Biogenetic cantharidin is a promising leading compound to manage insecticide resistance of Mythimna separata (Lepidoptera: Noctuidae)

Antimicrobial peptide cecropin B functions in pathogen resistance of Mythimna separata

Mythimna separata (Lepidoptera: Noctuidae) is an omnivorous pest that poses a great threat to food security. Insect antimicrobial peptides (AMPs) are small peptides that are important effector molecules of innate immunity. Here, we investigated the role of the AMP cecropin B in the growth, development, and immunity of M. separata. The gene encoding M. separata cecropin B (MscecropinB) was cloned. The expression of MscecropinB was determined in different developmental stages and tissues of M. separata. It was highest in the prepupal stage, followed by the pupal stage. Among larval stages, the highest expression was observed in the fourth instar. Tissue expression analysis of fourth instar larvae showed that MscecropinB was highly expressed in the fat body and haemolymph. An increase in population density led to upregulation of MscecropinB expression. MscecropinB expression was also upregulated by the infection of third and fourth instar M. separata with Beauveria bassiana or Bacillus thuringiensis (Bt). RNA interference (RNAi) targeting MscecropinB inhibited the emergence rate and fecundity of M. separata, and resulted in an increased sensitivity to B. bassiana and Bt. The mortality of M. separata larvae was significantly higher in pathogen plus RNAi-treated M. separata than in controls treated with pathogens only. Our findings indicate that MscecropinB functions in the eclosion and fecundity of M. separata and plays an important role in resistance to infection by B. bassiana and Bt.

Long-term impact of rearing substrates on susceptibility to insecticides and metabolic enzyme activities in the house fly Musca domestica

Musca domestica Linnaeus is a devastating insect pest of medical and veterinary importance with reports of resistance development to commonly used insecticides worldwide. Rearing substrates usually play a crucial role in determining susceptibility to insecticides and control of insect pests. The aim of the present study was to investigate the effect of five rearing substrates of M. domestica on its susceptibility to different insecticides and activities of metabolic enzymes. After 30 generations of rearing, susceptibility of M. domestica to tested insecticides, viz., malathion, pirimiphos-methyl, alpha-cypermethrin, deltamethrin, methomyl, propoxur, spinetoram, and chlorfenapyr had evident differences. Musca domestica reared on hen liver exhibited reduced susceptibility to all insecticides followed by the strain reared on poultry manure. However, M. domestica reared on milk-based diet showed the highest susceptibility to tested insecticides followed by the strain reared on manures of buffalo and horse. In addition, M. domestica reared on different substrates exhibited significant differences (p < 0.01) in the activities of glutathione S-transferase (GST), cytochrome P450-dependent monooxygenase, and carboxylesterase (CarE). Overall, hen liver and poultry manure strains exhibited higher activities of metabolic enzymes than those of the milk-based diet, buffalo, and horse manure strains. In conclusion, the data of the present study exhibited a significant effect of rearing substrates on the susceptibility to insecticides and activities of metabolic enzymes in M. domestica. These results could be helpful for the sustainable management of M. domestica on different hosts by selecting appropriate insecticides.

Key Contributions of the Overexpressed Plutella xylostella Sigma Glutathione S-Transferase 1 Gene (PxGSTs1) in the Resistance Evolution to Multiple Insecticides

The overexpression of insect detoxification enzymes is a typical adaptive evolutionary strategy for insects to cope with insecticide pressure. In this study, we identified a glutathione S-transferase (GST) gene, PxGSTs1, that exhibited pronounced expression in the field-resistant population of Plutella xylostella. By using RNAi (RNA interference), the transgenic fly models, and quantitative real-time polymerase chain reaction (RT-qPCR) methods, we confirmed that the augmented expression of PxGSTs1 mediates the resistance of P. xylostella to various types of insecticides, including chlorantraniliprole, novaluron, λ-cyhalothrin, and abamectin. PxGSTs1 was found to bolster insecticide resistance in two ways: direct detoxification and enhancing antioxidative defenses. In addition, our findings demonstrated that pxy-miR-8528a exerts a pivotal influence on forming insecticide resistance in P. xylostella by downregulating PxGSTs1 expression. In summary, we elucidated the multifaceted molecular and biochemical underpinnings of PxGSTs1-driven insecticide resistance in P. xylostella. Our results provide a new perspective for understanding the insecticide resistance mechanism of P. xylostella.

