Bed bugs evolved unique adaptive strategy to resist pyrethroid insecticides

Comparative Insecticidal Activity and Mechanism of Isocycloseram versus Other GABAergic Insecticides against the Fall Armyworm

The fall armyworm (FAW) is a serious agricultural pest and has developed resistance to multiple insecticides. It is necessary to introduce novel insecticide(s) for controlling FAW. Isocycloseram is a completely novel isoxazoline insecticide. However, its activity and mode of action against FAW have not been reported. In this study, isocycloseram exhibited a higher insecticidal activity (LC50 = 0.26 mg/kg) than fipronil (LC50 = 7.72 mg/kg) against FAW. The median inhibitory concentration (IC50) of isocycloseram (IC50 = 8.52 nM) was almost equal to that of the desmethyl-broflanilide (IC50 = 7.32 nM) to the SfrRDL1 receptor. The IC50 of isocycloseram to the SfrRDL2 receptor was 11.13 nM, which was obviously less than that of desmethyl-broflanilide, dieldrin, fipronil, fluxametamide. Compared with the SfrRDL2 receptor, the SfrRDL1 receptor exhibited higher sensitivity to GABAergic insecticides. The recombinant SfrGluCl receptor was successfully stimulated by l-glutamate; however, the currents were low and weakly inhibited by isocycloseram at 10 μM. In conclusion, our results provided the theoretical basis for usage of GABAergic insecticides for controlling FAW.

Bed bug control with various dusts: Efficacy comparison between silicon dioxide, diatomaceous earth, and Sommières earth

Bed bugs are considered a major public health problem in industrialized countries. Usually, bed bug infestations are managed using a combination of physical and chemical methods. In recent years, new strategies for bed bug control have emerged, particularly the use of dusts like diatomaceous earth and silicon dioxide. However, in Europe, the use of silicon dioxide is restricted to professional, while diatomaceous earth can be harmful to the lungs. This study aimed to assess bed bug mortality rates associated with Sommières earth, green clay, talc, and sodium bicarbonate compared to silicon dioxide and diatomaceous earth from a pest management company, diatomaceous earth for litter conditioner, and diatomaceous earth from a supermarket. We tested permanent exposure, short exposure, horizontal transfer and repellent effect on two bed bug colonies. Sommières earth demonstrated efficacy ranging from 75% to 100% in permanent and short exposures, similar to the efficacy of diatomaceous earth from the pest management company. On the contrary, diatomaceous earth for litter conditioner and diatomaceous earth from a supermarket, green clay, talc, and sodium bicarbonate were found to be ineffective. This study demonstrates, for the first time, the efficacy of Sommières earth against bed bugs, but also highlights the variability in efficacy of diatomaceous earths on bed bugs depending on their quality.

Trends in poisoning associated with the use of insecticides for bed bug infestations: a 20-year retrospective study in France

Bed bugs are pervasive global pests that have reemerged in the last 20 years as a significant public health concern, especially in densely populated urban areas. Beyond financial losses, expenses, inconvenience, and psychological distress, bed bug infestations often necessitate chemical management, posing poisoning risks to those with an infestation. The French Poison Control Centers recorded 1056 cases of exposure to bed bug insecticide products between 1999 and 2021. This study followed cases over 2007-2021, with a notable surge in reports of adverse reactions from 2016 onwards. Data revealed an increased recurrent misuse of insecticides, including substances banned or not approved for this use. Our findings underscore the growing public reliance on chemical insecticides for home bed bug management. With this is the concern of increased poisoning risks, and potential long-term health consequences from non-professional efforts by the public to manage bed bugs in their homes. This escalating trend emphasizes the need for safer and more sustainable pest management strategies in urban environments.

Strong resistance to β-cyfluthrin in a strain of the beetle Alphitobius diaperinus: a de novo transcriptome analysis

The lesser mealworm, Alphitobius diaperinus, is an invasive tenebrionid beetle and a vector of pathogens. Due to the emergence of insecticide resistance and consequent outbreaks that generate significant phytosanitary and energy costs for poultry farmers, it has become a major insect pest worldwide. To better understand the molecular mechanisms behind this resistance, we studied a strain of A. diaperinus from a poultry house in Brittany that was found to be highly resistant to the β-cyfluthrin. The strain survived β-cyfluthrin exposures corresponding to more than 100 times the recommended dose. We used a comparative de novo RNA-Seq approach to explore genes expression in resistant versus sensitive strains. Our de novo transcriptomic analyses showed that responses to β-cyfluthrin likely involved a whole set of resistance mechanisms. Genes related to detoxification, metabolic resistance, cuticular hydrocarbon biosynthesis and proteolysis were found to be constitutively overexpressed in the resistant compared to the sensitive strain. Follow-up enzymatic assays confirmed that the resistant strain exhibited high basal activities for detoxification enzymes such as cytochrome P450 monooxygenase and glutathione-S-transferase. The in-depth analysis of differentially expressed genes suggests the involvement of complex regulation of signaling pathways. Detailed knowledge of these resistance mechanisms is essential for the establishment of effective pest control.

A cuticular protein, BgCPLCP1, contributes to insecticide resistance by thickening the cockroach endocuticle

Overuse of insecticides has led to severe environmental problems. Insect cuticle, which consists mainly of chitin, proteins and a thin outer lipid layer, serves multiple functions. Its prominent role is as a physical barrier that impedes the penetration of xenobiotics, including insecticides. Blattella germanica (L.) is a major worldwide indoor pest that causes allergic disease and asthma. Extensive use of pyrethroid insecticides, including β-cypermethrin, has selected for the rapid and independent evolution of resistance in cockroach populations on a global scale. We demonstrated that BgCPLCP1, the first CPLCP (cuticular proteins of low complexity with a highly repetitive proline-rich region) family cuticular protein in order Blattodea, contributes to insecticide penetration resistance. Silencing BgCPLCP1 resulted in 85.0 %-85.7 % and 81.0 %-82.0 % thinner cuticle (and especially thinner endocuticle) in the insecticide-susceptible (S) and β-cypermethrin-resistant (R) strains, respectively. The thinner and more permeable cuticles resulted in 14.4 % and 20.0 % lower survival of β-cypermethrin-treated S- and R-strain cockroaches, respectively. This study advances our understanding of cuticular penetration resistance in insects and opens opportunities for the development of new efficiently and environmentally friendly insecticides targeting the CPLCP family of cuticular proteins.

Roles of insect odorant binding proteins in communication and xenobiotic adaptation

Odorant binding proteins (OBPs) are small water-soluble proteins mainly associated with olfaction, facilitating the transport of odorant molecules to their relevant receptors in the sensillum lymph. While traditionally considered essential for olfaction, recent research has revealed that OBPs are engaged in a diverse range of physiological functions in modulating chemical communication and defense. Over the past 10 years, emerging evidence suggests that OBPs play vital roles in purifying the perireceptor space from unwanted xenobiotics including plant volatiles and pesticides, potentially facilitating xenobiotic adaptation, such as host location, adaptation, and pesticide resistance. This multifunctionality can be attributed, in part, to their structural variability and effectiveness in transporting, sequestering, and concealing numerous hydrophobic molecules. Here, we firstly overviewed the classification and structural properties of OBPs in diverse insect orders. Subsequently, we discussed the myriad of functional roles of insect OBPs in communication and their adaptation to xenobiotics. By synthesizing the current knowledge in this field, our review paper contributes to a comprehensive understanding of the significance of insect OBPs in chemical ecology, xenobiotic adaptation, paving the way for future research in this fascinating area of study.

RNAi turns 25:contributions and challenges in insect science

Since its discovery in 1998, RNA interference (RNAi), a Nobel prize-winning technology, made significant contributions to advances in biology because of its ability to mediate the knockdown of specific target genes. RNAi applications in medicine and agriculture have been explored with mixed success. The past 25 years of research on RNAi resulted in advances in our understanding of the mechanisms of its action, target specificity, and differential efficiency among animals and plants. RNAi played a major role in advances in insect biology. Did RNAi technology fully meet insect pest and disease vector management expectations? This review will discuss recent advances in the mechanisms of RNAi and its contributions to insect science. The remaining challenges, including delivery to the target site, differential efficiency, potential resistance development and possible solutions for the widespread use of this technology in insect management.

