biological insecticides or bioinsecticides are bio-based drugs that act with different action mechanisms to kill various insect pests. These are categorized into three major groups based on technical descriptions given by the US Environmental Protection Agency (EPA): (1) Natural biochemicals that operate under non-toxic process; (2) Entomopathogens of the microbial origin; and (3) Proteins originating from genetically modified plants introduced in plants (Kachhawa 2017).To date, over 3000 types of microorganisms have been reported to cause insect disorders leading to their mortality, and more than 100 bacteria were identified as insect pathogens, among which Bacillus thuringiensis Berliner (Bt) is regarded as the best microbial control agent (Koul 2011). Two species of bacteria, such as spore-forming Bacillus thuringiensis (Bizzarri et al. 2008; Porcar et al. 2008) and the non-spore-forming Serratia entomophila (Inglis and Lawrence 2001; O’Callaghan and Gerard 2005) have gained more attention as pest control agents. Other important EPB like B. popilliae, Pseudomonas alcaligenes, P. aureofaciens, Clostridium bifermentans, Streptomyces avermitilis, and Saccharopolyspora spinosa were also exploited as potential biocontrol agents.

Bt is most commonly used for the control of lepidopterans (Spodoptera exempta, Cydia pomonella, Helicoverpa armigera, etc.), Dipterans (Anopheles albimanus, Culex obscures, Aedes aegypti, etc.), Coleopterans (Popillia japonica, Leptinotarsa decemlineatam, Tribolium confusum, etc.), and Hymenopterans (Megachile frontalis, Megastigmus spermotrophus, Xylocopa aruana, etc.) (Bravo et al. 2007). Bt var. israelensis and B. sphaericus strains produce specific endotoxin, which is used worldwide to eradicate the mosquito larva, particularly in malaria and human lymphatic filariasis endemics. Moreover, black fly (Simulium spp) larva, which serves as a vector for the river blindness of man (onchocerciasis) in Africa’s tropical regions, is also controlled by israelensis. More than 40 Bt products, like Larvect 50, Mosquito Dunks, Monterey Bt, are used to control caterpillars, beetles, and mosquitoes, which constitutes 1% of the overall insecticide business (Bizzarri et al. 2008). Beta-proteobacteria supports another group, including species with significant implications as biocontrol agents (BCA) against various insect pests. Burkholderia rinojensis strain has been used for insect control that works through ingestion and interaction with various insects and mites (Cordova et al. 2013). These insecticides’ action is based on different metabolites, and the commercial product mostly focuses on heat-killed cells and fermentation media. Another commercially active beta-proteobacterium is a strain of Chromobacterium subtsugae whose metabolites show wide-spectrum insecticide activity against Lepidoptera, Hemiptera, Coleoptera, and Diptera insect species (Martin et al. 2007). Several Streptomyces species possess various insecticidal toxins, particularly macrocyclic lactone derivatives, which act mostly on insects’ peripheral nerves in almost the same phylum. Similarly, Saccharopolyspora spinosa releases potent and wide-spectrum insecticidal toxins known as spinosin, which are natural and semi-synthetic compounds of economic importance (Kirst 2011).

3.2 Classification of Entomopathogenic Bacteria (EPB)

The details of EPB groups are shared below.

3.2.1 Family: Bacillaceae

3.2.1.1 Genus: Bacillus

This genus contains the catalase-positive bacteria that induce sporulation that protects them under unfavorable conditions. They can form oval-shaped endospores, which may be followed by parasporal bodies in some organisms. Bacillus contains the essential entomopathogenic species for insect control. Important species include B. thuringiensis, B. weihenstephanensis, B. pseudomycoides, and B. mycoides. The plasmids of Bacillus sp. contain genes that play a vital role in their pathogenicity. This leads to the development of several bio-insecticides using Bacillus sp. (Raymond et al. 2010a). Conversely, increased saprophytic development is preferred when insecticidal toxin-containing plasmids are lacking in the bacterium (Gohar et al. 2005).

