Foccα6, a truncated nAChR subunit, positively correlates with spinosad resistance in the western flower thrips, Frankliniella occidentalis (Pergande)

The composition and function of bacterial communities in Bombyx mori (Lepidoptera: Bombycidae) changed dramatically with infected fungi: A new potential to culture Cordyceps cicadae

Cordyceps cicadae (Hypocreales: Cordycipitaceae) is a renowned entomopathogenic fungus used as herbal medicine in China. However, wild C. cicadae resources have been threatened by heavy harvesting. We hypothesised that Bombyx mori L. (Lepidoptera: Bombycidae) could be a new alternative to cultivate C. cicadae due to the low cost of rearing. Bacterial communities are crucial for the formation of Cordyceps and for promoting the production of metabolites. To better understand the bacterial community structure associated with Cordyceps, three Claviciptaceae fungi were used to explore the pathogenicity of the silkworms. Here, fifth-instar silkworms were infected with C. cicadae, Cordyceps cateniannulata (Hypocreales: Cordycipitaceae) and Beauveria bassiana (Hypocreales: Cordycipitaceae). Subsequently, we applied high-throughput sequencing to explore the composition of bacterial communities in silkworms. Our results showed that all three fungi were highly pathogenic to silkworms, which suggests that silkworms have the potential to cultivate Cordyceps. After fungal infection, the diversity of bacterial communities in silkworms decreased significantly, and the abundance of Staphylococcus increased in mummified larvae, which may play a role in the death process when the host suffers infection by entomopathogenic fungi. Furthermore, there were high similarities in the bacterial community composition and function in the C. cicadae and C. cateniannulata infected samples, and the phylogenetic analysis suggested that these similarities may be related to the fungal phylogenetic relationship. Our findings reveal that infection with different entomopathogenic fungi affects the composition and function of bacterial communities in silkworms and that the bacterial species associated with Cordyceps are primarily host dependent, while fungal infection affects bacterial abundance.

Mutation V65I in the β1 Subunit of the Nicotinic Acetylcholine Receptor Confers Neonicotinoid and Sulfoxaflor Resistance in Insects

Neonicotinoids have been widely used to control pests with remarkable effectiveness. Excessive insecticides have led to serious insect resistance. Mutations of the nicotinic acetylcholine receptor (nAChR) are one of the reasons for neonicotinoid resistance conferred in various agricultural pests. Two mutations, V65I and V104I, were found in the nAChR β1 subunit of two neonicotinoid-resistant aphid populations. However, the specific functions of the two mutations remain unclear. In this study, we cloned and identified four nAChR subunits (α1, α2, α8, and β1) of thrips and found them to be highly homologous to the nAChR subunits of other insects. Subsequently, we successfully expressed two subtypes nAChR (α1/α2/α8/β1 and α1/α8/β1) by coinjecting three cofactors for the first time in thrips, and α1/α8/β1 showed abundant current rapidly. Acetylcholine, neonicotinoids, and sulfoxaflor exhibited different activation capacities for the two subtypes of nAChRs. Finally, V65I was found to significantly reduce the binding ability of nAChR to neonicotinoids and sulfoxaflor through electrophysiology and computer simulations. V104I caused a decrease in agonist affinity (pEC50) but an increase in the efficacy (Imax) of nAChR against neonicotinoids and reduced the binding ability of nAChR to sulfoxaflor. This study provides theoretical and technical support for studying the molecular mechanisms of neonicotinoid resistance in pests.