Identification and functional analysis of ABC transporter genes related to deltamethrin resistance in Culex pipiens pallens

BACKGROUND

Pathogens that reproduce or develop in mosquitoes can transmit several diseases, endanger human health, and overwhelm health systems. Synthetic pyrethroids are the most widely used insecticides against adult mosquitoes, but their widespread use has led to resistance. The adenosine triphosphate (ATP)-binding cassette (ABC) transporters are involved in the resistance monitoring of insects, but their role and underlying mechanisms in insecticide resistance have not been fully elucidated. In the present study, we identified ABC transporter genes in Culex pipiens and investigated their role in the development of insecticide resistance.

RESULTS

We identified 63 ABC transporter genes in Cx. pipiens and classified them as per the ABC transporter subfamilies. We also performed phylogenetic analysis. The knockdown rate of the mosquitoes orally fed with the ABC transporter inhibitor verapamil increased after deltamethrin treatment compared with that of the control group. Several genes from the ABCB, ABCC, and ABCG subfamilies were highly expressed in resistant mosquitoes. Immunofluorescence analysis revealed that ABCG6032427 was expressed in the head, chest, abdomen, wings, and legs, and the expression was the highest in the legs. Subsequently, knockdown of ABCG6032427 using RNA interference (RNAi) increased the sensitivity of the mosquitoes to deltamethrin, and scanning and transmission electron microscopy revealed that ABCG6032427 knockdown reduced cuticle thickness and the cuticle became loose and irregular.

CONCLUSIONS

ABCG6032427 may modulate cuticle thickness and structure, thus play an important role in the development of deltamethrin resistance in mosquitoes. Thus, it could be a potential target for deltamethrin resistance management in Cx. pipiens. © 2023 Society of Chemical Industry.

A major 6 Mb superlocus is involved in pyrethroid resistance in the common bed bug Cimex lectularius

In the last few years, the bed bug Cimex lectularius has been an increasing problem worldwide, mainly due to the development of insecticide resistance to pyrethroids. The characterization of resistance alleles is a prerequisite to improve surveillance and resistance management. To identify genomic variants associated with pyrethroid resistance in Cimex lectularius, we compared the genetic composition of two recent and resistant populations with that of two ancient-susceptible strains using a genome-wide pool-seq design. We identified a large 6 Mb "superlocus" showing particularly high genetic differentiation and association with the resistance phenotype. This superlocus contained several clustered resistance genes and was also characterized by a high density of structural variants (inversions, duplications). The possibility that this superlocus constitutes a resistance "supergene" that evolved after the clustering of alleles adapted to insecticide and after reduction in recombination is discussed.

Effect of the Insecticide Chlorpyrifos on Behavioral and Metabolic Aspects of the Spider Polybetes pythagoricus

The toxicity of pesticides to organisms depends on the total amount of chemical exposure. Toxicity can be minimized if the organism recognizes the pesticide and alters its behavior. Furthermore, the physical barrier of cuticular hydrocarbons can prevent the entrance of the pesticide into the organism. Finally, if the pesticide enters the body, the organism experiences physiological changes favoring detoxification and the maintenance of homeostasis. We analyzed the behavioral and metabolic response of the spider Polybetes pythagoricus at different times of exposure to the organophosphate pesticide chlorpyrifos. First we observed that the individuals are capable of recognizing and avoiding surfaces treated with pesticides based on a behavioral analysis. Subsequently, we characterized cuticular hydrocarbons as a possible barrier against pesticides. Then we observed that the pesticide provoked histological damage, mainly at the level of the midgut diverticula. Finally, we analyzed the activity of several of the spider's enzymes linked to oxidative stress after exposure to chlorpyrifos for different lengths of time (6, 24, and 48 h). We observed that catalase activity was high at the start, whereas the activity of superoxide dismutase and glutathione S-transferase changed significantly at 48 h. Lipid peroxidation became high at 6 h, but decreased at 48 h. In conclusion, although P. pythagoricus can avoid contact with chlorpyrifos, this pesticide causes activation of the antioxidant system when it enters the body. Our results make a significant contribution to the ecotoxicology of spiders. Environ Toxicol Chem 2023;00:1-16. © 2023 SETAC.

Dynamic Roles of Insect Carboxyl/Cholinesterases in Chemical Adaptation

Insects have evolved several intricate defense mechanisms to adapt to their chemical environment. Due to their versatile capabilities in hydrolytic biotransformation, insect carboxyl/cholinesterases (CCEs) play vital roles in the development of pesticide resistance, facilitating the adaptation of insects to their host plants, and manipulating insect behaviors through the olfaction system. CCEs confer insecticide resistance through the mechanisms of qualitative or quantitative changes of CCE-mediated enhanced metabolism or target-site insensitivity, and may contribute to the host plant adaptation. CCEs represent the first odorant-degrading enzymes (ODEs) discovered to degrade insect pheromones and plant odors and remain the most promising ODE candidates. Here, we summarize insect CCE classification, currently characterized insect CCE protein structure characteristics, and the dynamic roles of insect CCEs in chemical adaptation.

Insecticide Resistance of Cimex lectularius L. Populations and the Performance of Selected Neonicotinoid-Pyrethroid Mixture Sprays and an Inorganic Dust

Insecticide resistance is one of the factors contributing to the resurgence of the common bed bug, Cimex lectularius L. This study aimed to profile the resistance levels of field-collected C. lectularius populations to two neonicotinoids and one pyrethroid insecticide and the performance of selected insecticide sprays and an inorganic dust. The susceptibility of 13 field-collected C. lectularius populations from the United States to acetamiprid, imidacloprid, and deltamethrin was assessed by topical application using a discriminating dose (10 × LD₉₀ of the respective chemical against a laboratory strain). The RR₅₀ based on KT₅₀ values for acetamiprid and imidacloprid ranged from 1.0-4.7 except for the Linden 2019 population which had RR₅₀ of ≥ 76.9. Seven populations had RR₅₀ values of > 160 for deltamethrin. The performance of three insecticide mixture sprays and an inorganic dust were evaluated against three C. lectularius field populations. The performance ratio of Transport GHP (acetamiprid + bifenthrin), Temprid SC (imidacloprid + β-cyfluthrin), and Tandem (thiamethoxam + λ-cyhalothrin) based on LC₉₀ were 900-2017, 55-129, and 100-196, respectively. Five minute exposure to CimeXa (92.1% amorphous silica) caused > 95% mortality to all populations at 72 h post-treatment.

Dual assessment of transcriptional and metabolomic responses in the American dog tick following exposure to different pesticides and repellents

The American dog tick, Dermacentor variabilis, is a major pest to humans and animals, serving as a vector to Rickettsia rickettsii, a bacterium responsible for Rocky Mountain spotted fever, and Francisella tularensis, which is responsible for tularemia. Although several tactics for management have been deployed, very little is known about the molecular response following pesticidal treatments in ticks. In this study, we used a combined approach utilizing transcriptomics and metabolomics to understand the response of the American dog tick to five common pesticides (amitraz, chlorpyrifos, fipronil, permethrin, and propoxur), and analyzed previous experimental data utilizing DEET repellent. Exposure to different chemicals led to significant differential expression of a varying number of transcripts, where 42 were downregulated and only one was upregulated across all treatments. A metabolomic analysis identified significant changes in acetate and aspartate levels following exposure to chlorpyrifos and propoxur, which was attributed to reduced cholinesterase activity. Integrating the metabolomics study with RNA-seq analysis, we found the physiological manifestations of the combined metabolic and transcriptional differences, revealing several novel biomolecular pathways. In particular, we discovered the downregulation of amino sugar metabolism and methylhistidine metabolism after permethrin exposure, as well as an upregulation of glutamate metabolism in amitraz treated samples. Understanding these altered biochemical pathways following pesticide and repellent exposure can help us formulate more effective chemical treatments to reduce the burden of ticks.

Functional and Structural Diversity of Insect Glutathione S-transferases in Xenobiotic Adaptation

As a superfamily of multifunctional enzymes that is mainly associated with xenobiotic adaptation, glutathione S-transferases (GSTs) facilitate insects' survival under chemical stresses in their environment. GSTs confer xenobiotic adaptation through direct metabolism or sequestration of xenobiotics, and/or indirectly by providing protection against oxidative stress induced by xenobiotic exposure. In this article, a comprehensive overview of current understanding on the versatile functions of insect GSTs in detoxifying chemical compounds is presented. The diverse structures of different classes of insect GSTs, specifically the spatial localization and composition of their amino acid residues constituted in their active sites are also summarized. Recent availability of whole genome sequences of numerous insect species, accompanied by RNA interference, X-ray crystallography, enzyme kinetics and site-directed mutagenesis techniques have significantly enhanced our understanding of functional and structural diversity of insect GSTs.