3.2.1.1.1 Bacillus thuringiensis (Bt)

Several authors have reviewed Bacillus thuringiensis (Bt) and its toxins (Burges 2001; Sanchis 2011). The pathogen was previously derived from diseased silkworm (Bombyx mori) larvae by Shigetane Ishiwata in 1901 (Ishiwata 1901). Furthermore, studies of Ishiwata have shown that its toxicity is correlated with proteins in sporozoite cultures but not in plant cells (Aoki and Chigasaki 1916). Ernst Berliner extracted it from the Mediterranean flour moth’s diseased larvae, formally described and named the bacterium in Thuringia, Germany (Berliner 1915). Berliner reported the existence of a spindle cell’s body or crystalline participation that was later identified as protein and solubilized in an alkali solution (Hannay and Fitz-James 1955). The only phenotypic characteristic unique to Bt appears to be forming these structural proteins in spindle cells. This feature is used to differentiate this bacterium from many other Bacillus (Vilas-Boas et al. 2007). To identify and classify new Bt segregates, several biochemical, morphological, and antigenic strategies were used (Heimpel and Angus 1958). The existence of the Pasteur Institute serotyping infrastructure has contributed significantly to the widespread implementation of the H-serotyping method to identify Bt isolates (Dulmage et al. 1981; Lecadet et al. 1999). Identification of mutations based on the fundamental susceptibility to the different phages of bacterial strains and Ribosome RNA gene typing was also used to identify strains (Akhurst et al. 1997; Ackermann et al. 1995). Conservation of certain extragenic palindromic elements, DNA interbreeding analysis, and improved spontaneous polymorphism analysis with appropriate systems have been suggested to endorse H-serotyping, which remains the most widely accepted method for the identification of Bt isolates (Reyes-Ramırez and Ibarra 2005; Hansen et al. 1998). A maximum of 82 Bt serotypes or even more commonly utilized subspecies have been described in its most current version of an H-antigen category (Lecadet et al. 1999). This category contained 69 classes of receptors and 13 subsets. Serotype mogi extracted through leaf litter and displaying virulence to mosquito larvae (Roh et al. 2009). All known Bt isolates initially showed pathogenicity towards Lepidoptera embryos (pathotype A) (Goldberg and Margalit 1977). Bt strains have also been pathogenic to the Hemiptera, Hymenoptera, Isoptera, Orthoptera, Coleoptera, and nematodes (Quesada-Moraga et al. 2004; Lima et al. 2008; Garcia-Robles et al. 2001; de Castilhos-Fortes et al. 2002; Quesada-Moraga et al. 2004; Bottjer et al. 1985). The potential for distinguishing between pathotype-based Bt varieties has been reported to be highly threatened by the development of numerous poisons with different isolation characteristics and the growing number of recorded pathotypes. Consequently, many Bt serotypes produce strains that are pathogenic to species of various taxonomic groups. Bt toxins are highly selective, and even a specific Bt gene or crystal toxin, even within the genus, can only be effective against such a limited number of insect species. For example, Tolworthi (Bt) is exceptionally infectious to military worm embryos (Spodoptera frugiperda) and East Asian leafworm (Spodoptera litura) (Hernandez 1988; Amonkar et al. 1985).

3.2.1.1.2 Bacillus cereus

Most of the strains under Bacillus cereus associated with insects are saprophytic or symbiotic bacteria inhabiting the insect digestive system. They showed high genetic similarity to Bt; however, B. cereus does not produce a crystalline parasporal poison that restricts its toxicity to arthropod hosts. B. cereus isolates can induce natural or artificial diseases in different pest species such as scarab beetle larvae (Selvakumar et al. 2007); flour beetle Tribolium castaneum (Kumari and Neelgund 1985); spruce budworms C. Fumiferana (Strongman et al. 1997); Anopheles mosquito (Chatterjee et al. 2010); Trichoplusiani (Wai Nam et al. 1975); and Glossina morsitans (Kaaya and Darji 1989). The vegetative insecticide proteins (Vip) were initially known to be present in the B. cereus strain that causes pathogenesis to maize rootworms (Diabrotica spp.) (Warren et al. 1996). Vip in B. cereus poisons are similar to Clostridium’s secondary toxins and consist of a subunit of a toxin bound to the target polymer toxin cell (Vip1 toxin) and a second toxin displaying patterns of actin-ADP-ribosylating activity (Vip2 toxin) to avoid actin polymerization and kill the host insect tissues (Han et al. 1999). The Vip1 and Vip 2 toxins are crucial for their toxicity. Recently, the Vip2 toxin receptor expression in plants as a non-active proenzyme obtained after ingestion by the rootworm helped develop transgenic crops to control pests (Jucovic et al. 2008). Heat tolerant B. cereus variants have been identified for cotton boll weevil (Anthonomus grandis). However, less effect has been observed for cotton leafworm (Spodoptera littoralis) or black bean aphid, Aphis fabaee (Perchat et al. 2005; Yuan et al. 2007; Luxananil et al. 2001; Jucovic et al. 2008). Transgenic plants that transmit toxin Vip from B. cereus were created; however, such productions’ protection would require careful consideration before commercialization.