Investigation of sex expression profiles and the cantharidin biosynthesis genes in two blister beetles

Cantharidin (CTD) is a well-established defensive toxin synthesized by blister beetles, displaying both therapeutic potential and toxicity. Among these beetles, Hycleus cichorii and Hycleus phaleratus are the two most commercially significant species due to their capacity to produce CTD in males. In this investigation, we conducted a gene expression profiling analysis of male and female individuals of these two species, utilizing the Illumina Hiseq4000 platform. We identified 7,983 expressed genes, including 2,823 differentially expressed genes (DEGs) shared by both male and female blister beetles. Nineteen genes related to CTD biosynthesis in the terpenoid backbone biosynthesis pathway were identified, including hydroxymethylglutaryl-CoA reductase (HMGR; EC:1.1.1.34), which demonstrated a significant correlation with CTD content. Furthermore, hydroxymethylglutaryl-CoA synthase (HMGS; EC:2.3.3.10) and isopentenyl-diphosphate Delta-isomerase (IDI; EC:5.3.3.2) were also found to be significantly up-regulated in males. Comparative analysis revealed that NADP+-dependent farnesol dehydrogenase (FOHSDR; EC:1.1.1.216) and farnesyl diphosphate synthase (FDPS; EC:2.5.1.1) had the highest copy number in these beetles, significantly higher than the copy number of the other four non-Meloidae insects. The analysis of the protein-protein interaction network of genes related to CTD biosynthesis revealed that the acetyl-CoA C-acetyltransferase (ACAT; EC:2.3.1.9) gene was the central gene, exhibiting greater expression in male blister beetles than in females. This study offers novel insights into the mechanisms of CTD biosynthesis in blister beetles and enhances our comprehensions of the association between particular genes and CTD content.

Molecular targets of insecticides and herbicides - Are there useful overlaps?

New insecticide modes of action are needed for insecticide resistance management strategies. The number of molecular targets of commercial herbicides and insecticides are fewer than 35 for both. Few commercial insecticide targets are found in plants, but ten targets of commercial herbicides are found in insects. For several of these commonly held targets, some compounds kill both plants and insects. For example, herbicidal inhibitors of p-hydroxyphenylpyruvate dioxygenase are effective insecticides on blood-fed insects. The glutamine synthetase-inhibiting herbicide glufosinate is insecticidal by the same mechanism of action, inhibition of glutamine synthetase. These and other examples of shared activities of commercial herbicides with insecticides through the same target site are discussed. Compounds with novel herbicide targets shared by insects that are not commercialized as pesticides (such as statins) are also discussed. Compounds that are both herbicidal and insecticidal can be used for insect pests not associated with crops or with crops made resistant to the compounds.

Risk Assessment of Fluxametamide Resistance and Fitness Costs in Fall Armyworm (Spodoptera frugiperda)

The fall armyworm (FAW), Spodoptera frugiperda, is one of the most devastating invasive polyphagous pests, which has attracted recent global attention by developing resistance to various insecticidal active ingredients with independent mode of action. Fluxametamide, a newly commercialized isoxazoline insecticide, is exceptionally selective towards several lepidopteran pests. The present study aimed to evaluate resistance risk in FAW to fluxametamide and the fitness costs associated with fluxametamide resistance. A field-collected and genetically mixed population of FAW was artificially selected through continuous exposure to fluxametamide. After successive selection of 10 generations, there was no obvious increase in the LC₅₀ (RF: 2.63-fold). The realized heritability (h²) of fluxametamide resistance was estimated as h² = 0.084 using a quantitative genetic approach. Compared with the susceptible F0 strain, the Flux-SEL (F10) strain of FAW displayed no significant cross-resistance to broflanilide, chlorantraniliprole, fipronil, indoxacarb, lambda cyhalothrin, spinetoram, and tetraniliprole, except emamectin benzoate (RF: 2.08-fold). Increased activity of glutathione S-transferase (ratio 1.94) was observed in the Flux-SEL (F10) strain of FAW, while the cytochrome P450 and carboxylesterase activities were not altered. The fluxametamide-selection significantly affected the development and reproductive traits of FAW with a lower R₀, T and relative fitness (R_f = 0.353). The results alluded that the risk of fluxametamide resistance evolution in FAW is relatively lower; however, proactive implementation of resistance management approaches should be done to maintain the field efficacy of fluxametamide against FAW.