Functional analysis of cyantraniliprole tolerance ability mediated by ATP-binding cassette transporters in Aphis gossypii glover

Cyantraniliprole, a second-generation anthranilic diamide insecticide, is widely used to control chewing and sucking pests. ATP-binding cassette transporters (ABCs) are a ubiquitous family of membrane proteins that play important roles in insect detoxification mechanisms. However, the potential effects of ABCs on cyantraniliprole-resistance remain unclear. In the present study, synergism bioassays revealed that verapamil, an ABC inhibitor, increased the toxicity of cyantraniliprole by 2.00- and 12.25-fold in the susceptible and cyantraniliprole-resistant strains of Aphis gossypii. Based on transcriptome data, the expression levels of ABCB4, ABCB5, ABCD1, ABCG4, ABCG7, ABCG13, ABCG16, ABCG17, ABCG26 and MRP12 were upregulated 1.56-, 1.32-, 1.51-, 2.03-, 1.65-, 1.50-, 4.18-, 6.07-, 4.68- and 4.69-fold, respectively, in the cyantraniliprole-resistant strain (CyR) compared to the susceptible strain (SS), as determined using RT-qPCR. Drosophila melanogaster ectopically overexpressing ABCB5, ABCG4, ABCG7, ABCG16, ABCG17, ABCG26 and MRP12 exhibited significantly increased tolerance to cyantraniliprole by 11.71-, 2.39-, 4.85-, 2.06-, 3.75-, 4.20- and 3.50-fold, respectively, with ABCB5 and ABCG family members being the most effective. Furthermore, the suppression of ABCB5, ABCG4, ABCG7, ABCG16, ABCG17, ABCG26 and MRP12 significantly increased the sensitivity of the CyR strain to cyantraniliprole. These results indicate that ABCs may play crucial roles in cyantraniliprole resistance and may provide information for shaping resistance management strategies.

Bacterial symbionts influence host susceptibility to fenitrothion and imidacloprid in the obligate hematophagous bed bug, Cimex hemipterus

The use of insecticides remains important in managing pest insects. Over the years, many insects manifested physiological and behavioral modifications resulting in reduced efficacy of insecticides targeted against them. Emerging evidence suggests that bacterial symbionts could modulate susceptibility of host insects against insecticides. Here, we explore the influence of host microbiota in affecting the susceptibility of insect host against different insecticides in the blood-sucking bed bug, Cimex hemipterus. Rifampicin antibiotic treatment resulted in increased susceptibility to fenitrothion and imidacloprid, but not against deltamethrin. Meanwhile, the host fitness parameters measured in the present study were not significantly affected by rifampicin treatment, suggesting the role of bacterial symbionts influencing susceptibility against the insecticides. 16S metagenomics sequencing revealed a drastic shift in the composition of several bacterial taxa following rifampicin treatment. The highly abundant Alphaproteobacteria (Wolbachia > 90%) and Gammaproteobacteria (Yersinia > 6%) in control bed bugs were significantly suppressed and replaced by Actinobacteria, Bacilli, and Betaproteobacteria in the rifampicin treated F1 bed bugs, suggesting possibilities of Wolbachia mediating insecticide susceptibility in C. hemipterus. However, no significant changes in the total esterase, GST, and P450 activities were observed following rifampicin treatment, indicating yet unknown bacterial mechanisms explaining the observed phenomena. Re-inoculation of microbial content from control individuals regained the tolerance of rifampicin treated bed bugs to imidacloprid and fenitrothion. This study provides a foundation for a symbiont-mediated mechanism in influencing insecticide susceptibility that was previously unknown to bed bugs.

Knockout of ABC Transporter ABCG4 Gene Confers Resistance to Cry1 Proteins in Ostrinia furnacalis

Ostrinia furnacalis is an important borer on maize. Long-term and large-scale planting of transgenic corn has led O. furnacalis evolving resistance and reducing the control effect. Recently, high levels of resistance to Bt Cry1 toxins have been reported to be genetically linked to the mutation or down-regulation of ABC transporter subfamily G gene ABCG4 in O. furnacalis. In order to further determine the relationship between ABCG4 gene and the resistance to Cry1 toxins in O. furnacalis, the novel CRISPR/Cas9 genome engineering system was utilized to successfully construct ABCG4-KO knockout homozygous strain. Bioassay results indicated that an ABCG4-KO strain had a higher resistance to Cry1 proteins compared with a susceptible strain (ACB-BtS). The result indicates that the ABCG4 gene may act as a receptor of the Bt Cry1 toxin in O. furnacalis. Furthermore, the development time was significantly changed in the early stage ABCG4-KO larvae, and the population parameters were also significantly changed. In summary, our CRISPR/Cas9-mediated genome editing study presents evidence that ABCG4 gene is a functional receptor for Bt Cry1 toxins, laying the foundation for further clarification of the Bt resistance mechanism.

Nuclear receptor HR96 up-regulates cytochrome P450 for insecticide detoxification in Tribolium castaneum

BACKGROUND

Red flour beetle, Tribolium castaneum (T. castaneum), is a major agricultural pest that causes significant damage to stored grains and products. Although hormone receptor 96 (HR96) is known to be the single ortholog corresponding to mammalian constitutive androstane receptor and pregnane X receptor, the structural features of Tribolium HR96 (TcHR96) and its role in insecticide-mediated transcription control of cytochrome P450 enzyme genes in T. castaneum have not been elucidated yet.

RESULTS

We cloned full-length complementary DNA encoding TcHR96 and revealed the role of TcHR96 in transcriptional control of cytochrome P450 enzyme genes. Interestingly, genome-wide transcriptome analysis of HR96-deficient beetles using RNA sequencing showed a positive correlation between TcHR96 and gene transcription of metabolizing enzymes involved in phase I detoxification processes. Moreover, TcHR96 overexpression significantly increased the promoter activity of genes encoding phase I P450 enzymes such as CYP4Q4, CYP4G7, CYP4BR3, and CYP345A1. Chromatin immunoprecipitation analysis showed that TcHR96 could directly bind to the promoter of gene encoding CYP345A1, an enzyme for metabolizing insecticides in T. castaneum. Furthermore, imidacloprid, a neonicotinoid insecticide, significantly increased gene expression of phase I P450 enzymes in old larvae of T. castaneum, which were reversed by TcHR96 knockdown. Finally, TcHR96 knockdown significantly decreased the resistance of old larvae to imidacloprid concomitant with reduction of imidacloprid-mediated phase I P450 enzyme gene expression.

CONCLUSION

TcHR96 plays a major role in transcriptional control of P450 enzyme for imidacloprid detoxification. Controlling TcHR96 might facilitate the regulation of insecticide tolerance in T. castaneum, thus providing a promising new strategy to manage pest beetle populations. © 2021 Society of Chemical Industry.

Multiple Mechanisms Conferring Broad-Spectrum Insecticide Resistance in the Tropical Bed Bug (Hemiptera: Cimicidae)

The modern resurgence of the common (Cimex lectularius L.) and tropical bed bugs (C. hemipterus [F.]) is thought to be primarily due to insecticide resistance. While there are many reports on insecticide resistance mechanisms in C. lectularius, such information in C. hemipterus is limited. We examined dichloro-diphenyl-trichloroethane (DDT), malathion, deltamethrin, permethrin, lambda-cyhalothrin resistance, and the underlying mechanisms in several C. hemipterus strains (Australia: Queensland [QLD-AU]; Malaysia: Kuala Lumpur [KL-MY], Tanjung Tokong [TT-MY], Christian [CH-MY], and Green Lane [GL-MY]). We used a surface contact method, synergism studies (utilizing piperonyl butoxide [PBO], S,S,S-tributyl phosphorotrithioate [DEF], and diethyl maleate [DEM]), and molecular detection of kdr mutations. Results demonstrated that all C. hemipterus strains possessed high resistance to DDT and the pyrethroids and moderate to high resistance to malathion. Synergism studies showed that deltamethrin resistance in all strains was significantly (P < 0.05) inhibited by PBO. In contrast, deltamethrin resistance was not affected in DEF or DEM. Similar findings were found with lambda-cyhalothrin resistance. Malathion resistance was significantly (P < 0.05) reduced by DEF in all strains. Resistance to DDT was not affected by DEM in all strains. Multiple kdr mutations (M918I, D953G, and L1014F) were detected by molecular analyses. TT-MY strain was found with individuals possessing three kdr mutation combinations; D953G + L1014F (homozygous susceptible: M918), M918I + D953G + L1014F (heterozygous resistant: I918), and M918I + D953G + L1014F (homozygous resistant: I918). Individuals with M918I + D953G + L1014F (homozygous resistant: I918) survived longer on deltamethrin (>12 h) than those (≤1 h) with other combinations. M918I + L1014F mutations most likely conferred super-kdr characteristic toward pyrethroids and DDT in C. hemipterus.