3.2.1.1.3 Lysinibacillus

Lysinibacillus are strictly aerobic and include both a saprophytic and pathogenic bacteria group (Hu et al. 2008). L. sphaericus is heterogenous species identical to B. sphaericus (Ahmed et al. 2007). The distinctive feature of this bacterium is the development of the spherical spore swollen sporangium in a terminal location. Strains of B. sphaericus have been well established for dipteran infections. Out of 49 serotypes, 9 are mosquitocidal strains (H1, H2, H3, H5, H6, H9, H25, H26, and H48), are recognized by sphaericus. However, serotype is not a good indicator of mosquitocidal behavior or the production of virulence factors (Priest and Dewar 2000; Hu et al. 2008). Strain 2362 and C-41 are the most active ingredients in commercial products of B. Sphaericus (Park et al. 2010). In mosquito species, the resistance of B. sphaericus appears to be highly variable depending on the strain (Wraight et al. 1987). This is attributable to toxin development (Davidson et al. 1975). Anopheles, Culex, Mansonia, and Aedes show the regular declining order of sensitivity against B. sphaericus. The large difference in sensitivity to Culex and Aedes mosquitoes may be due to variations in the binding poison to the intestinal epithelium (Davidson 1989). Histological research has shown that bacteria are contained in the peritrophic matrix when consumed by the host, which might be associated with the toxicity development (Davidson et al. 1975). The toxin appears to attach carbohydrate residue to intestinal receptors entrance and growth of mosquito larvae within midgut cells (Davidson 1989; Oliveira et al. 2009). Pathogens colonize to enter the body after host death, the foliage cells multiply, and the cycle ends with magnifying spore formation around the area. Mortality is delayed at low bacterial levels; however, long-term impacts on population growth are detected (Davidson 1989).

3.2.2 Family: Paenibacillaceae

3.2.2.1 Genus: Paenibacillus

Milk disease was identified as a mere infection of P. japonica, a Japanese beetle that entered the US in the 1930s via hemolymph of scarab larvae (Dutky 1940). There are two types of infectious bacteria: B. popilliae, a distinctive parasporal body inside the sporangium, and B. lentimorbus, without the parasporal body. Many other types of bacteria-causing milk disease concentrate on a particular dwelling phase with even a spore and parasporal structure embedded in a thick sporangium, giving a footprint-like appearance to these cells until seen by an optical microscope. Strains can be isolated from the infected morphotype (Milner 1981) based on the nature and shape of a spindle cell structure, including these specific entities. Spores are ingested by Scarabaeidae members that feed upon plant roots as well as organic material. Spores grow during higher pH in enzyme-rich intestine scarab conditions (Jackson et al. 2004). Inside the hemolymph, vegetative cells grow, causing little or no toxaemia, allowing the larva to remain active. Infection with milk disease typically consists of foliage rods and spores. When infected, a larva never molds, there is almost no evidence of the melanization response to an infection, and larval mortality is mostly due to the lack of nutrients and fat stores in the body. Hemocoel is filled with nearly 20 billion spores at the end of the infection duration (Sharpe and Detroy 1979). These are introduced to the soil after the death of a contaminated larva. While in the ground, spores live for even a more extended period and live till new susceptible larvae accessed at the same site (Franken et al. 1996). This is also proof that strain-host compatibility is characterized by a bacteria’s ability to cross an intestinal wall boundary before optimum haemolymph conditions are multiplied. Discrepancies of response to an infection can be due to problems in achieving apparent spore germination. Heat operation to excess nutrients (Stahly and Klein 1992) and pressure use with suspension in intestinal fluids (Krieger et al. 1996) help in germination. The resistance of vancomycin seems to be a common function for B. popilliae (Pettersson et al. 1999). Even so, the resistance to vancomycin to P. popilliae is shown to be contradictory as repeated studies have also shown that the resistance gene was not found throughout the American P. popilliae strains (Harrison et al. 2000). Therefore, neither the presence of paraspore nor vancomycin tolerance has been shown to become a decisive differentiating factor.