Glutathione S-Transferase May Contribute to the Detoxification of (S)-(-)-Palasonin in Plutella xylostella (L.) via Direct Metabolism

The control of P. xylostella primarily involves chemical insecticides, but overuse has brought about many negative effects. Our previous study reported that (S)-(-)-palasonin (PLN) is a plant-derived active substance with significant insecticidal activity against P. xylostella. However, we noticed a possible cross-resistance between (S)-(-)-palasonin and other insecticides which may be related to metabolic detoxification. In order to further explore the detoxification effect of detoxification enzymes on (S)-(-)-palasonin in P. xylostella, the effects of (S)-(-)-palasonin on enzyme activity and transcription level were determined, and the detoxification and metabolism of GSTs on (S)-(-)-palasonin were studied by in vitro inhibition and metabolism experiments. During this study, GST enzyme activity was significantly increased in P. xylostella after (S)-(-)-palasonin treatment. The expression levels of 19 GSTs genes were significantly increased whereas the expression levels of 1 gene decreased. Furthermore, (S)-(-)-palasonin is shown to be stabilized with GSTs and metabolized GSTs (GSTd1, GSTd2, GSTs1 and GSTs2) in vitro, with the highest metabolic rate of 80.59% for GSTs1. This study advances the beneficial utilization of (S)-(-)-palasonin as a botanical pesticide to control P. xylostella in the field.

The Detoxification Enzymatic Responses of Plutella xylostella (Lepidoptera: Plutellidae) to Cantharidin

Plutella xylostella (L.) (Lepidoptera: Plutellidae) is one of the most destructive pests of Brassicaceae vegetables. Cantharidin is an insect-derived defensive toxin, which has been reported to have toxicity to a variety of pests and especially lepidopteran pests. Although the toxicity of cantharidin on P. xylostella has been demonstrated, there is little information available on the specific detoxification response of P. xylostella against cantharidin. This study investigates the enzymatic response (including serine/threonine phosphatases [PSPs], carboxylesterases [CarEs], glutathione-S-transferases [GSTs], and cytochrome P450 monooxygenases [P450]) in P. xylostella to the sublethal and low lethal concentrations of cantharidin (LC10 and LC25). Results showed that the inhibitory activity of PSPs was increased and then decreased in vivo, while PSPs activity could be almost completely inhibited in vitro. Interestingly, the activities of detoxification enzymes (GST, CarE, and P450) in P. xylostella displayed a trend of decreasing and then increasing after exposure to the two concentrations of cantharidin. Notably, the increase in P450 enzyme activity was the most significant. The increasing trend of detoxification enzyme activity was congruent with the recovery trend of PSPs activity. This study contributes to our understanding of the detoxification mechanism of cantharidin in P. xylostella and helps in the further development of biogenic agents.

Draft genome of the blister beetle, Epicauta chinensis

Existence of cantharidin (CTD) in blister beetles is a significant ecological adaptive mechanism that defends against predators and regulates courtship and mating behaviors. To better understand CTD biosynthetic information as well as its biology and pharmacology, we assembled a genome of 151.88 Mb for Epicauta chinensis using PacBio sequencing technology. Gene annotation yielded 249,238 repeats, 527 non-coding RNAs and 12,520 protein-coding genes. Compared to other 11 insects, expansions of gene families in E. chinensis for most core gene families likely associated with environmental adaptation, such as chemoreception, immunity, and detoxification. We further annotated P450s and immune-related genes, a total of 117 putative P450s comprising 7 CYP2, 67 CYP3, 36 CYP4, and 7 mitochondrial P450s and 281 immune-related genes were identified. Comparative analysis of the insect immune repertoires indicated presence of immune genes detected only from Coleopteran insects such as MD2-like. This suggested a lineage-specific gene evolution for Coleopteran insects. Based on the gene family evolution analysis, we identified two probable candidate genes including CYP4TT1 and phytanoyl-CoA dioxygenase for CTD biosynthesis. The high-quality reference genome of E. chinensis provides the genetic basis for further investigation of CTD biosynthesis and in-depth studies of the development and evolution of blister beetles.