Contribution of Glutathione S-Transferases to Imidacloprid Resistance in Nilaparvata lugens

Characterization of two glutathione S-transferase genes involved in clothianidin resistance in Bradysia odoriphaga

BACKGROUND

Glutathione S-transferase (GST) is a key phase II detoxification enzyme involved in xenobiotics metabolism, and plays a pivotal role in the evolution of resistance to various types of insecticides. However, the specific functions of GST genes in clothianidin resistance remain obscure in Bradysia odoriphaga.

RESULTS

Here, a specific GST inhibitor, diethyl maleate (DEM), significantly increased the mortality of Bradysia odoriphaga larvae following exposure to clothianidin, and the activity of GST enzyme in clothianidin-resistant (CL-R) strain of Bradysia odoriphaga was markedly greater than that in the SS strain. Two sigma BoGSTs (BoGSTs1 and BoGSTs2) were markedly overexpressed in the CL-R strain and exhibited a higher abundance in the Malpighian tubules or midgut. Exposure to clothianidin resulted in a significant increased expression of BoGSTs1 and BoGSTs2. The knockdown of BoGSTs1 and BoGSTs2 increased sensitivity of larvae to clothianidin in the resistant strain. Furthermore, overexpression of BoGSTs1 and BoGSTs2 led to a significant increase in Escherichia coli cells tolerance to clothianidin. In vitro metabolic assays indicate that these two GSTs cannot directly metabolize clothianidin and its secondary metabolite desmethyl-clothianidin. Disk diffusion assays and fluorescence competitive binding assays indicated that BoGSTs1 and BoGSTs2 play a critical role in clothianidin resistance by antioxidant activity and non-catalytic binding activity. The docking results showed that BoGSTs1 and BoGSTs2 have strong binding affinity toward clothianidin.

CONCLUSION

Collectively, these findings pinpoint the potential role of BoGSTs1 and BoGSTs2 in conferring insecticide resistance in Bradysia odoriphaga and contribute to our understanding of the underlying mechanisms of insecticide resistance. © 2024 Society of Chemical Industry.

Chemosensory Proteins Protect Nilaparvata lugens from Imidacloprid by Sequestering the Insecticide and Facilitating Metabolic Detoxification

The involvement of chemosensory proteins (CSPs) in binding to insecticides has been implicated. However, our understanding of CSP-mediated insecticide resistance remains limited. Herein, 15 CSP genes were identified and characterized from Nilaparvata lugens. Expression analysis identified six CSPs with overexpression in the imidacloprid-resistant strain, whose involvement in imidacloprid resistance was validated by RNA interference. Among them, four CSPs were successfully expressed using a prokaryotic expression system, and their binding affinities to imidacloprid were confirmed through fluorescence competitive binding assays. Knockdown of them impaired the capacity of N. lugens to metabolize imidacloprid and inhibited the activity of metabolic detoxification pathways, while their overexpression in Escherichia coli enhanced bacterial metabolic efficiency toward imidacloprid. Furthermore, the transcriptional regulation of CSP2 and CSP15 was found to be mediated by AhR/ARNT and CncC/MafK. These findings suggest that the overexpression of CSPs in N. lugens promotes imidacloprid resistance by sequestering the insecticide and enhancing metabolic detoxification.

Roles of spermary-specific carboxylesterases in Nilaparvata lugens reproduction: Opposite between insecticide-induced upregulation and resistance-associated overexpression

Seminal fluid proteins (SFPs) in insect males are critical for reproduction, being transported into female ovary. In Nilaparvata lugens, a significant rice pest, seven spermary-specific carboxylesterases (CarEs) were found abundantly in SFPs, with over-expression in males of an imidacloprid-resistant (RES) strain compared to a susceptible (SUS) strain. This study aimed to evaluate roles of spermary-specific CarEs in N. lugens reproduction, especially in the reproduction disadvantage in RES. Sublethal doses of three insecticides significantly impacted N. lugens reproduction, concomitant with up-regulation of these CarE genes in males. When RNAi against these CarE genes in SUS males did not affect reproduction, RNAi suppression of insecticide-induced up-regulation worsened the reproductive decline caused by insecticides, indicating a reproduction benefit from the transient up-regulation induced by insecticides. However, constitutive overexpression of these CarEs in RES males did not enhance reproduction and instead imposed a fitness cost. RNAi against part CarE genes in RES males increased reproduction. Collectively, the transient up-regulation and constitutive overexpression of these spermary-expressed CarE genes had the opposite effects on N. lugens reproduction. We need to find a balance between these opposite effects in insect pest control and insecticide resistance management, especially for insecticides targeting reproduction.

Inhibition of transcriptional regulation of detoxification genes contributes to insecticide resistance management in Spodoptera exigua

Synthetic insecticides have been widely used for the prevention and control of disease vectors and agricultural pests. However, frequent uses of insecticides have resulted in the development of insecticide resistance in these insect pests. The resistance adversely affects the efficacy of insecticides, and seriously reduces the lifespan of insecticides. Therefore, resistance management requires new strategies to suppress insecticide resistance. Here, we confirm that CncC/Maf are the key regulators of various detoxification genes involved in insecticide resistance in Spodoptera exigua. Then, we develop a cell screening platform to identify the natural compound inhibitors of CncC/Maf and determine that sofalcone can act as a CncC/Maf inhibitor in vitro and in vivo. Bioassay results showed that sofalcone significantly enhanced the toxicity (more than 3-fold) of chlorpyrifos and lambda-cyhalothrin against S. exigua larvae. Finally, we demonstrate that sofalcone can greatly improve the susceptibility of S. exigua larvae to insecticides by inhibiting the activity of the ROS/CncC-dependent detoxifying enzymes and downregulating the expression levels of detoxification genes. CncC/Maf inhibitors can be used as broad-spectrum synergists to overcome insecticide resistance in pest populations. Altogether, our results demonstrate that reduced expression of detoxification genes resulting from suppression of transcriptional regulation of these genes contributes to controlling insecticide resistance, which provides a very novel and high-efficiency green resistance management strategy.

