New and emerging mechanisms of insecticide resistance

Proactive Resistance Management Studies Highlight the Role of Cytochrome P450 Genes in the Resistance of Tuta absoluta Against Tetraniliprole

The diamide insecticide tetraniliprole is a valuable tool for managing major insect pests like the invasive tomato pinworm, Tuta absoluta (Meyrick). However, the mechanisms underlying tetraniliprole resistance, as well as its associated fitness costs, remain unclear. In this study, we assessed the fitness of tetraniliprole-resistant (TetraRS) and susceptible (SS) strains of T. absoluta and conducted Illumina RNA-seq to compare their transcriptomes. We also used nanocarrier-mediated RNA interference (RNAi) to knockdown P450 genes and evaluate their role in tetraniliprole resistance. After eight generations of selection, T. absoluta developed a 20.80-fold resistance to tetraniliprole, accompanied by fitness costs. RNA-seq analysis revealed 3332 differentially expressed genes (DEGs), with 1707 upregulated and 1625 downregulated in the TetraRS compared to the SS strain. Gene Ontology (GO) annotations showed significant enrichment in categories related to metabolic processes, cellular processes, catalytic activity, cellular anatomical entity, and binding. These genes were also identified in key KEGG pathways such as cytochrome P450, drug metabolism, carbon metabolism, oxidative phosphorylation, fatty acid metabolism, and protein processing. RT-qPCR analysis confirmed that P450 genes (CYP405D1, CYP6AB269, and CYP4AU1) were upregulated in TetraRS insects, in line with the RNA-seq results. Cytochrome P450 activity was significantly higher in the TetraRS strain than in the SS strain. Notably, nano-encapsulated dsRNA targeting these overexpressed P450 genes increased the susceptibility of T. absoluta to tetraniliprole. Further, cytochrome P450 activity was significantly reduced following silencing of P450 genes. These findings suggest that multiple genes and pathways, particularly P450 genes, contribute to tetraniliprole resistance in T. absoluta. This study provides valuable insights into the molecular mechanisms underlying insecticide resistance in this key pest species.

The Role of Chemosensory Proteins in Insecticide Resistance: A Review

Chemosensory proteins (CSPs) are small soluble proteins found predominantly in insects, with a conserved structure that contains a hydrophobic cavity. While originally associated with chemosensation, they were soon implicated to several other functions related to their ability to bind hydrophobic molecules. Research in the last decade has shown that CSPs may play a role in insecticide resistance. Several CSP genes are upregulated upon induction by sublethal insecticide doses or are highly expressed in resistant populations. RNA interference of CSP genes can restore susceptibility to insecticides. In vitro binding assays and molecular docking simulations suggest that CSPs can strongly bind to insecticides and can accommodate even large molecules in their hydrophobic cavities. Some cases of CSP overexpression in transgenic insects conferring insecticide resistance are reported. Taken together, these results indicate a role for CSPs in insecticide resistance, presumably through a sequestration mechanism, perhaps in combination with other mechanisms like metabolic resistance. This article reviews the evidence for CSP involvement in resistance and discusses ongoing research in the field.

Precise Lineage Tracking Using Molecular Barcodes Demonstrates Fitness Trade-offs for Ivermectin Resistance in Nematodes