Comparative transcriptome analysis of thiamethoxam susceptible and resistant Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), using RNA-sequencing

Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), transmits the causal pathogen of huanglongbing and is a global pest of citrus. D. citri populations exhibit resistance to multiple insecticide modes of action in areas where these chemicals have been overused. We performed genome-wide transcriptional analysis for two field populations of D. citri (Wauchula and Lake Alfred, Florida, USA) that exhibit 1300-fold resistance to the neonicotinoid insecticide, thiamethoxam, and compared it to that of susceptible psyllids collected from the same area and without imposed selection. The Lake Alfred population responded to insecticide resistance by up-regulation of 240 genes and down-regulation of 148 others. The Wauchula population exhibited similar patterns to the Lake Alfred population with up-regulation of 253 genes and down-regulation of 115 others. Gene Ontology annotation associated with cellular processes, cell, and catalytic activity were assigned to differentially expressed genes (DEGs). The DEGs from Lake Alfred and Wauchula populations were mapped to Kyoto Encyclopedia of Gene and Genomes pathways and implicated enrichment of metabolic pathways, oxidative phosphorylation, extracellular matrix-receptor interaction, terpenoid backbone biosynthesis, and insect hormone biosynthesis in the resistant populations. Up-regulation of 60s ribosomal proteins, UDP-gluscoyltransferases, cytochrome c oxidases, and CYP and ABC transporters among thiamethoxam-resistant D. citri implicates a broad array of novel and conventionally understood resistance mechanisms.

Cuticle thickening associated with fenitrothion and imidacloprid resistance and influence of voltage-gated sodium channel mutations on pyrethroid resistance in the tropical bed bug, Cimex hemipterus

BACKGROUND

The common bed bug, Cimex lectularius L., and the tropical bed bug, Cimex hemipterus (F.), are now widely regarded as important public health pests following their rapid global resurgence, largely due to insecticide resistance and an increased rate of global travel. The insecticide resistance mechanisms are well documented in C. lectularius, however, only one mechanism is validated in C. hemipterus thus far. This demands further understanding on the resistance mechanisms involved in C. hemipterus.

RESULTS

Here, we identified differences in resistance to fenitrothion (organophosphate) and imidacloprid (neonicotinoid) related cuticle thickness in C. hemipterus. There is evidence of a possible association between cuticle thickness and resistance, but the association can be tenuous, likely because resistance is multifactorial in C. hemipterus. We also discovered a novel T1011 residue in domain IIS6 of the voltage-gated sodium channel that likely enhanced susceptibility to deltamethrin (pyrethroid) despite the presence of a L1014F mutation known to confer pyrethroid resistance in C. hemipterus. Our findings also confirmed that the M918I mutation enhanced resistance to pyrethroid when present with the L1014F mutation, which was consistent with a super-kdr phenotype, as reported previously. Multiple resistance mechanisms can be found within a single C. hemipterus population, and the presence of both M918I + L1014F mutations likely masked the influence of cuticle thickness in conferring resistance against deltamethrin. The elevated metabolic enzyme activities in some strains were not necessarily associated with increased insecticide resistance.

CONCLUSION

This study has enhanced our understanding on the penetration resistance mechanism and target site insensitivity of sodium channels in C. hemipterus.

High levels of pyrethroid resistance and super-kdr mutations in the populations of tropical bed bug, Cimex hemipterus, in Iran

Background

The tropical bed bug, Cimex hemipterus, is an important ectoparasite causing various health problems. This species is mainly confined to tropical regions; however, insecticide resistance, global warming, and globalization have changed its distribution map. Molecular information on pyrethroid resistance, which is essential for the development of control programs, is unknown for C. hemipterus in expanded areas. The present study was designed to determine the permethrin resistance status, characterize the pyrethroid receptor sites in voltage-gated sodium channel (vgsc) gene, and identify the resistance-related mutations in the populations of tropical bed bug in Iran.

Methods

Live bed bugs were collected, and adults of C. hemipterus were selected for bioassay and molecular surveys. Bioassay was performed by tarsal contact with permethrin 0.75% in mixed-sex of samples. Conventional and quantitative TaqMan and SYBR Green real-time PCR assays were conducted to characterize the vgsc gene and genotypes of studied populations.

Results

In the bioassay tests, the mortality rates were in the range of 30.7–38.7% and 56.2–77.4% in 24 and 48 h, respectively. The knockdown rates of studied populations were in the range of 32.2–46.6% and 61.5–83.8% in the first and second days, respectively. The KT₅₀ and KT₉₀ values in the Cimex lectularius Kh1 strain were presented as 5.39 and 15.55 h, respectively. These values in the selected populations of C. hemipterus varied from 27.9 to 29.5 and from 82.8 to 104.4 h, respectively. Knockdown time ratios (KR₅₀ and KR₉₀) in these populations varied from 5.17 to 6.17-fold compared with those of the C. lectularius Kh1 strain. Fragments of vgsc gene with 355 bp and 812 bp were amplified. Analysis of sequences revealed the A468T substitution, kdr-associated D953G, and super-kdr M918I and L1014F mutations in all populations.

Conclusions

The specific/sensitive, safe, and rapid diagnostic assays developed in this study are recommended for detection of kdr/super-kdr mutations and frequency of mutant alleles. The presence of super-kdr mutations and high resistance to permethrin in all the populations necessitate the reconsideration of control approaches against C. hemipterus.

Graphical Abstract

Early evidence of establishment of the tropical bedbug (Cimex hemipterus) in Central Europe

In recent decades, the world has witnessed a remarkable resurgence of bedbugs (Hemiptera: Cimicidae). Although populations of the common bedbug, Cimex lectularius L., expanded in temperate regions of its original distribution, the tropical bedbug, C. hemipterus (F.), increased its abundance in warmer regions, where it also had been historically distributed. However, C. hemipterus has recently been observed to be expanding to other areas, e.g. North Australia, Middle East, the United States and Russia. In other parts of Europe, few sporadic and ephemeral introductions of C. hemipterus were recorded until recently. We conducted an extensive sampling of European bedbug populations starting in 2002 and found that C. hemipterus has recently become locally established. Among 566 examined infestations, nearly all of which involved C. lectularius, C. hemipterus occurred in six infestations collected since 2019. In at least three cases, the social background of inhabitants of the infested properties indicated that tropical bedbugs likely spread within local communities. Using cytochrome oxidase subunit I, we linked five of the infestations to the most common haplotype found globally, and one to an African haplotype. In all infestations, we observed two kdr-associated mutations in the sodium channel gene, which are also commonly found across the world.

The Olfactory Chemosensation of Hematophagous Hemipteran Insects

As one of the most abundant insect orders on earth, most Hemipteran insects are phytophagous, with the few hematophagous exceptions falling into two families: Cimicidae, such as bed bugs, and Reduviidae, such as kissing bugs. Many of these blood-feeding hemipteran insects are known to be realistic or potential disease vectors, presenting both physical and psychological risks for public health. Considerable researches into the interactions between hemipteran insects such as kissing bugs and bed bugs and their human hosts have revealed important information that deepens our understanding of their chemical ecology and olfactory physiology. Sensory mechanisms in the peripheral olfactory system of both insects have now been characterized, with a particular emphasis on their olfactory sensory neurons and odorant receptors. This review summarizes the findings of recent studies of both kissing bugs (including Rhodnius prolixus and Triatoma infestans) and bed bugs (Cimex lectularius), focusing on their chemical ecology and peripheral olfactory systems. Potential chemosensation-based applications for the management of these Hemipteran insect vectors are also discussed.