Two detoxification enzyme genes, CYP6DA2 and CarFE4, mediate the susceptibility to afidopyropen in Semiaphis heraclei

Introduction

Semiaphis heraclei is an important economic pest affecting Caprifoliaceae and Apiaceae plants, and chemical control is still the main effective control method in the field. Afidopyropen is a new type of pyridine cyclopropyl insecticide, which can effectively control piercing-sucking mouthparts pests and is suitable for pest resistance management. However, the detoxification mechanism of S. heraclei to afidopyropen is still poorly cleared.

Methods

The insecticidal activity of afidopyropen against S. heraclei and the enzyme activity assay and synergism bioassay were evaluated. The detoxification enzyme genes were obtained by transcriptome and validated by quantitative real-time PCR (RT-qPCR). Furthermore, RNA interference was used to study the functions of detoxification enzyme genes.

Results

The activities of cytochrome P450 monooxygenases (P450s) and carboxylesterases (CarEs) were significantly increased under afidopyropen treatment. The toxicity of afidopyropen against S. heraclei was significantly increased after application the inhibitors of piperonyl butoxide and triphenyl phosphate. Sixteen P450 genes and three CarE genes were identified in the transcriptome of S. heraclei. The RT-qPCR results showed that eleven P450 genes and two CarE genes were significantly upregulated under afidopyropen treatment, and the expression of CYP6DA2 and CarFE4 was upregulated by more than 2.5 times. The expression pattern of CYP6DA2 and CarFE4 was further analyzed in different developmental stages of S. heraclei and knockdown of CYP6DA2 and CarFE4 significantly increased the susceptibility of S. heraclei to afidopyropen.

Conclusion

The results of this study uncover the key functions of CYP6DA2 and CarFE4 in the detoxification mechanism of S. heraclei to afidopyropen, and provide a theoretical basis for the scientific use of afidopyropen in the field.

Two critical detoxification enzyme genes, NlCYP301B1 and NlGSTm2 confer pymetrozine resistance in the brown planthopper (BPH), Nilaparvata lugens Stål

The brown planthopper (BPH), Nilaparvata lugens Stål, is a notorious pest that infests rice across Asia. The rapid evolution of chemical pesticide resistance in BPH poses an ongoing threat to agriculture and human health. Currently, pymetrozine has emerged as a viable alternative to imidacloprid for managing N. lugens. The detoxification of insecticides in insects includes three major metabolic gene families: cytochrome P450 monooxygenases (P450s), glutathione S-transferases (GSTs), and carboxylesterases (CarEs). In this study, highly resistant strains of BPH to pymetrozine (BPH-R40: 705-fold) were created from the susceptible BPH strain through continuous multi-selection. The activities of detoxifying enzymes, including P450s, GSTs, and CarEs were measured. Notably, P450s and GSTs exhibited significantly higher activity in the pymetrozine-resistance strain than that of the susceptible BPH strain. Hence, we characterized P450s and GSTs genes in N. lugens and revealed their phylogeny, structure, motif analysis, and chromosome location. Subsequently, the expression profiles of 53 P450s and 11 GSTs genes were quantified, and two crucial detoxifying enzyme genes, NlCYP301B1 and NlGSTm2, were identified as being involved in pymetrozine resistance. Furthermore, RNA interference (RNAi)-mediated silencing of NlCYP301B1 and NlGSTm2 gene expression significantly increased larval mortality of BPH in response to pymetrozine. To our knowledge, enhancing the activity of key detoxification enzymes to resist insecticides represents a widespread and vital defense mechanism in insects.

CYP4CE1 Metabolized Nitenpyram through Two Types of Oxidation Reaction, Hydroxylation, and N-Demethylation

Nitenpyram, taking the place of imidacloprid, is a widely used neonicotinoid insecticide to control Nilaparvata lugens in Asia. Two P450s, CYP4CE1 and CYP6ER1, are key factors in the metabolic resistance against nitenpyram and imidacloprid. In this study, we found that CYP4CE1 expression was strongly associated with nitenpyram resistance in 8 field-collected populations, whereas CYP6ER1 expression correlated with imidacloprid resistance. Hence, we focused on nitenpyram metabolism by CYP4CE1, due to that imidacloprid metabolism by CYP6ER1 has intensively investigated. Mass spectrometry analysis revealed that recombinant CYP4CE1 metabolized nitenpyram into three products, N-desmethyl nitenpyram, hydroxy-nitenpyram, and N-desmethyl hydroxy-nitenpyram, with a preference for hydroxylation. In contrast, CYP6ER1 metabolized nitenpyram into a single product, N-desmethyl nitenpyram. These results provide new insights into the specific catalytic mechanisms of P450 enzymes in neonicotinoid metabolism and underscore the importance of different catalytic reactions in neonicotinoid insecticide resistance.

Heterologous expression and characterization of two delta glutathione S-transferases genes involved in imidacloprid metabolism in Grapholita molesta

Glutathione S-transferases (GSTs) are multifunctional enzymes, and insect GSTs play a pivotal role in the metabolism of insecticides. Grapholita molesta is a worldwide pest that causes substantial economic losses to the fruit industry. However, it remains unclear how imidacloprid, a commonly used insecticide in orchards, is metabolized by G. molesta. In the present study, the synergist diethyl maleate (DEM), which inhibits the GST activity, exhibited a 22-fold synergistic ratio against imidacloprid. Two new GST genes, GmGSTD2 (OR096251) and GmGSTD3 (OR096252), were identified and successfully cloned, showing the highest expression in the Malpighian tubes. Knockdown of GmGSTD2 and GmGSTD3 by RNA interference, increased the mortality of G. molesta from 28% to 47% following imidacloprid treatment. Both recombinant GmGSTD2 and GmGSTD3 proteins exhibited 1-chloro-2,4-dinitrobenzene (CDNB) activity and could be inhibited by imidacloprid in vitro, with maximum inhibition was 60% for GmGSTD2 and 80% for GmGSTD3. These results suggested that GSTs participate in the metabolism of imidacloprid with GmGSTD2 and GmGSTD3 playing key roles in this process.