A fundamental tenet of evolutionary genetics is that the direction and strength of selection on individual loci varies with the environment. Barcoded evolutionary lineage tracking is a powerful approach for high-throughput measurement of selection within experimental evolution that to date has largely been restricted to studies within microbial systems, largely because the random integration of barcodes within animals is limited by physical and molecular protection of the germline. Here, we use the recently developed TARDIS barcoding system in Caenorhabditis elegans (Stevenson et al., 2023) to implement the first randomly inserted genomic-barcode fitness experiment within an animal model and use this system to precisely measure the influence of the concentration of the anthelmintic compound ivermectin on the strength of selection on an ivermectin resistance cassette. The combination of the trio of knockouts in neuronally expressed GluCl channels, avr-14, avr-15, and glc-1, has been previously demonstrated to provide resistance to ivermectin at high concentrations. Varying the concentration of ivermectin in liquid culture allows the strength of selection on these genes to be precisely controlled within populations of millions of individuals, with the frequency of each barcode then being measured at multiple time points via sequencing at deep coverage and used to estimate the fitness of the individual lineages in the population. The mutations display a high cost to resistance at low concentrations, rapidly losing out to wildtype genotypes, but the balance tips in their favor when the ivermectin concentration exceeds 2nM. This trade-off in resistance is likely generated by a hindered rate of development in resistant individuals. Our results demonstrate that C. elegans can be used to generate high precision estimates of fitness using a high-throughput barcoding approach to yield novel insights into evolutionarily and economically important traits.

JNK-ERK Synergistic Regulation of P450 Gene Expression Confers Nitenpyram Resistance in Nilaparvata lugens (Stål)

Cascading regulation of signaling pathways plays a crucial role in insect growth, development, and adaptation. However, how insects employ signaling cascades to regulate detoxification gene expression and enhance resistance is not well understood. In the current study, we investigated the MAPK signaling pathway in mediating nitenpyram resistance in Nilaparvata lugens. qRT-PCR and western-blot analyses revealed that both transcription and protein levels of NlJNK and NlERK were upregulated in the nitenpyram resistant strain, and these changes can be induced by exposure to nitenpyram. Moreover, the expression of P450 genes including NlCYP6ER1, NlCYP302A1, and NlCYP6AY1, which were closely associated with nitenpyram resistance, was significantly decreased following the silencing of NlJNK and NlERK or inhibitor treatments. Further studies showed that NlERK-NlJNK comediated transcription factors NlCREB and NlAP-1 to regulate P450 gene expression. These findings highlight the critical role of the MAPK pathway in N. lugens resistance and offer the potential targets for the insecticide resistance management.

Transcription factor CREB/ATF regulates overexpression of CYP6CY14 conferring resistance to cycloxaprid in Aphis gossypii

Aphis gossypii Glover as a destructive agricultural pest has evolved resistance to various insecticides. Cycloxaprid is a novel structure neonicotinoid insecticide with excellent toxicity against A. gossypii. However, the resistance mechanism of A. gossypii to cycloxaprid was unclear. In the present study, a cycloxaprid-resistant (Cpd-R) strain (80.1-fold) of A. gossypii was obtained by continuous selection. Bioassay results showed that piperonyl butoxide significantly increased the toxicity of cycloxaprid by 10.5-fold to the Cpd-R strain. The activity of P450s was significantly higher in Cpd-R strain than in susceptible (Cpd-S) strain. The transcriptomic and qRT-PCR results showed that CYP6CY14, CYP380C44 and CYP303A1 were significantly upregulated in Cpd-R strain compared with Cpd-S strain. Furthermore, knockdown of CYP6CY14, CYP380C44 and CYP303A1 via RNA interference (RNAi) significantly increased the sensitivity of Cpd-R strain to cycloxaprid. Based on the higher expression of CYP6CY14 and RNAi results, transgenic Drosophila assay was conducted to further clarify the role of CYP6CY14 in cycloxaprid resistance, and results showed a significant increase in resistance to cycloxaprid in D. melanogaster. Additionally, the results of RNAi, dual-luciferase reporter and yeast one-hybrid (Y1H) indicated that CREB/ATF directly regulates CYP6CY14 expression. These findings provide necessary basis for clarifying the resistance mechanism of cycloxaprid in A. gossypii.

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.

Regulation of insect cuticular hydrocarbon biosynthesis

Cuticular hydrocarbons (CHCs) play pleiotropic roles in insect survival and reproduction. They prevent desiccation and function as pheromones influencing different behaviors. While the genes in the CHC biosynthesis pathway have been extensively studied, the regulatory mechanisms that lead to different CHC compositions received far less attention. In this review, we present an overview of how different hormones and transcriptional factors regulate CHC synthesis genes, leading to different CHC compositions. Future research focusing on the regulatory mechanisms underlying CHC biosynthesis can lead to a better understanding of how insects could produce dynamic chemical profiles in response to different stimuli.