Plant essential oil constituents enhance deltamethrin toxicity in a resistant population of bed bugs (Cimex lectularius L.) by inhibiting cytochrome P450 enzymes

Plant essential oils (EOs) are secondary metabolites derived from aromatic plants that are composed of complex mixtures of chemical constituents. EOs have been proposed as one of the alternative methods for bed bug (Cimex lectularius L.) control. In insecticide resistant mosquitoes and tobacco cutworm, EOs synergize pyrethroid toxicity by inhibiting detoxification enzymes. However, whether EOs and their constituents enhance pyrethroid toxicity in C. lectularius has remained unknown. Therefore, this study was designed to (i) determine the effects of binary mixtures of deltamethrin (a pyrethroid insecticide) with EOs or EO constituents or EcoRaider® (an EO-based product) on mortality of insecticide resistant and susceptible bed bugs, and (ii) evaluate the effects of EO constituent pre-treatment on detoxification enzyme activities of resistant and susceptible populations. Topical bioassays with binary mixtures of deltamethrin and individual EOs (e.g., thyme, oregano, clove, geranium or coriander oils) or their major constituents (e.g., thymol, carvacrol, eugenol, geraniol or linalool) or EcoRaider® at doses that kill approximately 25% of bed bugs caused significant increases in mortality of resistant bed bugs. However, in the susceptible population, only coriander oil, EcoRaider®, thymol, and carvacrol significantly increased the toxicity of deltamethrin. Detoxification enzyme assays with protein extracts from bed bugs pre-treated with EO constituents suggested selective inhibition of cytochrome P450 activity in the resistant population, but no impacts were observed on esterase and glutathione transferase activities in either population. Inhibition of P450 activity by EO constituents thus appears to be one of the mechanisms of deltamethrin toxicity enhancement in resistant bed bugs.

Standing genetic variation in laboratory populations of insecticide-susceptible Phlebotomus papatasi and Lutzomyia longipalpis (Diptera: Psychodidae: Phlebotominae) for the evolution of resistance

Insecticides can exert strong selection on insect pest species, including those that vector diseases, and have led to rapid evolution of resistance. Despite such rapid evolution, relatively little is known about standing genetic variation for resistance in insecticide-susceptible populations of many species. To help fill this knowledge gap, we generated genotyping-by-sequencing data from insecticide-susceptible Phlebotomus papatasi and Lutzomyia longipalpis sand flies that survived or died from a sub-diagnostic exposure to either permethrin or malathion using a modified version of the Centers for Disease Control and Prevention bottle bioassay. Multi-locus genome-wide association mapping methods were used to quantify standing genetic variation for insecticide resistance in these populations and to identify specific alleles associated with insecticide survival. For each insecticide treatment, we estimated the proportion of the variation in survival explained by the genetic data (i.e., "chip" heritability) and the number and contribution of individual loci with measurable effects. For all treatments, survival to an insecticide exposure was heritable with a polygenic architecture. Both P. papatasi and L. longipalpis had alleles for survival that resided within many genes throughout their genomes. The implications for resistance conferred by many alleles, as well as inferences made about the utility of laboratory insecticide resistance association studies compared to field observations, are discussed.

Field Evaluations of Sulfuryl Fluoride Fumigation for Control of the Common Bed Bug (Hemiptera: Cimicidae), Using a 1.9× Dosage Factor in Motor Vehicles and Filled Cargo Trailers

This study investigated the efficacy of using Vikane gas fumigant (sulfuryl fluoride) at the 1.9× dosage rate for eliminating bed bugs (Cimex lectularius L.) in two challenging infestation situations: personal vehicles, and confined spaces densely packed with personal belongings. The vehicles used in this study were large minivans with seating that folded into the floor. The confined spaces were cargo trailers filled to 85% capacity with books, furniture, and other household items. Each van and trailer was equipped with ~90 sentinel bed bugs consisting of three groups of 9-11 bed bug eggs, 10 nymphs, and 10 adults. The Vikane Fumiguide calculator was used to determine the target dosage (g-h/m3) to apply in each replicate (e.g., one van or trailer). Sulfuryl fluoride concentrations were measured throughout the fumigation process using a Spectros SF-ReportIR. Concentration readings were input into the Fumiguide to determine when the accumulated dosage (g-h/m3) was achieved, and when aeration should be initiated. After aeration was complete, the sentinel bed bugs were removed from the replicates and bed bug nymph and adult mortality was recorded. Bed bug eggs were monitored for 23 d to determine latent mortality. Fumigated bed bug mortality for each replication was 100% regardless of life stage. Latent mortality was observed in a single bed bug egg, but the first instar never fully eclosed. This study determined that fumigation with sulfuryl fluoride at the 1.9× dosage factor is an effective method for eliminating resistant bed bugs from vehicles and personal belongings in densely packed situations.

Comparative proteomics reveals mechanisms that underlie insecticide resistance in Culex pipiens pallens Coquillett

Mosquito control based on chemical insecticides is considered as an important element of the current global strategies for the control of mosquito-borne diseases. Unfortunately, the development of insecticide resistance of important vector mosquito species jeopardizes the effectiveness of insecticide-based mosquito control. In contrast to target site resistance, other mechanisms are far from being fully understood. Global protein profiles among cypermethrin-resistant, propoxur-resistant, dimethyl-dichloro-vinyl-phosphate-resistant and susceptible strain of Culex pipiens pallens were obtained and proteomic differences were evaluated by using isobaric tags for relative and absolute quantification labeling coupled with liquid chromatography/tandem mass spectrometric analysis. A susceptible strain of Culex pipiens pallens showed elevated resistance levels after 25 generations of insecticide selection, through iTRAQ data analysis detected 2,502 proteins, of which 1,513 were differentially expressed in insecticide-selected strains compared to the susceptible strain. Finally, midgut differential protein expression profiles were analyzed, and 62 proteins were selected for verification of differential expression using iTRAQ and parallel reaction monitoring strategy, respectively. iTRAQ profiles of adaptation selection to three insecticide strains combined with midgut profiles revealed that multiple insecticide resistance mechanisms operate simultaneously in resistant insects of Culex pipiens pallens. Significant molecular resources were developed for Culex pipiens pallens, potential candidates were involved in metabolic resistance and reducing penetration or sequestering insecticide. Future research that is targeted towards RNA interference of the identified metabolic targets, such as cuticular proteins, cytochrome P450s, glutathione S-transferases and ribosomal proteins proteins and biological pathways (drug metabolism—cytochrome P450, metabolism of xenobiotics by cytochrome P450, oxidative phosphorylation, ribosome) could lay the foundation for a better understanding of the genetic basis of insecticide resistance in Culex pipiens pallens.

Global protein profiles were compared among a susceptible strain of Cx. pipiens pallens and strains that were cypermethrin-resistant, propoxur-resistant, and dimethyl-dichloro-vinyl-phosphate-resistant after 25 generations of selection by distinct chemical insecticide families, multiple mechanisms were found to operate simultaneously in resistant mosquitoes of Cx. pipiens pallens, including mechanisms to lower penetration of or sequester the insecticide or to increase biodegradation of the insecticide via subtle alterations in either the cuticular protein levels or the activities of detoxification enzymes (P450s and glutathione S-transferases).

Magnitude and Spread of Bed Bugs (Cimex lectularius) throughout Ohio (USA) Revealed by Surveys of Pest Management Industry

Simple Summary

Bed bugs are small blood-sucking insects that live indoors and feed on humans. They have become a problem in countries worldwide. In this study, the problem in Ohio (Midwest U.S.) was measured based on treatments by licensed pest control companies throughout the state. Results from 2005 showed that Ohio’s bed bug problem likely started in Hamilton County, which includes Cincinnati. Much larger numbers of bed bug treatments were performed in 2011 and again in 2016, especially in counties with large cities. Almost every Ohio county had numerous bed bug treatments in 2016. Most treatments were in apartments/condos and single-family homes. Residents misused many pesticides, especially over-the-counter “bug bombs” and household cleaners, trying to eliminate bed bugs. Many people also threw away unwrapped infested furniture, which may further spread these bugs. More public education is needed to stop such practices. This study shows that bed bug problems can grow and spread quickly. Federal, state, and local officials and the public should immediately deal with bed bugs rather than waiting until they become an even bigger problem.