Chlorantraniliprole Resistance in Spodoptera frugiperda: Resistance Monitoring, Resistance Risk, and Resistance Mechanisms

Spodoptera frugiperda is a significant global pest, and chlorantraniliprole (CAP) is extensively used in China for its control. Understanding CAP resistance in S. frugiperda is crucial for effective management of this pest. Field populations exhibited varying degrees of resistance to CAP (RR = 1.74-5.60-fold). After 10 generations of selection, the CAP-resistant strain developed over 10-fold resistance, with a realized heritability (h2) of 0.10. Genetic analysis reveals inheritance patterns as autosomal, incomplete recessive, and monofactorial. The CAP-resistant strain showed limited cross-resistance to lufenuron and tetrachlorantraniliprole, negative cross-resistance to spinetoram, and no observed cross-resistance to other insecticides. Biochemical analysis suggested that P450-mediated detoxification is the primary resistance mechanism, with 26 genes overexpressed in the CAP-resistant strain. Additionally, the knockdown of CYP4L13, CYP6B39, CYP6B40, and CYP4G74 significantly increased the sensitivity of the resistant larvae to CAP. These findings highlight the resistance risk of CAP in S. frugiperda and emphasize the crucial role of P450 enzymes in resistance.

Functional Analysis of Insecticide Inhibition and Metabolism of Six Glutathione S-Transferases in the Rice Stem Borer, Chilo suppressalis

The glutathione S-transferases (GSTs) are important detoxifying enzymes in insects. Our previous studies found that the susceptibility of Chilo suppressalis to abamectin was significantly increased when the CsGST activity was inhibited by glutathione (GSH) depletory. In this study, the potential detoxification mechanisms of CsGSTs to abamectin were explored. Six CsGSTs of C. suppressalis were expressed in vitro. Enzymatic kinetic parameters including Km and Vmax of recombinant CsGSTs were determined, and results showed that all of the six CsGSTs were catalytically active and displaying glutathione transferase activity. Insecticide inhibitions revealed that a low concentration of abamectin could effectively inhibit the activities of CsGSTs including CsGSTd1, CsGSTe4, CsGSTo2, CsGSTs3, and CsGSTu1. However, the in vitro metabolism assay found that the six CsGSTs could not metabolize abamectin directly. Additionally, the glutathione transferase activity of CsGSTs in C. suppressalis was significantly increased post-treatment with abamectin. Comprehensive analysis of the results in present and our previous studies demonstrated that CsGSTs play an important role in detoxification of abamectin by catalyzing the conjugation of GSH to abamectin in C. suppressalis, and the high binding affinities of CsGSTd1, CsGSTe4, CsGSTo2, CsGSTs3, and CsGSTu1 with abamectin might also suggest the involvement of CsGSTs in detoxification of abamectin via the noncatalytic passive binding and sequestration instead of direct metabolism. These studies are helpful to better understand the detoxification mechanisms of GSTs in insects.

Nicotinamide deficiency promotes imidacloprid resistance via activation of ROS/CncC signaling pathway-mediated UGT detoxification in Nilaparvata lugens

Metabolic alternation is a typical characteristic of insecticide resistance in insects. However, mechanisms underlying metabolic alternation and how altered metabolism in turn affects insecticide resistance are largely unknown. Here, we report that nicotinamide levels are decreased in the imidacloprid-resistant strain of Nilaparvata lugens, may due to reduced abundance of the symbiotic bacteria Arsenophonus. Importantly, the low levels of nicotinamide promote imidacloprid resistance via metabolic detoxification alternation, including elevations in UDP-glycosyltransferase enzymatic activity and enhancements in UGT386B2-mediated metabolism capability. Mechanistically, nicotinamide suppresses transcriptional regulatory activities of cap 'n' collar isoform C (CncC) and its partner small muscle aponeurosis fibromatosis isoform K (MafK) by scavenging the reactive oxygen species (ROS) and blocking the DNA binding domain of MafK. In imidacloprid-resistant N. lugens, nicotinamide deficiency re-activates the ROS/CncC signaling pathway to provoke UGT386B2 overexpression, thereby promoting imidacloprid detoxification. Thus, nicotinamide metabolism represents a promising target to counteract imidacloprid resistance in N. lugens.

miR-317-3p and miR-283-5p Play a Crucial Role in Regulating the Resistance to Indoxacarb in Spodoptera frugiperda by Targeting GSTs4

Spodoptera frugiperda is primarily controlled through chemical insecticides. Our RNA-seq data highlight the overexpression of GSTs4 in indoxacarb-resistant S. frugiperda. However, the exact role of GSTs4 in indoxacarb resistance and its regulatory mechanisms remains elusive. Therefore, we investigated the functional role of GSTs4 in S. frugiperda and explored the underlying post-transcriptional regulatory mechanisms. GSTs4 was highly overexpressed (27.6-fold) in the indoxacarb-resistant strain, and GSTs4 silencing significantly increases the susceptibility of S. frugiperda to indoxacarb, increasing mortality by 27.3%. miR-317-3p and miR-283-5p can bind to the 3'UTR of GSTs4, and the targeting relationship was confirmed by dual-luciferase reporter assays. Injecting miR-317-3p and miR-283-5p agomirs reduces GSTs4 levels by 64.8 and 42.3%, respectively, resulting in an increased susceptibility of S. frugiperda to indoxacarb. Conversely, the administration of miR-317-3p and miR-283-5pantagomirs increases GSTs4 expression and reduces larval susceptibility to indoxacarb. These findings demonstrate that miR-317-3p and miR-283-5p contribute to indoxacarb resistance in S. frugiperda by regulating the overexpression of GSTs4.

Unclassified glutathione-S-transferase AiGSTu1 confers chlorantraniliprole tolerance in Agrotis ipsilon

BACKGROUND

Chlorantraniliprole (CAP) is a diamide insecticide with high efficacy against many pest insects, including the black cutworm, Agrotis ipsilon. Agrotis ipsilon is a serious pest causing significant yield losses in crops. Glutathione-S-transferases (GSTs) belong to a family of metabolic enzymes that can detoxify a wide range of pesticides. However, little is known about the functions of GSTs in CAP tolerance in A. ipsilon.