Overexpression of multiple cytochrome P450 genes with and without knockdown resistance mutations confers high resistance to deltamethrin in Culex quinquefasciatus

BACKGROUND

The cytochrome P450s-mediated metabolic resistance and the target site insensitivity caused by the knockdown resistance (kdr) mutation in the voltage-gated sodium channel (vgsc) gene were the main mechanisms conferring resistance to deltamethrin in Culex quinquefasciatus from Thailand. This study aimed to investigate the expression levels of cytochrome P450 genes and detect mutations of the vgsc gene in deltamethrin-resistant Cx. quinquefasciatus populations in Thailand.

METHODS

Two field-collected strains of Cx. quinquefasciatus, Cq_SP and Cq_NiH, were selected with deltamethrin to generate the resistant strains Cq_SP-R and Cq_NiH-R, respectively. Bioassays were tested on larvae and adults of each strain according to WHO methods. Eight cytochrome P450 genes were analyzed for the expression level using quantitative real time-PCR. The cDNA of mosquitoes was amplified and sequenced for four fragments of vgsc gene. The kdr L1014F mutation and the haplotype of the CYP9M10 gene were detected in survivor and dead mosquitoes after exposure to the deltamethrin WHO test paper. Statistical analyses were performed using Fisher's exaction test.

RESULTS

Bioassay tests revealed a significantly higher resistance level in Cq_SP-R than in Cq_NiH-R strains in both larvae and adults. All eight cytochrome P450 genes were significantly overexpressed in larvae of Cq_NiH-R strain compared to the parent and susceptible Cq_Sus strains. The CYP6AA7 and CYP9J34 genes had the highest expression ratios, exceeding 24-fold in Cq_NiH-R larvae. In Cq_SP-R strain, the CYP4H34 and CYP9J34 genes were overexpressed in both stages. The kdr L1014F mutation was found in Cq_SP-R and its parent Cq_SP strains with a significantly higher mutant allele frequency in the survivor mosquitoes than in dead mosquitoes (P < 0.0001). The V240M and novel L925F mutations were found only in Cq_SP-R strain. Heterozygous genotype for the D-Cu( +)/Cu(-) of CYP9M10 gene was detected in Cq_NiH and Cq_NiH-R strains but other strains were mostly homozygous for the Cu(-)/Cu(-).

CONCLUSIONS

Overexpression of multiple cytochrome P450 genes alone has a relatively minor impact on resistance. The combined mechanisms of cytochrome P450- and kdr-mediated resistance result in significantly higher resistance to deltamethrin in Cx. quinquefasciatus. This study supports sustainable public health initiatives in Thailand to address the evolving challenges of insecticide resistance.

Function Analysis of Heme Peroxidase Genes, MpPxd2 and MpPxd4, Under Thiacloprid Exposure in the Neonicotinoid-Resistant Myzus persicae (Sulzer)

The green peach aphid, Myzus persicae, is a notorious pest worldwide. We collected a field population of the pest (FZQ-F) that exhibited high resistance to neonicotinoids. Exposure to neonicotinoids can induce oxidative damage in animals; however, it remains unclear whether antioxidant enzymes contribute to the innate immune response of neonicotinoid-resistant pests against high doses of insecticides. Treatment with sublethal doses of thiacloprid (LC10 and LC25) for 3, 6, 12, 24, 48, and 72 h resulted in significantly increased reactive oxygen species (ROS), including H2O2 content, in FZQ-F adults, indicating insecticide-induced oxidative stress. Additionally, the peroxidase activity in FZQ-F adults increased after thiacloprid exposure. Using comparative genomics, we identified 31 heme peroxidases in M. persicae with a typical "2Cys" structure, and phylogenetic analyses divided them into five groups. Comparative transcriptomes revealed that MpPxd2 and MpPxd4 were significantly upregulated in thiacloprid-treated aphids. Thiacloprid exposure significantly induced MpPxd2 and MpPxd4 expression levels, consistent with high H2O2 content and peroxidase activity. The knockdown of MpPxd2 or MpPxd4 in FZQ-F increased their susceptibility to imidacloprid, thiacloprid, and thiamethoxam, verifying the protective role of the heme peroxidases against neonicotinoids in aphids. The knockdown of MpPxd2 or MpPxd4 also led to shorter longevity and a low fecundity of adult aphids at 31 °C compared to controls. The results show that MpPxd2 or MpPxd4 is important in how cells respond to oxidative stress and may help resistant M. persicae pests to handle neonicotinoids.