3.2.2.2 Genus: Brevibacillus

Brevibacillus laterosporus, a spore-producing pathogen distinguished by a unique lamellar parasporal structure were reported. Based on the 16S rRNA, such species were primarily classified under Brevibacillus (Shida et al. 1996). Its insecticidal activity was first used to target the A. aegypti and Anopheles stephensi. However, insecticidal activities have also been reported against Coleopteran and other insect species (de Oliveira et al. 2004). Several B. laterosporus variants have crystalline additions, which are released through the lyses of sporangium (Smirnova et al. 1996). Crystals containing variants 921 and 615 are toxic to insects. Toxic effects among these strains were associated with parasporal crystals (special 130 kDa protein) poisonous to Aedes larvae (Zubasheva et al. 2010). Genetic findings indicate a high similarity between isolates in this population, suggesting a small type of genetic polymorphism (de Oliveira et al. 2004; Ruiu et al. 2007).

3.2.3 Family: Enterobacteriaceae

3.2.3.1 Genus: Serratia spp.

The genus Serratia consists of ten species widespread in soil and water (Grimont and Grimont 2006). S. marcescens is known to colonize an extensive range of insects’ digestive tract. It can produce a potent toxin that can kill insects with a lower lethal dose (Tan et al. 2006). Several insects are susceptible to this toxin, which enters via the oral pathway and acts against Glossina spp. (Flies tsetse) (Poinar et al. 1979); Lucilia sericata (blowfly) (O’Callaghan et al. 1996); and Melolontha spp. (May beetles) (Jackson et al. 2004). Another S. Sntomophilia or Serratia proteamaculans has been proven to be responsible for causing amber infection in New Zealand Grass grub larvae. S. proteamaculans and S. entomophilia. The LD50 for S. entomophila strain 154 was estimated to be 2–4 × 104 cells per larvae (Jackson et al. 2004). The inhalation of microbes has a significant impact mostly on the appearance of a diseased larva. After the intake of amber disease-causing bacteria, within 1–3 days, larvae of C. zealandica avoid eating; furthermore, the rates of stomach acid, trypsin, and chymotrypsin drop drastically throughout the intestinal tract (Jackson et al. 2001, 2004). During intake, S. entomophilia colonizes certain insects and attach to a cuticle of the foreskin (Jackson et al. 2001). The reduction in enzyme dilution in the intestine allowed transmission of serine protease enzymes responsible for ingestion into C. zealandica larvae (Marshall et al. 2008). Also, protein content, including protein synthesis concentrations, has been shown to rise throughout the intestinal tract of infected insects (Gatehouse et al. 2008).

3.2.3.2 Genus: Yersinia spp.

The species named, Yersinia pestis is best known for its invasion on the food track of rat flea (Xenopsylla cheopis) (Jarrett et al. 2004). Genetic polymorphisms have proved that the Tc gene’s genome homologs are normal among Yersinia strains and express a great diversity for insecticide actions (Fuchs et al. 2008). The Yersinia strain containing TC pathogenicity was obtained from the sixth larvae (Bresolin et al. 2006). Cooler temperatures are critical for both the activation of TC gene expression and infectivity of Y. enterocolitica (Champion et al. 2009). It has shown to have a diverse range of target insects (Hurst et al. 2011). About 106 colony-forming units (CFU) of Y. pseudotuberculosis IP32953 are thought to cause G. mellonella larvae mortality. A new entomopathogenic microbe, Y. entomophaga, has also been extracted from New Zealand Grass Grub (C. zealandica). Yersinia spp. is pathogenic to a wide range of Coleopteran, Lepidopteran, and Orthopteran insect species (Hurst et al. 2011). The targeted insects stop eating after consumption of this EPB. Following absorption, Y. entomophaga induces rapid epithelial membrane degradation in the digestive tract by hemocoel invasion, causing septicemia and destruction. Y. entomophaga can be distinguished from other forms of Yersinia spp. based on the 16S rRNA gene and hybridization of DNA (Hurst et al. 2011).