RESULTS

A cDNA sequence (designated AiGSTu1) encoding an unclassified GST was identified from A. ipsilon. AiGSTu1 is highly expressed during the 3rd -instar larval and the pupal stages. Most of the mRNA transcripts were found in larval Malpighian tubules. Exposure to CAP strongly enhanced AiGSTu1 expression, GST activity, hydrogen peroxide (H2 O2 ) and malondialdehyde levels in larvae. H2 O2 treatment upregulated the transcription level of AiGSTu1, suggesting that CAP-induced oxidative stress may activate AiGSTu1 expression. The activity of recombinant AiGSTu1 was inhibited by CAP in a dose-dependent manner. Metabolism assay results demonstrated that AiGSTu1 is capable of depleting CAP. Overexpression of AiGSTu1 enhanced the tolerance of Escherichia coli cells to H2 O2 and the oxidative stress inducer, cumene hydroperoxide. Silencing of AiGSTu1 by RNA interference increased the susceptibility of A. ipsilon larvae to CAP.

CONCLUSION

The findings of this study provide valuable insights into the potential role of AiGSTu1 in CAP detoxification and will improve our understanding of CAP tolerance in A. ipsilon. © 2023 Society of Chemical Industry.

Identification of differentially expressed miRNAs associated with diamide detoxification pathways in Spodoptera frugiperda

The fall armyworm (FAW) Spodoptera frugiperda is a severe economic pest of multiple crops globally. Control of this pest is often achieved using insecticides; however, over time, S. frugiperda has developed resistance to new mode of action compounds, including diamides. Previous studies have indicated diamide resistance is a complex developmental process involving multiple detoxification genes. Still, the mechanism underlying the possible involvement of microRNAs in post-transcriptional regulation of resistance has not yet been elucidated. In this study, a global screen of microRNAs (miRNAs) revealed 109 known and 63 novel miRNAs. Nine miRNAs (four known and five novel) were differentially expressed between insecticide-resistant and -susceptible strains. Gene Ontology analysis predicted putative target transcripts of the differentially expressed miRNAs encoding significant genes belonging to detoxification pathways. Additionally, miRNAs are involved in response to diamide exposure, indicating they are probably associated with the detoxification pathway. Thus, this study provides comprehensive evidence for the link between repressed miRNA expression and induced target transcripts that possibly mediate diamide resistance through post-transcriptional regulation. These findings highlight important clues for further research to unravel the roles and mechanisms of miRNAs in conferring diamide resistance.

Two Point Mutations in CYP4CE1 Promoter Contributed to the Differential Regulation of CYP4CE1 Expression by FoxO between Susceptible and Nitenpyram-Resistant Nilaparvata lugens

Four P450s were reported to be important for imidacloprid resistance in Nilaparvata lugens, a major insect pest on rice, which was confirmed in this study in an imidacloprid-resistant strain (ImiR). Here we found that only two (CYP4CE1 and CYP6ER1) from these four P450 genes were overexpressed in a nitenpyram-resistant strain (NitR) when compared to a susceptible strain (SUS). CYP4CE1 RNAi reduced nitenpyram and imidacloprid resistance in NitR and ImiR strains, with a greater reduction in nitenpyram resistance. The transcription factor FoxO mediated nitenpyram resistance in NitR and ImiR strains, but it was not differentially expressed among strains. The potential reason for the differential regulation of FoxO on CYP4CE1 expression was mainly from sequence differences in the CYP4CE1 promoter between susceptible and resistant insects. In six FoxO response elements predicted in the CYP4CE1 promoter, the single-nucleotide polymorphisms were frequently detected in over 50% of NitR and ImiR individuals. The luciferase reporter assays showed that two mutations, -650T/G and -2205T/A in two response elements at the positions of -648 and -2200 bp, mainly contributed to the enhanced regulation on CYP4CE1 expression by FoxO in resistant insects. The frequency was over 69% for both -650T/G and -2205T/A detected in NitR and ImiR individuals but less than 20% in SUS insects. In conclusion, CYP4CE1 overexpression importantly contributed to nitenpyram resistance in N. lugens, and two mutations in the CYP4CE1 promoter of resistant insects led to an enhanced regulation on CYP4CE1 expression by FoxO.

Role of the epsilon glutathione S-transferases in xanthotoxin tolerance in Spodoptera litura

Spodoptera litura, a polyphagous lepidopteran pest, demonstrates a remarkable capacity to adapt to varying host plants by efficiently detoxifying phytochemicals. However, the underlying mechanism for this adaptation is not well understood. Herein, twenty eplison glutathione S-transferase genes (GSTes) were characterized and their roles in phytochemical tolerance were analyzed in S. litura. Most of the GSTe genes were mainly expressed in the larval midgut and fat body. Exposure to the phytochemicals, especially xanthotoxin, induced the expression of most GSTe genes. Molecular docking analysis revealed that xanthotoxin could form stable bonds with six xanthotoxin-responsive GSTes, with binding free energies ranging from -36.44 to -68.83 kcal mol-1. Knockdown of these six GSTe genes increased the larval susceptibility to xanthotoxin. Furthermore, xanthotoxin exposure significantly upregulated the expression of two transcription factor genes CncC and MafK. Silencing of either CncC or MafK reduced the expression of GSTe16, which exhibited the largest change in response to xanthotoxin. Additionally, analysis of the promoter sequence of GSTe16 revealed the presence of seven CncC/Maf binding sites. Luciferase reporter assays showed that CncC and MafK enhanced the expression of GSTe16, leading to the increased xanthotoxin tolerance in S. litura. These findings provide insight into the functions and transcriptional regulatory mechanisms of GSTes, thereby enhancing our understanding of the role of GSTs in the adaptation of lepidopteran pests to phytochemicals.

Assessment of cytisine as an insecticide candidate for Hyphantria cunea management: Toxicological, biochemical, and control potential insights

In the present study, the toxicological effects of cytisine on the H. cunea larvae were investigated, and the potential of cytisine as a botanical insecticide through field simulation experiments was evaluated. The results showed that cytisine treatment (0.25-2.5%) exerted significant biotoxic effects on the H. cunea larvae, including diminished weight, disruption of both positive (HcCKS1, HcPLK, HcCCNA) and negative (HcGADD and HcCDKN) regulatory genes associated with larval growth, increased mortality, and heightened oxidative damage (H2O2 and MDA). Cytisine treatment significantly reduced glucose content and inhibited the expression of key rate-limiting enzyme genes (HcPFK, HcPK, HcHK1, HcCS, and HcIDH2) within glycolysis and the tricarboxylic acid cycle pathways. Under cytisine treatment, detoxification enzyme activities (CarE and GST) and expression of detoxification genes (HcCarE1, HcCarE2, HcCarE3, HcGST1, and HcGST3) were inhibited in H. cunea larvae. An increased contents of SOD, CAT, ASA and T-AOC, as well as expression of antioxidant enzyme genes HcSOD1 and HcCAT2, was found in cytisine-treated H. cunea larvae. Simultaneously, this is accompanied by a significant reduction in the expression of four antioxidant enzyme genes (e.g., HcPOD1 and HcPOD2). In the field experiment, a cytisine aqueous solution (25 g/L) with pre-sprayed and directly sprayed ways demonstrated potent insecticidal activity against H. cunea larvae, achieving a mortality rate of 53.75% and 100% at 24 h, respectively. Taken together, cytisine has significantly weight inhibition and lethal toxicity on the H. cunea larvae, and can be developed as a botanical insecticide for H. cunea control.