Abstract

Bed bugs have recently re-emerged as human pests worldwide. In this study, two surveys queried licensed pest management companies in Ohio (Midwest USA) about their experiences managing bed bugs. A primary objective was to assess the magnitude and spread of bed bug infestations statewide based on companies’ treatment records from 2005 and 2011 (first survey) and 2016 (second survey). The survey response rates were 35.6% and 31.6%, respectively. Treatment data from 2005 indicated that Ohio’s bed bug problem likely started in the SW corner of the state in Hamilton County (includes city of Cincinnati), since it totaled five times more treatments (approximately 4500) than second-ranking, centrally located Franklin County (Columbus). In the first half of 2011, more than 15,000 treatments were performed in these two counties. In 2016, treatments reached nearly 38,000 in Franklin County and in NE Ohio in the three combined counties that include Cleveland-Akron-Canton. Bed bug problems expanded statewide during an 11 y period, with an estimated 100+ treatments in 7 counties in 2005, 45 counties in 2011, and nearly all 88 counties in 2016. Apartments/condos and single-family residences comprised the largest share of bed bug work. Residents misused many pesticides and household cleaners trying to eliminate bed bugs. Many also discarded unwrapped infested furniture, which may further spread these bugs. More public education is needed to stop such practices. This study shows that bed bug problems grow and spread quickly; federal, state, and local officials and the public should immediately deal with bed bugs.

Detection of target-site and metabolic resistance to pyrethroids in the bed bug Cimex lectularius in Berlin, Germany

Knockdown-resistance (kdr) against pyrethroids in bed bugs (Cimex lectularis) is associated with the presence of several point mutations in the voltage-sensitive sodium channel α-subunit gene and/or an increased metabolic detoxification by cytochrome P450 monooxygenases (CYPs). In the present study, pyrosequencing assays were developed to quantify the presence of the kdr substitutions (V419L or L925I substitution) in bed bugs in Berlin, Germany. In 14 of 17 bed bug field strains, pyrosequencing revealed the presence of the substitution L925I with allele frequencies between 30% and 100%. One field strain additionally carried the substitution V419L with allele frequencies of 40% in males and 96% in females. In seven of the 17 field strains, mRNA levels of four CYP genes were examined using RT-qPCR. Relative to a susceptible laboratory reference strain, five field strains showed significantly higher mRNA levels of cyp397a1 with 7.1 to 56-fold increases. One of these strains additionally showed a 4.9-fold higher mRNA level of cyp398a1 compared to the reference strain, while cyp4cm1 and cyp6dn1 showed no significant differences. Our findings indicate that multiple resistance mechanisms are present in German C. lectularius populations simultaneously.

Developmental changes in larvae of the oriental latrine fly, Chrysomya megacephala, exposed to deltamethrin

The Chrysomya megacephala (Fabricius, 1794) (Diptera: Calliphoridae) has a worldwide distribution and is considered of medical and veterinary importance. Its synanthropic behavior combined with the visitation of decaying organic matter allows it to mechanically transmit pathogens to human and animals. Pyrethroids are widely used pesticides, effective in low doses against a broad spectrum of insects. Our objective was to evaluate the action of K-Othrine® on the post-embryonic development of C. megacephala. First instar larvae were exposed to different concentrations of K-Othrine® (3.75 × 10^-5 g a.i/mL (low), 7.5 × 10^-5 g a.i/mL (intermediate low), 1.125 × 10^-4 g a.i/mL (intermediate), 1.5 × 10^-4 g a.i/mL (intermediate high), and 2.5 × 10^-4 g a.i/mL (high)) mixed into artificial diets and a control with no pesticide. We measured weight, time of development, sex ratio, larval and pupal mortality, and esterase patterns. The mean weight values at intermediate low and intermediate concentrations were significantly lower compared with the control; additionally, the development time was delayed compared with the control. The larval and pupal mortality differed among the treatments. The larvae had six esterase regions, which altered its relative activity at 48 h and 72 h after insecticide ingestion. Further studies must investigate the effects of different insecticides on the post-embryonic development of C. megacephala.

Molecular analysis of pyrethroid resistance in Cimex hemipterus (Hemiptera: Cimicidae) collected from different parts of Iran

The present study was aimed to assess the bedbugs susceptibility to pyrethroid insecticides using molecular analysis. With the aid of pest control companies, adult bedbugs were collected from various places such as hotels, residential houses, and industrial buildings in seven cities highly crowded with domestic and foreign tourists in Iran from May 2016 to August 2017. Bedbugs were colonized in the laboratory to evaluate their resistance to pyrethroid using insecticide resistance bioassay. Genomic DNA was extracted from susceptible and resistant bedbugs. At first, specie specific primers targeting cytochrome oxidase subunit I (COI) gene was performed to confirm Cimexhemipterus species. Then, kdr-like gene was examined for point mutation using PCR and nucleotide sequencing. Bioassay showed that 11 out of 35 examined bedbugs were resistant to pyrethroids (31.43%; 95.00% confidence interval: 29.48-33.08%). The DNA sequencing showed that all examined bedbugs collected from Tehran province had homozygous V419L kdr-like gene mutations. The level of pyrethroid resistance found in the collected bugs from Tehran province indicated that this phenomenon has already been prevailed in the site and prompts the need to reevaluate the large use of pyrethroids to control the bedbugs.

The double-mutation（M918I + L1014F）kdr allele is fixed in Cimex hemipterus populations in Guangxi, China

Four putative knockdown resistance (kdr) mutations have been documented in the voltage-gated sodium channel (VGSC) gene of Cimex hemipterus from several countries. However, no information regarding kdr mutations in any Chinese tropical bed bug population is available to date. In this study, a double-mutation（M918I + L1014F）kdr allele was identified in six C. hemipterus populations across Guangxi Zhuang Autonomous Region of China. The frequency of this allele was 100% in all the six examined populations. In addition, only two cytochrome c oxidase I (COI) gene haplotypes, with one synonymous nucleotide variation, were identified in a total of 48 individuals from six locations. The fixation and broad geographic distribution of this resistant allele questions the continued use of pyrethroids in the treatment of tropical bed bug infestations. The very low genetic diversity within and among these populations indicates that these bed bugs may have a single origin.

Evaluation of a Non-Chemical Compared to a Non-Chemical Plus Silica Gel Approach to Bed Bug Management

Bed bug resistance to commonly used pesticide sprays has led to exploring new pesticides and other strategies for bed bug management. Non-chemical methods are effective in bed bug management; however, they do not provide residual protection. Compared to insecticide sprays, dust formulations are considered to provide longer residual control. We evaluated two bed bug management programs in apartment buildings. A building-wide inspection was initially conducted to identify bed bug infested apartments. Selected apartments were divided into two treatment groups: non-chemical plus silica gel dust treatment (10 apartments) and non-chemical treatment (11 apartments). After initial treatment, apartments were re-visited monthly for up to 6 months. During each visit, the total bed bug count per apartment was obtained by examining interceptor traps placed in the apartments and conducting a visual inspection. Mean bed bug count was reduced by 99% and 89% in non-chemical plus silica gel dust and non-chemical treatment, respectively. Non-chemical plus silica gel dust treatment caused significantly higher bed bug count reduction than the non-chemical treatment at 6 months. Bed bugs were eradicated from 40% and 36% of apartments treated with non-chemical plus silica gel dust treatment and non-chemical treatment, respectively.

Multiple ATP-binding cassette transporters genes are involved in thiamethoxam resistance in Aphis gossypii glover

ATP-binding cassette (ABC) transporters represent the largest known group of efflux pumps, utilizing ATP to translocate a broad spectrum of substrates across lipid membranes, which play an important role in phase III of the detoxification process. The presence of ABC transporters and their potential association with insecticide resistance have not been investigated in Aphis gossypii, one of the most economically important agricultural pests worldwide. In this study, the ABC transporter inhibitor-verapamil significantly increased thiamethoxam toxicity against resistant cotton aphids, suggesting that ABCs are involved in thiamethoxam resistance. ABC transporter genes were identified using the A. gossypii genome database and transcriptome data. A total of 69 ABC transporters were identified and grouped into seven subfamilies (A-G), including 4 ABCAs, 5 ABCBs, 25 ABCCs, 2 ABCDs, 1 ABCE, 4 ABCFs and 30 ABCGs. Of these ABC transporters, 53 were predicted to be functional, 19 were full transporters, 30 were half-transporters and 4 had two NBDs. Subfamilies C and G accounted for 77% (32 and 45%, respectively) of the genes. The transcripts of 20 of 26 ABCs based on the transcriptome were upregulated, and ABCA1, ABCA2, ABCB1, ABCB4, ABCB8, ABCD1, ABCD2, ABCE1, ABCF1, ABCF3, ABCG7, ABCG15, ABCG17, ABCG24, ABCG27, ABCG30, MRP1, MRP7, MRP14 and MRP21 transcripts were significantly increased in the thiamethoxan resistant strain compared to the susceptible strain with qRT-PCR. The suppression of overexpressed ABCs (ABCA2, ABCD1, ABCD2, ABCE1 and ABCG15) significantly increased the thiamethoxam sensitivity of resistant aphids. These results suggest that ABC transporters might be involved in thiamethoxam resistance in A. gossypii and will facilitate further work to validate the functional roles of these ABCs in thiamethoxam resistance. These results are useful for understanding the multiple resistance mechanisms of thiamethoxam and the management of insecticide-resistant cotton aphids.