3.3 Mechanism of Infection, Replication, and Transmission of Entomopathogenic Bacteria (EPB)

Most types of EPB typically have specific pathogenicity characterized by certain virulence factors consisting of insecticidal proteins (IPs), which play a vital role in the mortality of infected insects. The IPs attaches receptor throughout the midgut that interprets to kill gut cells. Disruption of the intestinal endothelial lining allows bacteria to grow a nutrient-rich lymph vessel. The IPs largely determine their accuracy by binding to specific midgut epithelial membrane receptors of the host insects (Djukic et al. 2011). Many Bt strains also synthesize cytolytic (Cyt) proteins that bind to lipid sites in the midgut membrane to generate detergent-like defects contributing towards cells cytolysis (Adang et al. 2014; Lee et al. 2003). A range of receptors was identified as Cry toxin receptors, including alkaline phosphatase (ALP), ATP binding cassette (ABC), aminopeptidase N (APN), and cadherin carriers and glycolipids (Adang et al. 2014). Prominent IP families produced by the EPB are the Cry, Cyt, Vip, and Bin protein toxins (Adang et al. 2014).

3.3.1 Cry Toxins

Many microbes can produce an insecticidal toxin that kills the insects and are specifically toxic to the insect orders Lepidoptera Coleoptera, Hymenoptera, and Diptera, and also to nematodes (Bravo et al. 2007). Such microbes may not always be infectious in certain instances but do not always inhabit the recipient insect cadaver. Insecticidal Cry toxins are well researched and characterized toxins reported from a vast number of EPB. Cry toxins have been recorded from many bacteria as a secretory protein (Varani et al. 2013; Barloy et al. 1996; Crickmore et al. 1994). Identifying the amino acid composition is presently used as a base for Cry toxin classification (Crickmore et al. 2015). On consumption of susceptible individuals, parasporal Bt crystals become dissolved in intestine physicochemical conditions but converted through an effective poison center of Bt proteins or proteases within the host gastrointestinal tract (Waterfield et al. 2004) the immediate action of cry toxins to lyse midgut epithelial cells in the target insect by forming pores in the apical microvilli membrane of the cells (Bravo et al. 2002). Cry proteins pass from crystal inclusion protoxins into membrane-inserted oligomers that cause ion leakage and cell lysis. After cell lysis and the midgut disrupt, epithelium releases the cell contents providing spores a germinating medium leading to severe septicemia and insect death by osmotic shock (Bravo et al. 2005).

3.3.2 Vegetative Insecticidal Proteins (Vip)

Vip protein is produced during Bt’s vegetative growth stage and includes Vip1, Vip2, and Vip3. The pathogenicity of Vip protein is based on the insect gut paralysis and lysis of gut epithelium. That available Vip toxicity data shows the Vip1, and Vip2 function as just dual poisons While Vip3 toxin was dependent upon pore formation of cytotoxic effects (Lee et al. 2003; Leuber et al. 2006; Liu et al. 2011; Singh et al. 2010; Barth et al. 2004). Vip3 have high larvicidal activity against Agrotis ipsilon and S. frugiperda whereas, S. litura and P. xylostella dramatically depend on pore formation for cytotoxicity (Lee et al. 2003). Unlike cry proteins, Vip3 toxins do not seem to have a protease-resistant toxin core, yet the mechanism leading to enterocyte death remains mostly unknown (Liu et al. 2011).