Chlorpyrifos and chlorfenapyr resistance in Spodoptera frugiperda (Lepidoptera: Noctuidae) relies on UDP-glucuronosyltransferases

Fall armyworm, Spodoptera frugiperda (J. E. Smith), has become an important agricultural pest worldwide. S. frugiperda is mainly controlled by the chemical insecticides, whereas the frequent application of insecticides would result in the resistance development. Insect uridine diphosphate-glucuronosyltransferases (UGTs), as phase II metabolism enzymes, play vital roles in the breakdown of endobiotic and xenobiotics. In this study, 42 UGT genes were identified by RNA-seq, including 29 UGT genes were elevated compared to the susceptible population, and the transcript levels of 3 UGTs (UGT40F20, UGT40R18, and UGT40D17) were increased by more than 2.0-fold in the field populations. Expression pattern analysis revealed that S. frugiperda UGT40F20, UGT40R18, and UGT40D17 were increased by 6.34-, 4.26-, and 8.28-fold, compared the susceptible populations, respectively. The expression of UGT40D17, UGT40F20, and UGT40R18 was affected after exposure to phenobarbital, chlorpyrifos, chlorfenapyr, sulfinpyrazone, and 5-nitrouracil. The induced expression of UGT genes may have improved UGT enzymatic activity, while the inhibition of UGTs genes expression may decreased UGT enzymatic activity. Sulfinpyrazone, and 5-nitrouracil, significantly increased the toxicity of chlorpyrifos and chlorfenapyr, as well as phenobarbital significantly reduced the toxicity of chlorpyrifos and chlorfenapyr against the susceptible populations and field populations of S. frugiperda. The suppression of UGTs (UGT40D17, UGT40F20, and UGT40R18) significantly increased the insensitivity of the field populations to chlorpyrifos and chlorfenapyr. These findings strongly supported our viewpoint that UGTs may play a critical role in insecticide detoxification. This study provides a scientific basis for the management of S. frugiperda.

Functional analysis of an overexpressed glutathione S-transferase BdGSTd5 involved in malathion and malaoxon detoxification in Bactrocera dorsalis

Glutathione S-transferases (GSTs) are one of the three detoxification enzyme families. The constitutive and inducible overexpression of GSTs genes plays an important role in insecticide resistance. Previous study showed that malathion resistance was polygenic, and elevated GSTs activity was one of the important factor participating in malathion resistance of Bactrocera dorsalis (Hendel), a serious economic pest worldwide. BdGSTd5 overexpression was inducible upon exposure to malathion. However, the involvement of BdGSTd5 in malathion resistance has not been clarified. In this study, we found that BdGSTd5 sequence harbored the conserved region of delta class GSTs, which were overexpressed in malathion resistant strain of B. dorsalis compared to malathion susceptible strain. The highest mRNA expression level of BdGSTd5 was found in 1-day-old adult, and the levels decreased with aging. The dsBdGSTd5 injection effectively silenced (73.4% reduction) the expression of BdGSTd5 and caused significant increase in susceptibility to malathion with a cumulative mortality increasing of 13.5% at 72 h post malathion treatment (p < 0.05). Cytotoxicity assay demonstrated that BdGSTd5 was capable of malathion detoxification. Molecular docking analysis further indicated the interactive potential of BdGSTd5 with malathion and its toxic oxide malaoxon. The recombinant BdGSTd5 exhibited glutathione-conjugating activity toward 1-chloro-2, 4-dinitrobenzene and malathion and malaoxon metabolic capacity with significant reduction (p < 0.05) of the peak areas by 90.0% and 73.1%, respectively. Taken together, the overexpressed BdGSTd5 contributes to malathion metabolism and resistance, which detoxify the malathion in B. dorsalis via directly depleting malathion and malaoxon.

Enantioselective activity and toxicity of chiral acaricide cyflumetofen toward target and non-target organisms

Cyflumetofen (CYF), a novel chiral acaricide, exert enantiomer-specific effects on target organisms by binding to glutathione S-transferase. However, there is limited knowledge regarding the response of non-target organisms to CYF, including enantioselective toxicity. In this study, we investigated the effects of racemic CYF (rac-CYF) and its two enantiomers (+)-CYF and (-)-CYF on MCF-7 cells and non-target (honeybees) and target (bee mites and red spider mites) organisms. The results showed that similar to estradiol, 1 μM (+)-CYF promoted the proliferation and disturbed the redox homeostasis of MCF-7 cells, whereas at high concentrations (≥100 μM) it exerted a negative effect on cell viability that was substantially stronger than that of (-)-CYF or rac-CYF. (-)-CYF and rac-CYF at 1 μM concentration did not significantly affect cell proliferation, but caused cell damage at high concentrations (≥100 μM). Analysis of acute CYF toxicity against non-target and target organisms revealed that for honeybees, all CYF samples had high lethal dose (LD₅₀) values, indicating low toxicity. In contrast, for bee mites and red spider mites, LD₅₀ values were low, whereas those of (+)-CYF were the lowest, suggesting higher toxicity of (+)-CYF than that of the other CYF samples. Proteomics profiling revealed potential CYF-targeted proteins in honeybees related to energy metabolism, stress responses, and protein synthesis. Upregulation of estrogen-induced FAM102A protein analog indicated that CYF might exert estrogenic effects by dysregulating estradiol production and altering estrogen-dependent protein expression in bees. Our findings suggest that CYF functions as an endocrine disruptor in non-target organisms in an enantiomer-specific manner, indicating the necessity for general ecological risk assessment for chiral pesticides.