Uridine diphosphate glucosyltransferases are involved in spinosad resistance in western flower thrips Frankliniella occidentalis (Pergande)

Uridine diphosphate glucosyltransferases (UGTs) play crucial roles in the insect detoxification system and are associated with pesticide resistance. Our previous transcriptomic analysis of spinosad-susceptible (Ivf03) and resistant (NIL-R) Frankliniella occidentalis revealed numerous upregulated UGT genes in the NIL-R strain, suggesting their potential contribution to spinosad resistance. To investigate this hypothesis, here we conducted UGT activity assays and spinosad induction experiments, employing RNA interference (RNAi) techniques for gene function validation. We found significantly elevated UGT activity in the NIL-R strain compared to Ivf03, with 5-nitrouracil showing a substantial synergistic effect on the resistant strain. Eighteen UGT genes were identified in F. occidentalis, with gene expansion and duplication observed within families UGT466, 467, and 468. Ten out of the eighteen UGTs exhibited higher expression levels in NIL-R, specifically FoUGT466B1, FoUGT468A3, and FoUGT468A4 consistently being upregulated across nymphs, males, and females. RNAi-based functional validation targeting these three UGT genes led to increased susceptibility to spinosad in a life stage-, sex-, and dose-dependent manner. These results indicate that UGTs are indeed involved in spinosad resistance in F. occidentalis, and the effects are dependent on life stage, sex, and dose. Therefore, sustainable control for F. occidentalis resistance should always consider these differential responses.

Transcriptional regulation and overexpression of GST cluster enhances pesticide resistance in the cotton bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae)

The rapid evolution of resistance in agricultural pest poses a serious threat to global food security. However, the mechanisms of resistance through metabolic regulation are largely unknown. Here, we found that a GST gene cluster was strongly selected in North China (NTC) population, and it was significantly genetically-linked to lambda-cyhalothrin resistance. Knockout of the GST cluster using CRISPR/Cas9 significantly increased the sensitivity of the knockout strain to lambda-cyhalothrin. Haplotype analysis revealed no non-synonymous mutations or structural variations in the GST cluster, whereas GST_119 and GST_121 were significantly overexpressed in the NTC population. Silencing of GST_119 or co-silencing of GST_119 and GST_121 with RNAi significantly increased larval sensitivity to lambda-cyhalothrin. We also identified additional GATAe transcription factor binding sites in the promoter of NTC_GST_119. Transient expression of GATAe in Hi5 cells activated NTC_GST_119 and Xinjiang (XJ)_GST_119 transcription, but the transcriptional activity of NTC_GST_119 was significantly higher than that of XJ_GST_119. These results demonstrate that variations in the regulatory region result in complex expression changes in the GST cluster, which enhances lambda-cyhalothrin resistance in field-populations. This study deepens our knowledge of the evolutionary mechanism of pest adaptation under environmental stress and provides potential targets for monitoring pest resistance and integrated management.

Molecular diagnostics for monitoring insecticide resistance in the western flower thrips Frankliniella occidentalis

BACKGROUND

Insecticide resistance has emerged in various western flower thrips (WFT) populations across the world, threatening the efficiency of chemical control applications. Elucidation of insecticide resistance mechanisms at the molecular level provides markers for the development of diagnostics to monitor the trait and support evidence-based resistance management.

RESULTS

TaqMan and Droplet Digital polymerase chain reaction (ddPCR) diagnostics were developed and validated, against Sanger sequencing, in individual and pooled WFT samples respectively, for the G275E mutation (nicotinic acetylcholine receptor α6 gene, nAChR α6) associated with resistance to nAChR allosteric modulators, site I (spinosyns); L1014F, T929I, T929C and T292V mutations (voltage-gated sodium channel gene, vgsc) linked with pyrethroid resistance; and I1017M (chitin synthase 1 gene, chs1) conferring resistance to growth inhibitors affecting CHS1 (benzoylureas). The detection limits of ddPCR assays for mutant allelic frequencies (MAF) were in the range of 0.1%-0.2%. The assays were applied in nine WFT field populations from Crete, Greece. The G275E (MAF = 29.66%-100.0%), T929I and T929V (combined MAF = 100%), L1014F (MAF = 11.01%-37.29%), and I1017M (MAF = 17.74%-51.07%) mutations were present in all populations.