3.3.3 Bin Toxins

Binary or bin toxins, produced by L. sphaericus, are shown as one needle-shaped crystal comprising equimolar quantities of two protein component, 42-kDa (BinA and P42) and 51-kDa (BinB as well as P51) (Broadwell et al. 1990) and has been widely used for control of mosquitoes. These two subunits (Bin A and Bin B) work together to exert maximal toxicity against mosquito larva through pore formation and induction of apoptosis (Boonserm et al. 2006). Both Bin-A and bin-B toxins get a comparatively low sequence similarity, yet they shared several domains that are essential for the activity of the toxin.). Gastrointestinal liquids converted both Bin-A and Bin-B via 40 kDa (Bin-A) and 43 kDa (Bin-B) proteins (Broadwell et al. 1990). The Bin-B protein includes a lectin-like N-terminal domain and a C terminal domain (Srisucharitpanit et al. 2014). It is associated with the initial receptor binding followed by Bin A’s interaction before internalization of the toxin complex (Boonserm et al. 2006).

3.3.4 Mtx Toxins

Several L. Sphaericus strains develop mosquitocidal toxin (Mtx) and are highly toxic to mosquito larvae, causes morphological changes that lead to loss of the typical cell shape and cluster formation (Carpusca et al. 2006). Mtx matured proteins contain an N-terminal 27 kDa segment of ADP ribosyl transferase action and a C-terminal 70 kDa segment with even a sequence identical to a lectin-like ricin bind component (Thanabalu et al. 1993; Hazes and Read 1995). It is also assumed that after attaching via associations between both the 70 kDa segment and unidentified receptor, the toxin becomes internalized through endosomes whose lower pH emission help the diffusion of a 27 kDa segment into ADP ribosylate proteins cytosol (Schirmer et al. 2002). The mixture of Mtx and Cry toxin through Bt spp. Israelensis has shown slight synergy towards C. quinquefasciatus (Wirth et al. 2014).

3.3.5 Toxin Complex

High molecular weight insecticidal toxins secreted by bacteria comprised of multiple protein subunits, termed the Toxin Complexes or Tc’s. They usually consist of three protein subunits (TcA, TcB and TcC) that effectively harm the insects (Ffrench-Constant et al. 2007). Information of a Tc toxicity mechanism of action has recently been clarified. “TcC” subunit contains, in addition to cytotoxicity, an ADP ribosyl transferase activity which is placed inside such a groove developed through TcB and TcC subsets (Busby et al. 2013). Such (TcB and TcC) heterodimer is bound to a p-start formation through TcA subsets, which bind the unknown heretofore receptors mostly to the central cell surface. Tc toxins are composed of three subunits that perforate the host membrane, similar to a syringe, and translocate toxic enzymes into the host cell. The reactions of the toxic enzymes lead to deterioration and, ultimately, the cell (Meusch et al. 2014; Gatsogiannis et al. 2013).

