The destruxins (Dxs) are structurally composed of an alpha-hydroxy acid and five amino acid residues joined together by amide and ester linkages to form a cyclic structure. There are 38 Destruxins or Dx analogs (Pedras et al., 2002), which is double the earlier reported, 19 types (Gupta et al., 1989). They are divided chemically into five basic groups labeled as A through E, plus several subgroups of each. Destruxins A and B were first reported in the Metarhizium isolates from Japan during 1960’s and were synonymously named as “oospora destructor” or destruxins. These cyclic depsipeptides have extensively been studied by plant pathologists, microbiologists, and natural products chemists for their toxic biochemical activities. The Dx – biosynthetic pathway is assumed to be a non-ribosomal multifunctional enzyme system (Kleinkauf and von Döhren, 1987, 1990; Turner, 2000). The insect specific toxicity of Dxs A, B, and E is well documented with reported induction of acute muscular paralysis in arthropods through the reversible opening of their muscle cell membrane Ca⁺² channels (Samuels et al., 1988; Dumas et al., 1996). The ionophoric properties of Dx A also allow Ca⁺² mobilizations across liposomal membrane barriers (Hinaje et al., 2002). Alternatively, Dxs are also reported to cause the inhibition of vacuolar (V-type) ATPase activity in the brush bordered midgut (BBM) membrane of Galleria mellonella, however, its structural stability is significantly compromised under the altered physiological conditions inside the host (Bandani et al., 2001). The Dx variants are further known for triggering the oxidative stress mechanisms in host (Spodoptera litura) through up regulating the levels of superoxide radicals and the systemic suppression of insect immunity (Pal et al., 2007; Sowjanya Sree et al., 2008). More recently, Meng et al. (2013) have described a yet another toxicity mechanism for Dx A on Spodoptera litura with deleterious effects on its wing disc like proteins (SLAWD) expressions, hence inducing the developmental abnormalities in larval stages of development.

These are the complex mixture of peptide toxins reported from the entomopathogenic soil hyphomycetes fungi, Tolypocladium spp. (Gupta et al., 1992). The variant form, Efrapeptin F is also known to significantly inhibit the activity of V-type ATPases, which regulates the protons gradient (K⁺/H⁺) across the brush bordered epithelium in insect mid-gut (Bandani et al., 2001). Simultaneously, the toxin has been reported to adversely affect the cell mediated immune mechanism in the host insect – G. mellonella (Bandani, 2008).

It’s an ionophoric cyclohexadepsipeptide of enniatin antibiotic family which can permeate through biological membranes with enhanced entomotoxic activities (Gupta et al., 1991). Structurally, a beauvericin molecule is consists of the alternating amino acid units of the three D-hydroxyisovaleryl and three N-methylphenylalanyl residues (Hamill et al., 1969). The compound was first isolated from B. bassiana, but was later reported from Fusarium, Verticillium, and Paecilomyces spp. (Suzuki et al., 1977). The various diastereoisomers of beauvericin (A, B, and C etc.) shows the insecticidal activities against a spectrum of pest species viz., Artemia salina, Calliphora erythrocephala, Aedes aegypti, Lygus, S. frugiperda, and Schizaphis graminum (Grove and Pople, 1980; Jestoi, 2008; Wang and Xu, 2012). Functionally, the ionophoric nature of beauvericin allows the compound to form reversible complexes with divalent (Ca⁺², Ba⁺²) and monovalent (Na⁺¹, K⁺¹) ions, which effectively disturb ionic equilibrium and pH potential across the lipid bilayers resulting in loss of membrane associated functions in insect host cells (Tang et al., 2005; Tonshin et al., 2010).

The bassianolides are structurally similar to beauvericin except the alternate four subunit repeats of d-2-hydroxyisovaleric acid (D-α-Hiv) and N-methyl Leucine (L-N-Me-Leu) in the cyclooligomer depsipeptide COD making it an octadepsipeptide (Suzuki et al., 1977). As an entomotoxic COD, bassianolide from B. bassiana and V. lecanii is known for inducing acute muscular atony among silkworm larvae through the inhibition of acetyl-choline mediated muscle contractions (Kanaoka et al., 1978; Nakajyo et al., 1983). Few of the reports have suggested the predominant role of bassianolides in fungal virulence as compared to other cyclodepsipeptides i.e., beauvericin, (Champlin and Grula, 1979; Xu et al., 2009). The high efficacy of these compounds as an effective entomotoxin can also be attributed to its structural conformation with hydrophobic exterior and relatively hydrophilic interior making it an ionophoric molecule. A wide range of pest species are reportedly affected through bassianolide exposure viz., Helicoverpa zea (Champlin and Grula, 1979), chagas vector Triatoma infestans (Lobo et al., 2015), and livestock pest Culicoides spp. (Narladkar et al., 2015) etc.