Transcriptional Response of ATP-Binding Cassette (ABC) Transporters to Insecticide in the Brown Planthopper, Nilaparvata lugens (Stål)

The ATP-binding cassette (ABC) transporter superfamily is one of the largest groups of proteins and plays a non-negligible role in phase III of the detoxification process, which is highly involved in the response of insects to environmental stress (plant secondary metabolites and insecticides). In the present study, in Nilaparvata lugens, we identified 32 ABC transporters, which are grouped into eight subfamilies (ABCA–H) based on phylogenetic analysis. The temporal and spatial expression profiles suggested that the nymphal stages (1st–5th) and adult males showed similarity, which was different from eggs and adult females, and NlABCA1, NlABCA2, NlABCB6, NlABCD2, NlABCG4, NlABCG12, NlABCG15, and NlABCH1 were highly expressed in the midgut and Malpighian tubules. In addition, ABCG12, which belongs to the ABC transporter G subfamily, was significantly upregulated after exposure to sulfoxaflor, nitenpyram, clothianidin, etofenprox, chlorpyrifos, and isoprocarb. Moreover, verapamil significantly increased the sensitivity of N. lugens to nitenpyram, clothianidin, etofenprox, chlorpyrifos, and isoprocarb. These results provide a basis for further research on ABC transporters involved in detoxification in N. lugens, and for a more comprehensive understanding of the response of N. lugens to environmental stress.

RNA interference of NADPH-cytochrome P450 reductase increases susceptibilities to multiple acaricides in Tetranychus urticae

The two-spotted spider mite, Tetranychus urticae, is a polyphagous pest feeding on over 1100 plant species, including numerous highly valued economic crops. The control of T. urticae largely depends on the use of acaricides, which leads to pervasive development of acaricide resistance. Cytochrome P450-mediated metabolic detoxification is one of the major mechanisms of acaricide resistance in T. urticae. NADPH-cytochrome P450 reductase (CPR) plays as a crucial co-factor protein that donates electron(s) to microsomal cytochrome P450s to complete their catalytic cycle. This study seeks to understand the involvement of CPR/P450 in acaricide resistance in T. urticae. The full-length cDNA sequence of T. urticae's CPR (TuCPR) was cloned and characterized. TuCPR was ubiquitously transcribed in different life stages of T. urticae and the highest transcription was observed in the nymph and adult stages. TuCPR was constitutively over-expressed in six acaricide resistant populations compared to a susceptible one. TuCPR transcriptional expression was also induced by multiple acaricides in a time-dependent manner. Down-regulation of TuCPR via RNA interference (RNAi) in T. urticae led to reduced enzymatic activities of TuCPR and cytochrome P450s, as well as a reduction of resistance to multiple acaricides, abamectin, bifenthrin, and fenpyroximate. The outcome of this study highlights CPR as a potential novel target for eco-friendly control of T. urticae and other related plant-feeding pests.

Insecticidal Activity of Doxycycline against the Common Bedbug

There is an ongoing need for safe and effective anti-bedbug compounds. Here, we tested the toxicity of three antimicrobial agents against bedbugs when administered orally. We reveal that doxycycline has direct insecticidal activity at 250 μg/ml (0.025%) that is particularly strong against immature bedbugs and appears to be independent of antimicrobial activity. Future studies to determine the mechanisms behind this property could be useful for the development of orally active insecticides or anti-bedbug therapeutics.

Synergistic Toxicity Interactions between Plant Essential Oil Components Against the Common Bed Bug (Cimex lectularius L.)

Management of the common bed bug (Cimex lectularius L.) necessitates the use of multiple control techniques. In addition to synthetic pesticides and mechanical interventions, plant-derived essential oils represent one of the control options. Mixtures of two or more essential oil components (monoterpenoids) exhibit synergistic toxicity effects against insects due to increased cuticular penetration. Monoterpenoids, such as carvacrol, eugenol and thymol, are neurologically active and inhibit the nerve firing activity of C. lectularius. However, the effects of mixtures of these monoterpenoids on their toxicity and neuroinhibitory potential against C. lectularius are not known. In this study, the toxicity levels of a tertiary mixture of carvacrol, eugenol and thymol (1:1:1 ratio) and a binary mixture of synthetic insecticides, bifenthrin and imidacloprid (1:1 ratio) were evaluated against C. lectularius through topical bioassays and electrophysiology experiments. Both a mixture of monoterpenoids and the mixture of synthetic insecticides exhibited synergistic effects in topical bioassays. In electrophysiology experiments, the monoterpenoid mixture led to greater neuroinhibitory effects, whereas a mixture of synthetic insecticides caused higher neuroexcitatory effects in comparison to single compounds. This study shows evidence for neurological mechanisms of synergistic interactions between monoterpenoids and provides information regarding the utilization of natural compound mixtures for C. lectularius management.

Mechanisms, Applications, and Challenges of Insect RNA Interference

The RNA interference (RNAi) triggered by short/small interfering RNA (siRNA) was discovered in nematodes and found to function in most living organisms. RNAi has been widely used as a research tool to study gene functions and has shown great potential for the development of novel pest management strategies. RNAi is highly efficient and systemic in coleopterans but highly variable or inefficient in many other insects. Differences in double-stranded RNA (dsRNA) degradation, cellular uptake, inter- and intracellular transports, processing of dsRNA to siRNA, and RNA-induced silencing complex formation influence RNAi efficiency. The basic dsRNA delivery methods include microinjection, feeding, and soaking. To improve dsRNA delivery, various new technologies, including cationic liposome-assisted, nanoparticle-enabled, symbiont-mediated, and plant-mediated deliveries, have been developed. Major challenges to widespread use of RNAi in insect pest management include variable RNAi efficiency among insects, lack of reliable dsRNA delivery methods, off-target and nontarget effects, and potential development of resistance in insect populations.

Involvement of integument-rich CYP4G19 in hydrocarbon biosynthesis and cuticular penetration resistance in Blattella germanica (L.)

BACKGROUND

Cuticle penetration plays an important role as a mechanism of insecticide resistance, but the underlying molecular mechanism remains poorly understood. In Blattella germanica, the cytochrome P450 gene, CYP4G19, is overexpressed in a pyrethroid-resistant strain. Here, we investigated whether CYP4G19 is involved in the biosynthesis of hydrocarbons and further contributes to cuticular penetration resistance in B. germanica.

RESULTS

Compared with the susceptible strain, pyrethroid-resistant cockroaches showed lower cuticular permeability with Eosin Y staining. Removal of epicuticular lipids, mainly nonpolar hydrocarbons, with a hexane wash intensified the cuticular permeability and decreased the resistance index of the resistant strain. CYP4G19 was predominately expressed in the abdominal integument and could be upregulated by desiccation stress or short exposure to beta-cypermethrin. Overexpression of CYP4G19 in the resistant strain was positively correlated with a higher level of cuticular hydrocarbons (CHCs). RNAi-mediated knockdown of CYP4G19 significantly decreased its expression and caused a reduction in CHCs. Meanwhile, CYP4G19 suppression resulted in a non-uniform array of the lipid layer, enhanced cuticle permeability, and compromised insecticide tolerance.

CONCLUSION

Our findings confirm that CYP4G19 is involved in hydrocarbon production and appears to contribute to hydrocarbon-based penetration resistance in B. germanica. This study highlights the lipid-based penetration resistance, advancing our understanding of the molecular mechanism underlying P450-mediated cuticular penetration resistance in insects. © 2019 Society of Chemical Industry.