Characterization of neutral lipases revealed the tissue-specific triacylglycerol hydrolytic activity in Nilaparvata lugens

The lipid metabolism plays an essential role in the development and reproduction of insects, and lipases are important enzymes in lipid metabolism. In Nilaparvata lugens, an important insect pest on rice, triacylglycerol hydrolytic activities were different among tissues, with high activity in integument, ovary, and fat body, but low activity in intestine. To figure out the tissue-specific triacylglycerol hydrolytic activity, we identified 43 lipases in N. lugens. Of these 43 lipases, 23 belonged to neutral lipases, so this group was selected to perform further experiments on triacylglycerol hydrolysis. The complete motifs of catalytic triads, β9 loop, and lid motif, are required for the triacylglycerol hydrolytic activity in neutral lipases, which were found in some neutral lipases with high gene expression levels in integument and ovary, but not in intestine. The recombinant proteins of 3 neutral lipases with or without 3 complete motifs were obtained, and the activity determination confirmed the importance of 3 motifs. Silencing XM_022331066.1, which is highly expressed in ovary and with 3 complete motifs, significantly decreased the egg production and hatchability of N. lugens, partially through decline of the lipid metabolism. In summary, at least one-third of important motifs were incomplete in all neutral lipases with high gene expression in intestine, which could partially explain why the lipase activity in intestine was much lower than that in other tissues. The low activity to hydrolyze triacylglycerol in N. lugens intestine might be associated with its food resource and nutrient components, and the ovary-specific neutral lipases were important for N. lugens reproduction.

Performance of Pyrethroid-Neonicotinoid Mixture Formulations Against Field-Collected Strains of the Tropical Bed Bug (Hemiptera: Cimicidae) on Different Substrates

The residual performance of two pyrethroid-neonicotinoid mixture formulations: Temprid SC (10.5% beta-cyfluthrin and 21% imidacloprid) and Tandem (3.5% lambda-cyhalothrin and 11.6% thiamethoxam) on two substrates (glass and filter paper) against eight pyrethroid-resistant strains (BM-MY, BP-MY, CH-MY, GL-MY, KL-MY, SAJ-MY, TT-MY, and QLD-AU) of the tropical bed bug, Cimex hemipterus (F.) (Hemiptera: Cimicidae) collected from Malaysia, and Australia were evaluated. The aging effect of treatment residues on glass was also investigated. A susceptible C. lectularius L. strain (Monheim) was used for comparison. Temprid SC showed varying levels of performance against all C. hemipterus strains: TT-MY (PR50 = 6.5-fold, high performance), BM-MY, GL-MY, SAJ-MY, and QLD-AU (12.8-21.6-fold, moderate performance), BP-MY, and KL-MY (48.2-49-fold, poor performance), CH-MY (128.2-fold, very poor performance). On the other hand, Tandem displayed high performance against all C. hemipterus strains (1.8-8.3-fold). Tandem caused faster mortality than Temprid SC for all strains. Temprid SC and Tandem residues killed C. hemipterus significantly faster on glass than filter paper. Compared with fresh residues, the efficacy of Temprid SC residues significantly declined after one week of aging, while the effectiveness of Tandem residues declined after two weeks of aging. Further investigations using the topical assay method with a diagnostic dose of imidacloprid found two strains (CH-MY and GL-MY) resistant to imidacloprid. The six other strains (BM-MY, BP-MY, KL-MY, SAJ-MY, TT-MY, and QLD-AU) were susceptible.

Contribution of UDP-glycosyltransferases to chlorpyrifos resistance in Nilaparvata lugens

As a multigene superfamily of Phase II detoxification enzymes, uridine diphosphate (UDP)-glycosyltransferases (UGTs) play important roles in the metabolism of xenobiotics including insecticides. In this study, 5-nitrouracil, an inhibitor of UGT enzyme activity, effectively increased the toxicity of chlorpyrifos to the chlorpyrifos-resistant strain of Nilaparvata lugens, one of the most resistant rice pests. The enzyme content of UGT in the resistant strain was significantly higher than that in the susceptible strain. Among 20 identified UGT genes, UGT386H2, UGT386J2, UGT386N2 and UGT386P1 were found significantly overexpressed in the resistant strain and can be effectively induced by chlorpyrifos. These four UGT genes were most highly expressed in the midgut and/or fat body, two main insect detoxification tissues. Amino acid sequence alignments revealed that these four UGTs contained a variable N-terminal substrate-binding domain and a conserved C-terminal sugar donor-binding domain. Furthermore, homology modeling and molecular docking analyses showed that these UGTs could stably bind to chlorpyrifos and chlorpyrifos oxon, with the binding free energies from -19.4 to -110.62 kcal mol^-1. Knockdown of UGT386H2 or UGT386P1 by RNA interference dramatically increased the susceptibility of the resistant strain to chlorpyrifos. These findings suggest that overexpression of these two UGT genes contributes to chlorpyrifos resistance in N. lugens.

Reciprocal interactions between anthropogenic stressors and insect microbiota

Insects play many important roles in nature due to their diversity, ecological role, and impact on agriculture or human health. They are directly influenced by environmental changes and in particular anthropic activities that constitute an important driver of change in the environmental characteristics. Insects face numerous anthropogenic stressors and have evolved various detoxication mechanisms to survive and/or resist to these compounds. Recent studies highligted the pressure exerted by xenobiotics on insect life-cycle and the important role of insect-associated bacterial microbiota in the insect responses to environmental changes. Stressor exposure can have various impacts on the composition and structure of insect microbiota that in turn may influence insect biology. Moreover, bacterial communities associated with insects can be directly or indirectly involved in detoxification processes with the selection of certain microorganisms capable of degrading xenobiotics. Further studies are needed to assess the role of insect-associated microbiota as key contributor to the xenobiotic metabolism and thus as a driver for insect adaptation to polluted habitats.