CONCLUSION

The molecular diagnostics panel that was developed in this study can facilitate the quick and sensitive resistance monitoring of WFT populations at the molecular level, to support evidence-based insecticide resistance management strategies. © 2022 Society of Chemical Industry.

Potential of UV-C for management of two-spotted spider mites and thrips in Florida strawberry

BACKGROUND

Major pests of Florida strawberry (two-spotted spider mites, Tetranychus urticae Koch; chilli thrips, Scirtothrips dorsalis Hood; and western flower thrips, Frankliniella occidentalis Pergande) are difficult to manage using only conventional chemical control. Previous research has demonstrated high mortality of T. urticae eggs after exposure to ultraviolet B (UV-B) light. Additionally, ultraviolet C (UV-C) light has been shown to suppress powdery mildew in strawberry at doses between 85 and 200 J m^-2 with no damage to the plant. Therefore, UV-C may also have the potential to be used as an integrated pest management tool for arthropod pests of strawberry. The objectives of this study were to: (i) determine the effect of UV-C on T. urticae, S. dorsalis, and F. occidentalis natural populations in open-field strawberry; and (ii) determine the effect of UV-C on T. urticae egg hatch after application in open-field strawberry. Field studies were conducted during the 2019-2020 and 2020-2021 strawberry seasons in Florida, USA. Four treatments were compared: (i) foliar application of spinetoram in response to natural pest pressure; (ii) application of UV-C 200 J m^-2 twice a week; (iii) application of UV-C 350 J m^-2 twice a week; and (iv) a non-treated control.

RESULTS

In the field trials, suppression of T. urticae was observed at 350 J m^-2 in 2020-2021. In the other field trials, no effect was observed due to low natural infestations. No negative impact on yield was observed from UV-C applications.

CONCLUSION

UV-C shows promise as a component of an integrated pest management program for T. urticae in strawberry. © 2022 Society of Chemical Industry.

The effects of the E3 ubiquitin-protein ligase UBR7 of Frankliniella occidentalis on the ability of insects to acquire and transmit TSWV

The interactions between plant viruses and insect vectors are very complex. In recent years, RNA sequencing data have been used to elucidate critical genes of Tomato spotted wilt ortho-tospovirus (TSWV) and Frankliniella occidentalis (F. occidentalis). However, very little is known about the essential genes involved in thrips acquisition and transmission of TSWV. Based on transcriptome data of F. occidentalis infected with TSWV, we verified the complete sequence of the E3 ubiquitin-protein ligase UBR7 gene (UBR7), which is closely related to virus transmission. Additionally, we found that UBR7 belongs to the E3 ubiquitin-protein ligase family that is highly expressed in adulthood in F. occidentalis. UBR7 could interfere with virus replication and thus affect the transmission efficiency of F. occidentalis. With low URB7 expression, TSWV transmission efficiency decreased, while TSWV acquisition efficiency was unaffected. Moreover, the direct interaction between UBR7 and the nucleocapsid (N) protein of TSWV was investigated through surface plasmon resonance and GST pull-down. In conclusion, we found that UBR7 is a crucial protein for TSWV transmission by F. occidentalis, as it directly interacts with TSWV N. This study provides a new direction for developing green pesticides targeting E3 ubiquitin to control TSWV and F. occidentalis.

Tomato spotted wilt orthotospovirus shifts sex ratio toward males in the western flower thrips, Frankliniella occidentalis, by down-regulating a FSCB-like gene

BACKGROUND

Plant viruses can facilitate their transmission by modulating the sex ratios of their insect vectors. Previously, we found that exposure to tomato spotted wilt orthotospovirus (TSWV) in the western flower thrips, Frankliniella occidentalis, led to a male-biased sex ratio in the offspring. TSWV, a generalist pathogen with a broad host range, is transmitted primarily by F. occidentalis in a circulative-propagative manner. Here, we integrated proteomic tools with RNAi to comprehensively investigate the genetic basis underlying the shift in vector sex ratio induced by the virus.