3.4 EPB Based Commercial Biopesticides

Biopesticides have great potential to control pests due to their target specificity. They are non-toxic to human health and environment-friendly, easily degradable with the right mortality level of insect pests (Kumar and Singh 2015). In plants protection strategy, the biopesticides cover just 2% of pesticides used for insect pest management, but their use is increasing every year. The global biopesticides market in 2021 is predicted to reach approximately 7.7 billion USD (Ruiu 2018). Among biopesticides, 90% are produced from EPB (Hubbard et al. 2014). Biopesticides are attracting the world due to their better pest control and management by pretending fewer effects on the environment and human health. These are classified into microbial, biochemical, and plant-incorporated protectants. The efficacy level of biopesticides is better than traditional pesticides (Kumar and Singh 2015). Bacterial pesticides reduce plant damage and maintain the pest population below the ETL. Bacterial entomopathogenic first enters the host body, avoids host defensive reaction, and produces virulence factors that produce diseases in the host and eventually kills the host (waterfield et al., 2004). EPB commercially has been developed for the control of insect pests of field crops. Bacterial species such as Lysinbacillus sphaericus, Paenibaccilus spp. Serrata entomophila and Bacillus thuringiensis subspecies kurstaki are mostly used to control the pests of field crops and forests (Lacey et al. 2015). Biopesticides, derived from B. thuringiensis, are very useful in the control of particular insect pests. Bt commercially used for forests and crops insect pests. Bt subspecies due to high host range (Lepidoptera, Diptera, Coleoptera, and other insects) are widely used to produce bacterial pesticides (van Frankenhuyze 2009). It is fast-reacting, readily available at a low price, long-lasting shelf life, and easy to formulate. It is also highly selective in target pests and has fewer negative impacts on the environment. Bt is used until harvesting begins, and it does not kill the beneficial insects. It is degraded in sunlight; therefore, it can be applied frequently (Glare et al. 2012). Bt is used to control pests in vegetables and lepidopteran pests in crops such as cotton, cucurbits, corn, and legumes (Lacey et al. 2015). Control of pests within Coleoptera by Bt is limited in the Chrysomelidae family (Wraight and Hajek 2009). Two main toxins are produced by Bt, which are Cry and Cyt. Vegetative insecticidal proteins (Vip) are also produced and secreted by Bt cells (Crickmore et al. 2015; Raymond et al. 2010b). Entomopathogenic Paenibacillus are classified into Paenibacillus larvae, P. popilliae, and P. lentimorbus. P. Popilliae and P. lentimorbus are very distinct at the molecular level. Parasporal body contains toxins that disrupt the gut epithelial barrier and facilitate the attack on the hemocoel (Zhang et al. 1997). The affected larvae cannot molt, become retorted, and eventually died. Paenibaccilus larvae are the etiological mediator of core bacteriological honey bee pathology (Gende et al. 2011). L. sphaericus produces spherical spores present in the terminal within a sporangium (Nakamura 2000). Their attack depends on the production of toxins that invade midgut cells in host larvae. This Bacillus species are pathogenic to insect orders Coleoptera, Diptera, and Lepidoptera (de Oliveira et al. 2004). Serratia entomophila and S. proteamaculans are non-spore-forming bacteria with limited stable life stages. These are used to control damaging pests of pasture and grass grub. These bacteria cause amber disease in C. zealandica larvae with chronic pathology. When larvae feed on this bacterium, their feeding stops, and the larval midgut is cleared, which results in the coloration of amber disease (Jackson et al. 2004). Table 3.1 described the Bt subspecies topical insecticidal products based on various transconjugant and recombinant strains (Sanahuja et al. 2011).

Table 3.1 Entomopathogenic bacteria (EPB) based commercial insecticidal products
Full size table

3.5 Potential of EPB as a Biological Control Agent

Biopesticides and their by-products are used for plant protection with less injurious effects. The biological control system of crop pests is changed after discovering the EPB (Glare and O’Callaghan 2000). They can control the target pest population as a natural enemy (Mampallil et al. 2017). Once enter into the insect cell, they affect the midgut epithelial cells and cause the host’s death by producing different toxins (Mampallil et al. 2017). Most of the bacterial pathogens of insect pests are present under families such as Bacillaceae, Pseudomonadaceae, Enterobacteriaceae, Streptococcaceae, and Micrococcaceae. The species belonging to the family Bacillaceae are highly effective against arthropods. Among EPB, mainly the spore developing species such as Bt, B. sphaericus, and B. popilliae is mostly used for biological control of pests (Lacey et al. 2015). Bt is the most successful biological control agent used to control insects. It comprises species that are naturally existing and are supplementary in the ecosystem for the control of insect pests. Bt produces toxins that contain an insecticidal protein called endotoxin. It attacks the host in larval condition, inserts in the host’s body, invades the midgut tissue, which causes death (Bravo et al. 2007). B. popilliae, which forms spores, causes milky disease in phytophagous coleopteran larvae. Spores are ingested by the host, which germinate into the midgut. It causes milky spore disease in the Scarabaeidae family (Evans 2008). Gram-negative bacteria such as Serratia and Enterobacter within the family Enterobacteriaceae were reported to possess entomopathogenic activity. Serratia is a facultative, anaerobic bacteria and proliferates in insects’ midgut and causes septicemia, leading to insect death. This bacterium is also secluded from diseased insects. Many bacteria interconnected with plants in the soil, which utilize beneficial effects like development, encouraged conflict to pathogens and pest control ability. Bacteria that subordinate with plants have different names like rhizosphere bacteria, endophytic bacteria present in their natural environmental conditions (Bostock et al. 2001).