Genome-wide identification and analysis of genes associated with RNA interference in Bemisia tabaci

BACKGROUND

As a method of RNA-mediated gene silencing, RNA interference (RNAi) is a useful reverse genetic tool with which to study gene function, and holds great promise for pest management. Bemisia tabaci is a cosmopolitan pest that causes extensive damage to crops. The mechanism underlying RNAi efficiency in B. tabaci is not well known. We identified and analyzed candidate genes in the RNAi pathway to understand the RNAi mechanism and provide a basis for the application of RNAi in pest management.

RESULTS

We identified 33 genes putatively involved in the RNAi pathway from the B. tabaci Q genome. Phylogenetic and structural analyses confirmed the characteristics of these genes. Furthermore, quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) and transcriptomic analysis profiled gene expression patterns during different developmental stages. Gene expression levels estimated by qRT-PCR and RNA-seq analyses were significantly correlated. Moreover, gene functions were verified by RNAi. When accompanied by knockdown of AGO2, Dicer2 and Sid1, the efficiency of CYP6DB3 RNAi decreased correspondingly.

CONCLUSION

In this study, we annotated and validated genes involved in B. tabaci RNAi. A better understanding of the building blocks of the RNAi process in B. tabaci facilitates integration of this novel biotechnology into the management of this emerging pest, either directly or indirectly. © 2019 Society of Chemical Industry.

Knockdown Resistance-Associated Mutations Dominate Populations of the Common Bed Bug (Hemiptera: Cimicidae) Across the South Central United States

Despite awareness of the mutations conferring insecticide resistance in the bed bug, Cimex lectularius L. (Hemiptera: Cimicidae), within the United States few studies address the distribution and frequency of these. Within the United States, studies have focused on collections made along the East Coast and Midwest, documenting the occurrence of two mutations (V419L and L925I) within the voltage-gated sodium channel α-subunit gene shown to be associated with knockdown resistance (kdr) to pyrethroids. Here, the distribution and frequency of the V419L and L925I site variants is reported from infestations sampled within Oklahoma and its immediately adjacent states. Additionally, the presence of a mutation previously undocumented in the United States (I935F) is noted. While novel in the United States, this mutation has previously been reported in Australian and Old World populations. No infestations were found to harbor wild-type individuals, and hence susceptible, at each of the three sites. Instead, ~21% were found to possess the resistant mutation at the L925I site (haplotype B), ~77% had mutations at both the V419L and L925I sites (haplotype C), and 2% possessed the mutation at the L936F site (haplotype Ab). The high frequency of haplotype C corresponds to previous studies in the United States, and contrasts dramatically with those of the Old World and Australia. The data presented here provide insight into the contemporary occurrence of kdr-associated insecticide resistance in the South Central United States, a region for which data have previously been absent. These data suggest that New World and Old World/Australian infestations are likely to have originated from different origins.

Physiological functions of a methuselah-like G protein coupled receptor in Lymantria dispar Linnaeus

Insect G protein coupled receptors (GPCRs) have been identified as a highly attractive target for new generation insecticides discovery due to their critical physiological functions. However, few insect GPCRs have been functionally characterized. Here, we cloned the full length of a methuselah-like GPCR gene (Ldmthl1) from the Asian gypsy moth, Lymantria dispar. We then characterized the secondary and tertiary structures of Ldmthl1. We also predicted the global structure of this insect GPCR protein which is composed of three major domains. RNA interference of Ldmthl1 resulted in a reduction of gypsy moths' resistance to deltamethrin and suppressed expression of downstream stress-associated genes, such as P450s, glutathione S transferases, and heat shock proteins. The function of Ldmthl1 was further investigated using transgenic lines of Drosophila melanogaster. Drosophila with overexpression of Ldmthl1 showed significantly longer lifespan than control flies. Taken together, our studies revealed that the physiological functions of Ldmthl1 in L. dispar are associated with longevity and resistance to insecticide stresses. Potentially, Ldmthl1 can be used as a target for new insecticide discovery in order to manage this notorious forest pest.

Ocular Albinism Type 1 Regulates Deltamethrin Tolerance in Lymantria dispar and Drosophila melanogaster

The ocular albinism type 1 (OA1), a pigment cell-specific integral membrane glycoprotein, is a member of the G-protein-coupled receptor (GPCR) superfamily that binds to heterotrimeric G proteins in mammalian cells. We aimed to characterize the physiological functions an insect OA1 from Lymantria dispar (LdOA1) employs in the regulation of insecticide tolerance. In the present study, we investigated the roles of LdOA1 in response to deltamethrin exposure in both L. dispar and Drosophila melanogaster. LdOA1 was expressed at the lowest level during the 4^th instar stage, while LdOA1 was significantly upregulated in the 5^th instar and male stages. Knockdown of LdOA1 by injecting dsRNA of LdOA1 into gypsy moth larvae caused a 4.80-fold higher mortality than in control larvae microinjected with dsRNA of GFP under deltamethrin stress. Nine out of 11 L. dispar CYP genes were significantly downregulated under deltamethrin stress in LdOA1 silenced larvae as compared to control larvae. Moreover, the LdOA1 gene was successfully overexpressed in D. melanogaster using transgenic technique. The deltamethrin contact assay showed that the LdOA1 overexpression in flies significantly enhanced the tolerance to deltamethrin compared to the control flies. Furthermore, the downstream Drosophila CYP genes were upregulated in the LdOA1 overexpression flies, suggesting LdOA1 may play a master switch role in P450-mediated metabolic detoxification. This study is the first report of an insect OA1 gene regulating insecticide tolerance and potentially playing a role in the regulation of downstream cytochrome P450 expression. These results contribute to the future development of novel insecticides targeting insect GPCRs.

Circadian rhythms in insecticide susceptibility, metabolic enzyme activity, and gene expression in Cimex lectularius (Hemiptera: Cimicidae)

Many insect species display daily variation of sensitivity to insecticides when they are exposed to the same concentration at different times during the day. To date, this has not been investigated in bed bugs. To address this, we explored circadian rhythms in insecticide susceptibility, xenobiotic metabolizing (XM) gene expressions, and metabolic detoxification in the common bed bug, Cimex lectularius. An insecticide susceptible Monheim strain of C. lectularius was most tolerant of deltamethrin during the late photophase at ZT9 (i.e. nine hours after light is present in the light-dark cycle (LD) cycle) and similarly repeated at CT9 (i.e. nine hours into the subjective day in constant darkness (DD)) suggesting endogenous circadian involvement in susceptibility to deltamethrin. No diel rhythm was observed against imidacloprid insecticide despite significant daily susceptibility in both LD and DD conditions. Rhythmic expressions of metabolic detoxification genes, GSTs1 and CYP397A1 displayed similar expression patterns with total GST and P450 enzyme activities in LD and DD conditions, respectively. The oscillation of mRNA levels of GSTs1 and CYP397A1 was found consistent with peak phases of deltamethrin susceptibility in C. lectularius. This study demonstrates that circadian patterns of metabolic detoxification gene expression occur within C. lectularius. As a consequence, insecticide efficacy can vary dramatically throughout a 24 hour period.

Toxicity and neurophysiological impacts of plant essential oil components on bed bugs (Cimicidae: Hemiptera)

Bed bugs (Cimex lectularius L.) are globally important human parasites. Integrated pest management (IPM) approaches, which include the use of essential oil-based insecticidal compounds, have been proposed for their control. This study aimed to define insecticidal activity and neurophysiological impacts of plant essential oil constituents. The topical and fumigant toxicity of 15 compounds was evaluated against adult male bed bugs. Neurological effects of the 6 most toxicologically active compounds were also determined. In both topical and fumigant bioassays, carvacrol and thymol were the most active compounds. The potency of bifenthrin (a pyrethroid insecticide) in topical bioassays was 72,000 times higher than carvacrol, while vapors of dichlorvos (an organophosphate insecticide) were 445 times more potent than thymol. Spontaneous electrical activity measurements of the bed bug nervous system demonstrated neuroinhibitory effects of carvacrol, thymol and eugenol, whereas linalool produced an excitatory effect. Although citronellic acid and (±)-camphor increased baseline activity of the nervous system their effects were not statistically significant. Bifenthrin also caused neuroexcitation, which is consistent with its known mode of action. These comparative toxicity and neurological impact findings provide new information for formulating effective essential oil-based insecticides for bed bug IPM and conducting mode-of-action studies on individual essential oil components.