RESULTS

Proteomic analysis exhibited 104 differentially expressed proteins between F. occidentalis adult males with and without TSWV. The expression of the fiber sheath CABYR-binding-like (FSCB) protein, namely FoFSCB-like, a sperm-specific protein associated with sperm capacitation and motility, was decreased by 46%. The predicted FoFSCB-like protein includes 10 classic Pro-X-X-Pro motifs and 42 phosphorylation sites, which are key features for sperm capacitation. FoFSCB-like expression was gradually increased during the development and peaked at the pupal stage. After exposure to TSWV, FoFSCB-like expression was substantially down-regulated. Nanoparticle-mediated RNAi substantially suppressed FoFSCB-like expression and led to a significant male bias in the offspring.

CONCLUSION

These combined results suggest that down-regulation of FoFSCB-like in virus-exposed thrips leads to a male-biased sex ratio in the offspring. This study not only advances our understanding of virus-vector interactions, but also identifies a potential target for the genetic management of F. occidentalis, the primary vector of TSWV, by manipulating male fertility. © 2022 Society of Chemical Industry.

CRISPR-mediated knockout of nicotinic acetylcholine receptor (nAChR) α6 subunit confers high levels of resistance to spinosyns in Spodoptera frugiperda

Spinosyn insecticides (spinosad and spinetoram) have been widely used to control a number of agricultural pests including the fall armyworm, Spodoptera frugiperda. Mutations of the nicotinic acetylcholine receptor α6 subunit (nAChRα6) have been reported to confer high levels of resistance to spinosyns in several insect pests. Here we used CRISPR-mediated gene knockout to determine the involvement of S. frugiperda nAChRα6 (Sfα6) in spinosyns susceptibility. A Sfα6 knockout strain of S. frugiperda (Sfα6-KO) was established using dual single guide RNA (sgRNA) directed large fragment deletion with the CRISPR/Cas9 system. Sfα6-KO showed high levels of resistance to spinosad (307-fold) and spinetoram (517-fold) compared with the progenitor strain YJ-19, while no resistance was observed to emamectin benzoate, indoxacarb, chlorfenapyr, chlorantraniliprole and broflanilide. Genetic analyses confirmed that spinosad resistance in Sfα6-KO was autosomal, incompletely recessive and tightly linked to the edited deletion mutation of Sfα6. Our results provided in vivo functional evidence for Sfα6 as the major target of spinosyns against S. frugiperda, and demonstrated that disruption of Sfα6 causes high level resistance to spinosyns. Although no mutations of Sfα6 have yet been reported in any field populations of S. frugiperda, it is critical to develop F₁ screens and/or DNA-based methods to detect and monitor the mutant allele frequencies of Sfα6 across global populations of S. frugiperda.

Effects of nicotinic acetylcholine receptor subunit deletion mutants on insecticide susceptibility and fitness in Drosophila melanogaster

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) are major excitatory neurotransmitter receptors in insects and also the target site for many insecticides. Unfortunately, the effectiveness of these insecticides is diminishing as a consequence of the evolution of insecticide resistance. Further exploration of insecticide targets is important to sustainable pest management.

RESULTS

In order to validate the role of nAChR subunits in insecticide susceptibility and test whether the subunit's absence imposes the fitness cost on insects, we determined the susceptibility of eight nAChR subunit deletion mutants of Drosophila melanogaster to nine insecticides. These findings highlighted the specific resistance of the Dα6 deletion mutant to spinosyns. Although triflumezopyrim, dinotefuran and imidacloprid are competitive modulators of nAChRs, differences in susceptibility of the insect with different deletion mutants suggested that the target sites of these three insecticides do not overlap completely. Mutants showed decreased susceptibility to insecticides, accompanied by a reduction in fitness. The number of eggs produced by Dα1^attP , Dα2^attP , Dβ2^attP and Dβ3^attP females was significantly lesser than that of the vas-Cas9 strain as the control. In addition, adults of Dα2^attP , Dα3^attP and Dα7^attP strains showed lower climbing performance. Meanwhile, males of Dα3^attP , Dα5^attP , Dβ2^attP and Dβ3^attP , and females of Dβ2^attP showed significantly shorter longevity than those of the vas-Cas9 strain.