Due to environmental and socioeconomic advantages, globally acreage cultivation with herbicide-and pest-resistant genetically modified crops has drastically increased since 1996. Until 2018, 191.7 million hectares and 825 GM varieties were confirmed to be released and cultivated. Specifically, Bt was used in 103 million hectares. Out of 103 million hectares, 80 million hectares were cultivated by crops containing stacked Bt herbicide tolerance genes, while 23 million hectares were cultivated with those crops containing only the Bt gene for resistance to coleopterans and lepidopterans insects-pests (ISAAA 2018). Since 1996, 304 Bt-GM varieties and lines of 10 plant species, including 1 tomato (Lycopersicon esculentum), 30 potato (Solanum tuberosum), 208 maize (Zea mays), 49 cotton (Gossypium hirsutum), 3 rice (Oryza sativa), 6 soybean (Glycine max), 3 sugarcane (Saccharum sp.), 2 poplar (Populus sp.), 1 cowpea (Vigna unguiculata) and 1 eggplant (Solanum melongena) variety, have been authorized for commercial release in 27 different countries around the world (ISAAA’s GM Approval Database 2020). Among them, 243 crop varieties are resistant to lepidopteran pests and contain anti-lepidopteran cry and vip genes, including cry1Ab, cry1Ac, cry1C, cry1F, cry1Fa2, cry2Ab2, cry2Ae, cry9C, and vip3A.

3.6 Genetic Improvements of EPB

Genetic technology is a potent way to improve good characters in these living entities’ desire species for their better utilization. It comprised the selection of growing populations, revealing the chosen qualities, artificial assortment, cross-hybridization, and further genetic management like gene mutations and genetic engineering (Karabörklü et al. 2018). Genetically improved bioinsecticides comprised of genetically modified entomopathogens are among the supreme essentials with fast and durable pest control (Azizglu et al. 2020). They are less toxic and easily degradable and decompose very fast than conventional pesticides (Arora et al. 2016). Scientists are continually developing and discovering new, environmentally friendly biopesticides that can be used alone or in association with chemical pesticides (Ruiu et al. 2013). Recombinant DNA technology has been widely used to make novel fusion proteins and bind toxin genes to transporter proteins that can show lethal effects after entering the pest cell (Fitches et al. 2004). EPB haves been utilized to manage insect pests of crops and control the mosquitoes that cause serious disease (Fitches et al. 2004). However, the main problem associated with the application of EPB is their poor stability and are commonly affected by adverse ecological aspects that affect their pathogenicity. EPB can be genetically modified to resist adverse environmental conditions (Karabörklü et al. 2018). Moreover, these genetically modified bacteria have fewer adverse effects on the beneficial organism (Azizglu et al. 2020). Genetically modified strains of EPBs have shown higher efficiency against target pests (Arora et al. 2016). Several EPB such as Photorhabdus, Bacillus, Serratia, Pseudomonas, Lysinibacillus has already been genetically modified.

The limitation of field stability is one of the biggest problems of EPB applications. Furthermore, pathogenicity is also affected by adverse environmental factors. In this case, genetic modification can increase the resistance of EPB to adverse environmental conditions (Karabörklü et al. 2018). Genetic modification of wild-type EPB is very important to increase effectiveness against the target insect and develop broad-spectrum insecticides (Wang et al. 2008; Patel et al. 2015). Lower insect resistance, higher pathogenicity, low spraying requirements, and longer-term efficacy are the advantages of genetically modified EPB over wild type EPBs (Sharma 2009; Castagnola and Jurat-Fuentes 2012; Karabörklü et al. 2018). There are several concerns and risks regarding the use of genetically modified EPBs. The major concern is the effects of modified EPBs on human health and the environment. Others include the development of resistance in target insect-pest, possible gene flow to wild species, and effects on non-target beneficial species, last but not the least impact on the rhizospheric microbial population (Castagnola and Jurat-Fuentes 2012; Karabörklü et al. 2018; Amarger 2002).

3.7 Conclusion

Future research in agriculture is mainly looking for eco-friendly pest and disease management practices. Such tools are being established and assessed globally to lessen the environmental and health-risks because the higher use of synthetic chemicals in agriculture is alarming the natural biodiversity. Under this scenario, EPBs have a massive scope as bio-control agents and an excellent source for exploring insecticidal toxin genes. Many Bt strains have already been approved, whereas many other strains are being described but not commercially developed. There is a broader scope for identifying new bacterial toxin genes from the under-research strains for their development into successful EPB based biopesticides.