Precise Lineage Tracking Using Molecular Barcodes Demonstrates Fitness Trade-offs for Ivermectin Resistance in Nematodes

A fundamental tenet of evolutionary genetics is that the direction and strength of selection on individual loci varies with the environment. Barcoded evolutionary lineage tracking is a powerful approach for high-throughput measurement of selection within experimental evolution that to date has largely been restricted to studies within microbial systems, largely because the random integration of barcodes within animals is limited by physical and molecular protection of the germline. Here, we use the recently developed TARDIS barcoding system in Caenorhabditis elegans (Stevenson et al., 2023) to implement the first randomly inserted genomic-barcode experimental evolution animal model and use this system to precisely measure the influence of the concentration of the anthelmintic compound ivermectin on the strength of selection on an ivermectin resistance cassette. The combination of the trio of knockouts in neuronally expressed GluCl channels, avr-14, avr-15, and glc-1, has been previously demonstrated to provide resistance to ivermectin at high concentrations. Varying the concentration of ivermectin in liquid culture allows the strength of selection on these genes to be precisely controlled within populations of millions of individuals, yielding the largest animal experimental evolution study to date. The frequency of each barcode was determined at multiple time points via sequencing at deep coverage and then used to estimate the fitness of the individual lineages in the population. The mutations display a high cost to resistance at low concentrations, rapidly losing out to wildtype genotypes, but the balance tips in their favor when the ivermectin concentration exceeds 2nM. This trade-off in resistance is likely generated by a hindered rate of development in resistant individuals. Our results demonstrate that C. elegans can be used to generate high precision estimates of fitness using a high-throughput barcoding approach to yield novel insights into evolutionarily and economically important traits.

A comparative approach for life history and functional response demonstrates similar survival strategies for Trichogramma evanescens and T. pintoi

BACKGROUND

Egg parasitoids are important biological control agents of lepidopteran pests of agricultural crops. Trichogramma evanescens Westwood and T. pintoi Voegele (Hymenoptera: Trichogrammatidae) are egg parasitoids with worldwide importance. The parasitoid selection necessitates comparative assessment of the life table traits and functional response analysis to provide insights into their effectiveness in pest control. In this study, we examined their life table traits including survivorship and reproductivity, and functional response and associated parameters i.e., attack coefficient and handling time.

RESULTS

Life table parameters, using age-stage, two-sex theory, revealed similar survival and reproductive strategies for both species. For example, the female longevity, oviposition days and fecundity did not differ between both species. Exceptionally, the male longevity of T. evanescens was shorter than that of T. pintoi. The population growth parameters such as gross reproductive rate (GRR), net reproductive rate (R0), intrinsic rate of increase (r), finite rate of increase (λ), and mean generation time (T) did not differ between species. The polynomial logistic regression yielded a type III functional response and a non-linear least square analysis revealed different attack coefficient and similar handling time. However, their parasitism rate differed between the lowest (five eggs) and highest (80 eggs) initial host egg densities such that T. evanescens had a lower parasitism rate at the lowest density and higher parasitism rate at the highest density.