Typically known for their immunosuppressive application for organ transplant, cyclosporines were originally reported from Tolypocladium niveum, and more recently from T. inflatum (Weiser and Matha, 1988; Bushley et al., 2013). Specifically, cyclosporine A is known to suppress the insect’s humoral or innate immune responses (Fiolka, 2008; Kulkarni et al., 2013). Earlier, the potential suppression of cyclosporine sensitive glycoprotein based efflux pump system in insect cells was recognized as the probable mechanism of insect mycoses (Podsiadlowski et al., 1998). Concomitantly, Fiolka (2008) have further proposed an alternate mechanism of cyclosporine mediated entomoxicity i.e., decrease in the activity of insect’s antimicrobial peptides and lysozymes which adversely effects its survival.

A non-reduced polyketide commonly reported from Beauveria spp. (Vining et al., 1962; Strasser et al., 2000). Recently, Feng et al. (2015) have verified the role of oosporein (bibenzoquinone oosporein) in establishing the fungal virulence in host insects through the inhibition of insect defense mechanisms viz., PPO (pro-phenoloxidase) activity and down-regulation of antifungal peptide expressions in host.

It represents a hexaketide compound with a 2-pyridone core reported first from Beauveria spp. and associated with a broad range of biological activities (McInnes et al., 1974). Alternatively, the compound has been reported to inhibit the Ca⁺² dependent ATP’ase activities in mammalian erythrocytes (Jeffs and Khachatourians, 1997). However, no elaborated reports are yet available which discuss the specific entomotoxic effects associated with bassianin.

Swainsonine is chemically an indolizidine alkaloid molecule with a fused piperidine and pyrrolidine ring system. This sugar analog was first discovered in Australian native legume Swainsona canescens (Colegate et al., 1979) followed by Astragalus and Oxytropis (Molyneux and James, 1982) as a toxin metabolite responsible for locoweed poisoning among livestock. Later on, the compound was also reported from microbial sources viz., Rhizoctonia leguminicola (Schneider et al., 1983) and M. anisopliae (Hino et al., 1985). Functionally, swainsonine acts as the reversible inhibitor of both lysosomal α-mannosidase and mannosidase II enzymes which mainly catalyze the cellular degradation of polysaccharides and asparagine-linked glycoproteins, respectively (Elbein et al., 1981). These properties of swainsonine have been well documented and maneuvered to develop the anti-metastatic and anti-proliferative candidates through laboratory based and clinical trial studies (Sun et al., 2009; Li et al., 2012; Singh and Kaur, 2014b). Although many studies have described the therapeutic potentials of swainsonine, its entomotoxic properties and role in the entomopathogenic virulence of Metarhizium are still largely unexplored. Recently, Singh and Kaur (2014a) have reported the in vitro entomotoxic properties of swainsonine isolated from M. anisopliae against the lepidopteran target host (Spodoptera sp.) through the induction of apoptotic cell death mechanisms. However, the potential role of swainsonine as an entomotoxin for establishing the fungal – insect chemical ecology requires further studies.

The potent entomotoxic metabolite is variously been reported from numerous entomopathogenic genera viz., Beauveria, Paecilomyces, and Verticillum (Claydon and Grove, 1982). The pyridine derivative compound i.e., dipicolinic acid (DPA) or pyridine-2, 6-dicarboxylic acid is ubiquitously found in all bacterial spores and confers them thermal resistance (Setlow et al., 2006). In the context of entomotoxicity, the fungal DPA or their calcium salts are reportedly known to be active against white fly larvae (Bemisia) and blowflies (C. erythrocephala) with varying degrees of toxicity (Asaff et al., 2005).