CONCLUSION

This study provides new insights into the interactions of different insecticides with different nAChRs subunit in D. melanogaster as a research model, it could help better understand such interaction in agricultural pests whose genetic manipulations for toxicological research are often challenging. © 2022 Society of Chemical Industry.

The use of insecticide mixtures containing neonicotinoids as a strategy to limit insect pests: Efficiency and mode of action

Synthetic insecticides continue to be the main strategy for managing insect pests, which are a major concern for both crop protection and public health. As nicotinic acetylcholine receptors play a central role in insect neurotransmission, they are the molecular target of neurotoxic insecticides such as neonicotinoids. These insecticides are used worldwide and have shown high efficiency in culture protection. However, the emergence of insect resistance mechanisms, and negative side-effects on non-target species have highlighted the need for a new control strategy. In this context, the use of insecticide mixtures with synergistic effects have been used in order to decrease the insecticide dose, and thus delay the selection of resistance-strains, and limit their negative impact. In this review, we summarize the available data concerning the mode of action of neonicotinoid mixtures, as well as their toxicity to various insect pests and non-target species. We found that insecticide mixtures containing neonicotinoids may be an effective strategy for limiting insect pests, and in particular resistant strains, although they could also negatively impact non-target species such as pollinating insects.

Spinetoram resistance drives interspecific competition between Megalurothrips usitatus and Frankliniella intonsa

BACKGROUND

Species displacement by the outcome of interspecific competition is of particular importance to pest management. Over the past decade, spinetoram has been extensively applied in control of the two closely related thrips Megalurothrips usitatus and Frankliniella intonsa worldwide, while whether its resistance is implicated in mediating interspecific interplay of the two thrips remains elusive to date.

RESULTS

Field population dynamics (from 2017 to 2019) demonstrated a trend toward displacement of F. intonsa by M. usitatus on cowpea crops, supporting an existing interspecific competition. Following exposure to spinetoram, M. usitatus became the predominate species, which suggests the use of spinetoram appears to be responsible for mediating interspecific interactions of the two thrips. Further annual and seasonal analysis (from 2016 to 2020) of field-evolved resistance dynamics revealed that M. usitatus developed remarkably higher resistance to spinetoram compared to that of F. intonsa, implying a close relationship between evolution of spinetoram resistance and their competitive interactions. After 12 generations of laboratory selection, resistance to spinetoram in M. usitatus and F. intonsa increased up to 64.50-fold and 28.33-fold, and the average realized heritability (h² ) of resistance was calculated as 0.2550 and 0.1602, respectively. Interestingly, two-sex life table analysis showed that the spinetoram-resistant strain of F. intonsa exhibited existing fitness costs, but not the M. usitatus. These indicate that a rapid development of spinetoram resistance and the lack of associated fitness costs may be the mechanism underlying recent dominance of M. usitatus over F. intonsa.

CONCLUSION

Collectively, our results uncover the involvement of insecticide resistance in conferring displacement mechanism behind interspecific competition, providing a framework for understanding the significance of the evolutionary relationships among insects under ongoing changing environments. These findings also can be invaluable in proposing the most appropriate strategies for sustainable thrips control programs. © 2022 Society of Chemical Industry.