CONCLUSION

The similarity in survival strategies and minor differences in host handling of both parasitoids are discussed in terms of relevance to applied biological control applications and evolutionary traits. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Crossbreeding and Backcrossing in the Pyrethroid-Resistant Ladybird Beetle Eriopis connexa (Germar) Determines Resistance in Offspring

The conservation of the predatory ladybird beetle, Eriopis connexa (Germar) after its release also relies on its resistance and the performance of its progenies. When resistant individuals are released or evolve in the field through mating with susceptible pairs, we anticipate that they will generate a resistant progeny, inheriting the resistance to lambda-cyhalothrin through an autosomal mechanism. The susceptibility of a field-collected population (EcGA) was characterized and determined by the performance of their progenies generated through mating with a resistant parent (EcViR). We paired virgin adults from the EcGA and EcViR populations, observing how their progenies developed, reproduced, and survived when exposed to dried lambda-cyhalothrin residues applied at field rates. Adults from the EcGA population were ~200-fold more susceptible than those of the EcViR population. Developmental times from larva to adult emergence for EcGA individuals were delayed by approximately 6 days and generated smaller adults compared to EcViR and their progenies. The egg production did not differ across parents and progenies, but females from EcGA and EcGA × EcViR progenies produced 130 more eggs in comparison to EcViR females during the 35-day evaluation period. Exposure to lambda-cyhalothrin resulted in 77.4 to 100% survival for adults from EcViR × EcGA progenies and EcViR parents, while EcGA and the population standard for susceptibility maintained in the laboratory (EcFM) did not survive the insecticide exposure. These findings indicate that field crosses between EcViR and EcGA will improve their progenies' biological performance compared to the EcViR parents and will maintain a high lambda-cyhalothrin survival rate.

Functions and regulations of insect gut bacteria

The insect gut is a complicated ecosystem that inhabits a large number of symbiotic bacteria. As an important organ of the host insect, the symbiotic bacteria of the insect gut play very important roles in regulating physiological and metabolic processes. Recently, much progress has been made in the study of symbiotic bacteria in insect guts with the development of high-throughput sequencing technology and molecular biology. This review summarizes the primary functions of symbiotic bacteria in insect guts, such as enhancing insecticide resistance, facilitating food digestion, promoting detoxification, and regulating mating behavior and egg hatching. It also addresses some possible pathways of gut bacteria symbiont regulation governed by external habitats, physiological conditions and immunity of the host insect. This review provides solid foundations for further studies on novel theories, new technologies and practical applications of symbiotic bacteria in insect guts. © 2024 Society of Chemical Industry.

Insights into the role of non-coding RNAs in the development of insecticide resistance in insects

Pest control heavily relies on chemical pesticides has been going on for decades. However, the indiscriminate use of chemical pesticides often results in the development of resistance in pests. Almost all pests have developed some degree of resistance to pesticides. Research showed that the mechanisms of insecticide resistance in insects encompass metabolic resistance, behavioral resistance, penetration resistance and target-site resistance. Research on the these mechanisms has been mainly focused on the cis-regulatory or trans-regulatory for the insecticide resistance-related genes, with less attention paid to non-coding RNAs (ncRNAs), such as microRNA (miRNA), long non-coding RNA (lncRNA), and circular RNA (circRNA). There has been increased studies focus on understanding how these ncRNAs are involved in post-transcriptional regulation of insecticide resistance-related genes. Besides, the formatted endogenous RNA (ceRNA) regulatory networks (lncRNA/circRNA-miRNA-mRNA) has been identified as a key player in governing insect resistance formation. This review delves into the functions and underlying mechanisms of miRNA, lncRNA, and circRNA in regulating insect resistance. ncRNAs orchestrate insect resistance by modulating the expression of detoxification enzyme genes, insecticide target genes, as well as receptor genes, effectively regulating both target-site, metabolic and penetration resistance in insects. It also explores the regulatory mechanisms of ceRNA networks in the development of resistance. By enhancing our understanding of the mechanisms of ncRNAs in insecticide resistance, it will not only provide valuable insights into the new mechanisms of insecticide resistance but also help to enrich new directions in ncRNAs gene regulation research.