Sensitivity Differences and Biochemical Characteristics of Laodelphax striatellus (Fallén) to Seven Insecticides in Different Areas of Shandong, China

Laodelphax striatellus Fallén is one of the main pests that can severely harm rice, corn, and wheat. Insecticides acting on the nicotinic acetylcholine receptor (nAChR) are the main type of pesticides used for the control of L. striatellus in Shandong Province, a major grain-producing region in China. In this study, the rice seedling dipping method was used to determine the sensitivities of six field L. striatellus populations in Shandong to seven insecticides acting on nAChR. The results showed that all the field populations were sensitive to clothianidin, nitenpyram, and triflumezopyrim, and the Jiaxiang population exhibited the lowest resistance ratio (RR) to imidacloprid, dinotefuran, sulfoxaflor, and thiamethoxam. The Donggang population showed a medium-level resistance to imidacloprid, with the highest RR of 17.48-fold. The Yutai population showed low-level resistance to imidacloprid and thiamethoxam, with RRs of 7.23- and 7.02-fold, respectively. The contents of cytochrome P450 monooxygenase (P450s), carboxylesterase (CarE), and glutathione S-transferase (GST) were the highest in the Donggang population and the lowest in the Jiaxiang population. The P450 gene CYP314A1 and the CarE gene LsCarE12 were highly up-regulated in all populations. No mutations of V62I, R81T, and K265E in the nAChR β1 subunit were found in any of the populations. These results provide valuable information for the strategies of resistance management of L. striatellus in the field.

RNAi Mediated Gene Silencing of Detoxification Related Genes in the Ectropis oblique

Ectropis oblique is one of the main pests that feed on tea leaves. At present, the main control method is chemical control, but the long-term use of insecticides has been related to the development of insect resistance. One of the resistance mechanisms is the upregulation of relevant detoxification enzymes for defense. In this study, four genes with increased expression were screened from the gene sequences annotated from the transcriptome data of deltamethrin-treated larvae of E. oblique, which are acid phosphatase EoACP138, and cytochrome P450 EoCYP316, carboxylesterase EoCarE592 and acetylcholine esterase EoAchE989, respectively. The fourth instar larvae of E. oblique were stimulated by deltamethrin, chlorpyrifos and fenpropathrin respectively, and the expression levels of the genes were detected by qRT-PCR. The result showed that all four genes’ expression had significantly increased under the stimulation of three insecticides. RNAi technology was used to silence the expression of genes of EoACP138, EoCYP316, EoCarE592 and EoAchE989 in the fourth instar larvae of E. oblique. The change in the expression levels of the above genes in the larvae treated with dsRNA and stimulated with pesticides was determined by qRT-PCR. The target genes have been effectively silenced after feeding on dsRNA and higher sensitivity with higher mortality to pesticides was observed in the larvae interfered with dsRNA. The above genes are related to the detoxification and metabolism of resistance of E. oblique, which lays a foundation for further study on the mechanism of insecticide resistance in E. oblique.

Identification and characterization of glutathione S-transferases and their potential roles in detoxification of abamectin in the rice stem borer, Chilo suppressalis

The glutathione S-transferases (GSTs) are a kind of metabolic enzymes and participate in the detoxification metabolism of xenobiotics in various organisms. In insects, GSTs play important roles in the development of insecticide resistance and antioxidant protection. The rice stem borer Chilo suppressalis is one of the most damaging pests in rice and has developed high levels of resistance to abamectin in many areas of China, whereas the potential resistance mechanisms of C suppressalis to abamectin are still unclear. In the present study, a total of 23 CsGSTs genes were identified from the C. suppressalis transcriptome and genome, including 21 cytosolic and two microsomal CsGSTs. The cytosolic CsGSTs were further classified into seven categories based on phylogenetic analysis, and their sequence characteristics and genome structures were also analyzed. Synergism study revealed that the susceptibility of C. suppressalis to abamectin was increased significantly when the CsGSTs were inhibited by diethyl maleate (DEM). Sixteen CsGSTs genes were up-regulated in C. suppressalis larvae after treatment with abamectin, among which four CsGSTs genes including CsGSTe2, CsGSTe4, CsGSTo4 and CsGSTu1 were significantly induced in the midgut and fat body tissues. These results indicated that CsGSTs were associated with the detoxification of C. suppressalis to abamectin, and CsGSTe2, CsGSTe4, CsGSTo4 and CsGSTu1 might play important roles in the insecticide detoxification or antioxidant protection in C. suppressalis. Our present study provides valuable information on C. suppressalis GSTs, and are helpful in understanding the contributions of GSTs in abamectin detoxification in C. suppressalis and other insects.

A review of physiological resistance to insecticide stress in Nilaparvata lugens

Insecticides are widely used in agriculture as effective means to control pests. However, pests have not been completely mitigated with the increased use of insecticides. Instead, many side effects have arisen, especially the '3Rs' (resistance, resurgence, and residue). The brown planthopper, Nilaparvata lugens, is one of the most threatening rice pests. The main insecticides for controlling N. lugens belong to organochlorine, organophosphorus, carbamate, neonicotinoid and pyrethroid groups. However, metabolic enzymes, including cytochrome P450s, esterases, glutathione-S-transferases, and ATP-binding cassette transporters, effectively promote the detoxification of insecticides. Besides, mutations of neurological target sites, such as acetylcholinesterase, nicotinic acetylcholine, γ-aminobutyric acid receptor, and ryanodine receptor, result in insensitivity to insecticides. Here, we review the physiological metabolic resistance in N. lugens under insecticide stress to provide a theoretical basis for identifying and developing more effective and harmless insecticides.

Silencing of cytochrome P450 gene CYP321A1 effects tannin detoxification and metabolism in Spodoptera litura

Cytochrome P450 monooxygenase (P450 or CYP) plays an important role in the metabolism of insecticides and plant allelochemicals by insects. CYP321B1, a novel Spodoptera litura P450 gene, was identified and characterized. CYP321B1 contains a 1488 bp open reading frame (ORF) that encodes a 495 amino acid protein. In fourth instar larvae, the highest CYP321B1 expression levels were found in the midgut and fat body. In the tannin feeding test, tannin can significantly induce the expression of CYP321B1 in the midgut and fat body of 4th instar larvae. To verify the function of CYP321B1, RNA interference and metabolome analysis were performed. The results showed that silencing CYP321B1 significantly reduced the rate of weight gain under tannin induction. Metabolome analysis showed silencing affected 47 different metabolites, mainly involved in secondary metabolite biosynthesis and amino acid metabolism, including amino acids, lipid fatty acids, organic acids and their derivatives. Henoxyacetic acid and cysteamine are the most highly regulated metabolites, respectively. These findings demonstrate that CYP321B1 plays an important role in tannin detoxification and metabolism. Functional knowledge about metabolite detoxification genes in this major herbivorous insect pest can provide new insights into this biological process and provide new targets for agricultural pest control.