Yet another aspect of insect immunity can be framed using the concept of “immunity bioenergetics’ under the condition of insect mycoses. The estimated cost of immune response in vertebrates is variedly calculated approximately 32%, thus, by analogy, one can assume the respective metabolic expenditures in case of insect mycoses or infections (Martin et al., 2008). The humoral (PO-mediated) and cellular (hemocyte mediated) immunity in infected insects entails bioenergetic cost among the host which can be tracked in its metabolome. A recent progress has led to the identification of the metabolic cues associated with insect’s altered metabolomic response to the co-evolutionary selection pressure by entomopathogenic fungi. Xu et al. (2015) have reported the alteration in the levels of energy and nutrient metabolism of silkworm moths infected with B. bassiana. As indicated in Figure 1, the group has observed an upregulation in the levels of carbohydrates, fatty acids, lipids, and amino acids with simultaneous down regulation in eicosanoids and amines. The proposed elevation in the amounts of carbohydrates (sugars) was correlated to the increased ratio of monosaccharide to disaccharide sugars owing to possible bio-conversion to meet the heightened energy cost for immunity and metabolism (Ardia et al., 2012). Another vital component of insect immune response tractable in its metabolome is comprised of lipids, fatty acids, and eicosanoids i.e., prostanoids. Intriguingly, the levels of various lipids viz., Phosphatidylethanolamine, triglycerides, glycerophosphocholine, and 1-oleoyl-2-palmitoylphosphocholine are reportedly increase in case of both the fungal as well as bacterial infections of host insects (Hoxmeier et al., 2015; Xu et al., 2015). The atypical elevations of the lipid levels thus fulfill the heightened biomolecular demands for energy generations, membrane repair, and signaling pathway intermediates (Atella and Shahabuddin, 2002). Hence, the alterations in the lipid levels may also serve as the potent and generic biomarkers of insect immunity under immune compromised conditions. Similarly, the fatty acids released from triglycerides viz., hexadecenoic acid, heptadecenoic acid, and octadecenoic acid further fulfill the energy demands through β-oxidation (Athenstaedt and Daum, 2006).

The upregulation of amino acid metabolism (asparagine, glutamine, lysine) and transglutaminase activity (amino acid cross links forming target clots) are often corresponded to the enhanced humoral immune response among the infected arthropods (Figure 1) (Bulet et al., 1991; Toubarro et al., 2013). In contrast, the elevation in the levels of the free amino acids in hemocoel might also be attributed to the proteolytic activities of the invading microbial parasites (Harrison and Bonning, 2010). An avant-garde experiment published by Graham et al. (2014) describes the different diet preferences of locusts (Chortoicetes terminifera) subjected to Metarhizium infection in vitro. The authors observed that the locust which switched their feeding to high carbohydrate diets survived the fungal infection more effectively than their counterparts fed upon high protein diets. Hence, a logical conjecture was drawn that the entomopathogenic fungi can more efficiently harness the protein contents from the insect hemocoel than the host themselves, and thus the high mortality was observed among the protein rich diet fed insects. Therefore, the meticulous and more robust metabolomic experimental design is required to differentiate the free amino acids and related metabotypes in hemocoel characterized for their origin while the stages of mycosis.

They are the signaling metabolites produced from the oxygenated poly-unsaturated fatty acids and functionally important for the immune responses in insects (Stanley, 2006). In particular, the insects infected with entomopathogenic fungi are reportedly known to have the reduced levels of eicosanoids viz., 17-hydroxyeicosatetraenoic acid (17-HETE) and protaglandins - E2 (PGE2) (Xu et al., 2015). The mechanism seems more pertinent as the suppression of eicosanoids and prostanoids is analogously reported in case of entomopathogenic bacteria and nematode infections thus establishing their vital role in the insect’s innate immunity (Hyrsl et al., 2010; Hwang et al., 2013; Hoxmeier et al., 2015).

Additionally, the reduced host immunity can further be implicated based on the upregulation of cytosine (nitrogenous bases) and trimethyluric acid, a purine alkaloid (Xu et al., 2015). Further, Diaz-Albiter et al. (2012) have suggested the deleterious effects of uric acid components on insect’s ROS mediated immunity, though the phenomena is quite unclear in case of in vivo metabolome alterations following mycoses. Nonetheless, there are many more esoteric facets of insect immunity besides the intrinsic immune response and metabolomes which needs to be delineated (Box 2). Quintessentially, the relative selection pressure induced by the host-pathogen interaction can thus be called as the main driving force behind the altered insect immunity and adaptation in the challenged environments.