Role of Insect Gut Microbiota in Pesticide Degradation: A Review

Insect pests cause significant agricultural and economic losses to crops worldwide due to their destructive activities. Pesticides are designed to be poisonous and are intentionally released into the environment to combat the menace caused by these noxious pests. To survive, these insects can resist toxic substances introduced by humans in the form of pesticides. According to recent findings, microbes that live in insect as symbionts have recently been found to protect their hosts against toxins. Symbioses that have been formed are between the pests and various microbes, a defensive mechanism against pathogens and pesticides. Insects' guts provide unique conditions for microbial colonization, and resident bacteria can deliver numerous benefits to their hosts. Insects vary significantly in their reliance on gut microbes for basic functions. Insect digestive tracts are very different in shape and chemical properties, which have a big impact on the structure and composition of the microbial community. Insect gut microbiota has been found to contribute to feeding, parasite and pathogen protection, immune response modulation, and pesticide breakdown. The current review will examine the roles of gut microbiota in pesticide detoxification and the mechanisms behind the development of resistance in insects to various pesticides. To better understand the detoxifying microbiota in agriculturally significant pest insects, we provided comprehensive information regarding the role of gut microbiota in the detoxification of pesticides.

Proteus, Matrine, and Pyridalyl Toxicity and Their Sublethal Effects on Orius laevigatus (Hemiptera: Anthocoridae)

Proteus, matrine, and pyridalyl were tested in the laboratory for their effects upon Orius laevigatus (Fieber), which is a polyphagous predator used for IPM programs of Frankliniella occidentalis (Pergande). Against female adults, the most toxic insecticide was Proteus (LC50 = 44.3 µl L-1), followed by pyridalyl (LC50 = 83.8 µl L-1) and matrine (LC50 = 102.7 µl L-1). The mortality of female adults was checked 24, 48, 72, and 96 h after exposure to 14-d residues of the treatments on strawberry leaves. The residual of Proteus was less toxic; the remaining products caused the lowest mortality at different times after exposure. Sublethal treatments (LC25) significantly prolonged the developmental duration of total immature stages from 17.6 d in control to 21.6 and 20.0 d in Proteus and pyridalyl treatments, respectively. Also, the fecundity of O. laevigatus treated with Proteus, pyridalyl, and matrine decreased to 58.8%, 75.6%, and 96.7%, respectively, in comparison to the control. Compared with the control population (0.118 d-1), the intrinsic rate of increase (r) of F1 generation decreased by 0.053, 0.095, and 0.110 d-1 in Proteus, pyridalyl, and matrine treatments, respectively. The consumption rate of control bugs reached 14.0 thrips during 24 h. The adults fed on Proteus treatment had the lowest consumption rate in this period (9.4 preys). Overall, matrine proved to be harmless with reproductive capacity and r similar to what was recorded in control bugs. We concluded that matrine can be used as an alternative for the synthetic insecticide to integrate with O. laevigatus.

Increasing Frequency of G275E Mutation in the Nicotinic Acetylcholine Receptor α6 Subunit Conferring Spinetoram Resistance in Invading Populations of Western Flower Thrips in China

Simple Summary

The western flower thrips (WFT) Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) is an important invasive pest in agriculture and forestry. It has developed resistance to a frequently used pesticide spinetoram world widely, including the invading area of China. However, the mechanism of resistance to spinetoram is unclear in China. In this study, we found the presence of the G275E mutation in the nicotinic acetylcholine receptor Foα6 in the early invading populations, which has now increased to a high frequency in China. There was a correlation between the frequency of the G275E mutation and resistance to spinetoram as characterized by median lethal concentration. Our results showed that G275E mutation is one of the mechanisms conferring spinetoram resistance in invading populations in China, as in many other countries. Our study highlights the rapid spread of the G275E mutation in China in the 2009–2021 period.

Abstract

The western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) is an important invasive pest worldwide. Field-evolved resistance to the pesticide spinetoram is an increasing problem in the chemical control of this pest. Here, we examined changes in the frequency of a genetic mutation associated with spinetoram resistance, the G275E mutation in the acetylcholine receptor Foα6, in 62 field populations collected from 2009 to 2021 across areas of China invaded by this pest. We found a low frequency of the G275E mutation in populations collected at the early invasion stage, in contrast to a high frequency in native USA populations. However, the frequency of the G275E mutation has increased to a high level in recently collected populations, with the mutation becoming fixed in some populations. There was a correlation between the frequency of the G275E mutation and resistance to spinetoram as characterized by median lethal concentration, although two populations were outliers. These results showed that G275E mutation is one of the mechanisms conferring spinetoram resistance in many invading populations in China. Ongoing dispersal of the WFT may have facilitated a rapid increase in the G275E mutation across China. Our study highlights the rapid evolution of pesticide resistance in an invasive species and points to a useful marker for molecular diagnostics of spinetoram resistance.