Sulfur deficiency exacerbates phytotoxicity and residues of imidacloprid through suppression of thiol-dependent detoxification in lettuce seedlings

Sulfur, an essential macronutrient, plays important roles in plant development and stress mitigation. Sulfur deficiency, a common problem in agricultural soils, may disturb plant stress resistance and xenobiotic detoxification. In the present study, the function and mechanism of limited sulfur nutrition on the residues and phtotoxicity of imidacloprid were investigated in lettuce plants. Sulfur deficiency significantly increased imidacloprid accumulation in lettuce tissues, exacerbated imidacloprid biological toxicity by enhancing the accumulation of toxic metabolites, like imidacloprid-olefin. Simultaneously, imidacloprid-induced detoxification enzymes including cytochromes P450, glutathione S-transferases (GSTs) and glycosyltransferases were inhibited under limited sulfur supply. On the other hand, sulfur deficiency further enhanced the generation of reactive oxygen species and exacerbated lipid peroxidation in lettuce tissues. Sulfur deficiency mainly reduced the abundance of thiol groups, which are essential redox modulators as well as xenobiotic conjugators, and significantly inhibited GSTs expression. These results clearly suggested that sulfur deficiency inhibited the synthesis of sulfur-containing compounds, leading to increased accumulation of pesticide residues and toxic metabolites as well as reduced detoxification capacity, consequently leading to oxidative damage to plants. Therefore, moderate sulfur supply in regions where neonicotinoid insecticides are intensively and indiscriminately used may be an efficient strategy to reduce pesticide residues and the potential risk to ecosystem.

The Key Glutathione S-Transferase Family Genes Involved in the Detoxification of Rice Gramine in Brown Planthopper Nilaparvata lugens

Phytochemical toxins are considered a defense measure for herbivore invasion. To adapt this defensive strategy, herbivores use glutathione S-transferases (GSTs) as an important detoxification enzyme to cope with toxic compounds, but the underlying molecular basis for GST genes in this process remains unclear. Here, we investigated the basis of how GST genes in brown planthopper (BPH, Nilaparvata lugens (Stål)) participated in the detoxification of gramine by RNA interference. For BPH, the LC25 and LC50 concentrations of gramine were 7.11 and 14.99 μg/mL at 72 h after feeding, respectively. The transcriptions of seven of eight GST genes in BPH were induced by a low concentration of gramine, and GST activity was activated. Although interferences of seven genes reduced BPH tolerance to gramine, only the expression of NlGST1-1, NlGSTD2, and NlGSTE1 was positively correlated with GST activities, and silencing of these three genes inhibited GST activities in BPH. Our findings reveal that two new key genes, NlGSTD2 and NlGSTE1, play an essential role in the detoxification of gramine such as NlGST1-1 does in BPH, which not only provides the molecular evidence for the coevolution theory, but also provides new insight into the development of an environmentally friendly strategy for herbivore population management.

Effects of camptothecin on histological structures and gene expression profiles of fat bodies in Spodoptera frugiperda

Spodoptera frugiperda is a serious threat to global food production. Our previous study demonstrated that Camptothecin (CPT), a bioactive secondary metabolite from Camptotheca acuminata (Decne: Nyssaceae), exhibits adverse impact on the larval midgut of S. frugiperda and inhibits insect growth. However, effects of CPT on fat bodies of S. frugiperda larvae have not been examined yet. In the present study, we found that histological structures of fat bodies of S. frugiperda larvae were damaged in insects treated with CPT. Comparative transcriptomic analyses among different fat body samples from controls and insects treated with 1.0 and 5.0 μg/g CPT were performed. A total of 4212 and 5044 differentially expressed genes (DEGs) were identified in the samples treated with 1.0 and 5.0 μg/g CPT, respectively. Our data indicated that the pathways of detoxification, immune response, fatty acids, chitin, and hormone biosynthesis in fat bodies were affected by CPT treatments based on DEGs. These results provided a comprehensive view of the damage and gene expression changes in fat bodies of S. frugiperda after CPT exposure, which shall be useful to reveal the mechanism of CPT toxicity against S. frugiperda in future.

Identification and Characterization of an Antennae-Specific Glutathione S-Transferase From the Indian Meal Moth

Insect glutathione-S-transferases (GSTs) play essential roles in metabolizing endogenous and exogenous compounds. GSTs that are uniquely expressed in antennae are assumed to function as scavengers of pheromones and host volatiles in the odorant detection system. Based on this assumption, antennae-specific GSTs have been identified and functionally characterized in increasing number of insect species. In the present study, 17 putative GSTs were identified from the antennal transcriptomic dataset of the Indian meal moth, Plodia interpunctella, a severe stored-grain pest worldwide. Among the GSTs, only PiGSTd1 is antennae-specific according to both Fragments Per Kilobase Million (FPKM) and quantitative real-time PCR (qRT-PCR) analysis. Sequence analysis revealed that PiGSTd1 has a similar identity as many delta GSTs from other moths. Enzyme kinetic assays using 1-chloro-2,4-dinitrobenzene (CDNB) as substrates showed that the recombinant PiGSTd1 gave a K_m of 0.2292 ± 0.01805 mM and a V_max of 14.02 ± 0.2545 μmol·mg⁻¹·min⁻¹ under the optimal catalytic conditions (35°C and pH = 7.5). Further analysis revealed that the recombinant PiGSTd1 could efficiently degrade the sex pheromone component Z9-12:Ac (75.63 ± 5.52%), as well as aldehyde volatiles, including hexanal (89.10 ± 2.21%), heptanal (63.19 ± 5.36%), (E)-2-octenal (73.58 ± 3.92%), (E)-2-nonenal (75.81 ± 1.90%), and (E)-2-decenal (61.13 ± 5.24%). Taken together, our findings suggest that PiGSTd1 may play essential roles in degrading and inactivating a variety of odorants, especially sex pheromones and host volatiles of P. interpunctella.