Lethal and Sublethal Effects of Proteus, Matrine, and Pyridalyl on Frankliniella occidentalis (Thysanoptera: Thripidae)

The western flower thrips (WFT), Frankliniella occidentalis (Pergande) is one of the most harmful pests of crops in greenhouses and fields. Considering the need for studies that introduce new insecticides for control of the WFT, the leaf dip method was carried out to study the acute toxicity of Proteus, matrine, and pyridalyl to adult thrips, and life tables were constructed to assess the impacts of sublethal concentrations (LC25) of these insecticides on the development and reproduction of the F1 generation. Bioassays showed that the toxicity of matrine (LC50: 45.9 µl ml-1) and Proteus (LC50: 54.5 µl ml-1) was higher than pyridalyl (LC50: 176.5 µl ml-1). At LC25 concentration, both Proteus and matrine prolonged the development period and reduced the survival rate of eggs, larval stages, and pupae in the F1 generation. Also, the adults' longevity, oviposition duration, and the cumulative number of eggs laid per female (fecundity) were decreased significantly. Sublethal concentrations of Proteus and matrine inhibited the population growth rate relative to the control based on the predicted number of offspring. The lowest net reproductive rate (R0), intrinsic rate of increase (r), and finite rate of increase (λ) were estimated for Proteus (7.02 offspring/individual, 0. 0838 d-1, and 1. 08 d-1, respectively). In contrast, the WFT F1 generation that resulted from parent adults treated with pyridalyl was neither affected in their developmental time, nor fecundity, or the intrinsic rate of increase. According to our findings, all tested insecticides, especially Proteus, showed good potential for use in integrated pest management strategies against F. occidentalis.

Mutation (G275E) of nAChR subunit Foα6 associated with spinetoram resistance in Australian western flower thrips, Frankliniella occidentalis (Pergande)

Western flower thrips, Frankliniella occidentalis is an economically important agricultural pest. It causes damage by feeding and oviposition or indirectly by plant virus transmission. Australian F. occidentalis are resistant to many insecticides including spinosad and the related chemical spinetoram. Spinetoram resistance to F. occidentalis has been recently reported in three different Australian States, however, mechanisms conferring that resistance have not been investigated. To identify the mechanisms underlying resistance to spinetoram in F. occidentalis, we sequenced the genomic region of nicotinic acetylcholine receptor Foα6 in number of spinosad and spinetoram resistant field-populations. We found that a single nucleotide substitution (G to A) in exon 9 of the α6 subunit was present in resistant strains (G275E) and absent from susceptible. By examining field populations we consider the G275E mutation is the major cause of resistance to spinetoram in Australian F. occidentalis. We developed a real-time PCR diagnostic assay to quickly identify resistant alleles in field-populations. The method was used to test spinetoram resistant F. occidentalis collected from Australian cotton during the 2018-2019. Results show thrips tested carried the G275E mutation and the resistance allele was unusually widely distributed. The wide distribution of G275E mutation was not expected because spinetoram is not extensively used in Australian cotton. We speculate resistance may relate to extensive chemical use in crops nearby such as horticulture where thrips are often targeted for control. Our molecular diagnostic assay can provide timely and precise resistance frequency information that can support sustainable chemical use including spinetoram based IPM.

Identification of the Aedes aegypti nAChR gene family and molecular target of spinosad

BACKGROUND

Spinosad is an insecticide with unique mode of action (MOA) of disrupting nicotinic acetylcholine receptor and is efficacious against many insect species. Mutations in the nicotinic acetylcholine receptor (nAChR) α6 subunit have been identified that are associated with levels of spinosad resistance, but the molecular characterization of the nAChR gene family and a causative association between nAChR α6 and resistance to spinosad in Aedes aegypti, a primary vector of many arboviruses, have not yet been reported.

RESULTS

In this study, we identified 10 candidate nAChR subunits in Ae. Aegypti, nAChRα1-α9 and nAChRβ1, showing similarly orthologous relationships with Anopheles gambiae. With the application of the CRISPR/Cas9 genome editing system, we introduced a 32-bp deletion at the 5' end of the Aaeα6 (Ae. aegypti nAChR α6) gene in a homozygous mutant strain (Aaeα6-KO). The mutation produced two successive pre-mature stop codons, resulting in loss of function in the target receptor. The Aaeα6-KO mutant strain exhibited a 320-fold level of resistance to spinosad compared with wildtype. A recessive mode of inheritance for spinosad resistance was found in the Aaeα6-KO strain.

CONCLUSION

CRISPR/Cas9 introduced truncated Aaeα6 receptor in Ae. aegypti resulted in an increased level of resistance to spinosad, suggesting that the conserved nAChR α6 subunit is the target for spinosad insecticide. © 2020 Society of Chemical Industry.

Variable resistance to spinetoram in populations of Thrips palmi across a small area unconnected to genetic similarity

The melon thrips, Thrips palmi, is an increasingly important pest of vegetables in northern China. Some populations have developed resistance in the field to the insecticide spinetoram. Understanding the origin and dispersal of insecticide-resistant populations can shed light on resistance management strategies. In this study, we tested susceptibility of seven greenhouse populations of T. palmi to spinetoram collected from a small area of about 300 km2 in Shandong Province and examined population genetic structure across the area based on a segment of mitochondrial cox1 gene and 22 microsatellite loci to infer the possible origin and dispersal of insecticide resistance. Levels of resistance to spinetoram differed among seven populations, which included one population with high resistance (LC50 = 759.34 mg/L), three populations with medium resistance (LC50 ranged from 28.69 to 34.79 mg/L), and three populations with low resistance (LC50 ranged from 7.61 to 8.97 mg/L). The populations were genetically differentiated into two groups unrelated to both levels of resistance and geographic distance. The molecular data indicated high levels of gene flow between populations with different levels of resistance to spinetoram and low gene flow among populations with the same level of resistance, pointing to a likely separate history of resistance evolution. Resistance levels of two tested populations to spinetoram decreased 23 and 4.6 times after five generations without any exposure to the pesticide. We therefore suspect that resistance of T. palmi most likely evolved in response to local applications of the insecticide. Our study suggests that the development of resistance could be avoided or resistance even reversed by reducing usage of spinetoram.

Insights into the Toxicity and Degradation Mechanisms of Imidacloprid Via Physicochemical and Microbial Approaches

Imidacloprid is a neonicotinoid insecticide that has been widely used to control insect pests in agricultural fields for decades. It shows insecticidal activity mainly by blocking the normal conduction of the central nervous system in insects. However, in recent years, imidacloprid has been reported to be an emerging contaminant in all parts of the world, and has different toxic effects on a variety of non-target organisms, including human beings, due to its large-scale use. Hence, the removal of imidacloprid from the ecosystem has received widespread attention. Different remediation approaches have been studied to eliminate imidacloprid residues from the environment, such as oxidation, hydrolysis, adsorption, ultrasound, illumination, and biodegradation. In nature, microbial degradation is one of the most important processes controlling the fate of and transformation from imidacloprid use, and from an environmental point of view, it is the most promising means, as it is the most effective, least hazardous, and most environmentally friendly. To date, several imidacloprid-degrading microbes, including Bacillus, Pseudoxanthomonas, Mycobacterium, Rhizobium, Rhodococcus, and Stenotrophomonas, have been characterized for biodegradation. In addition, previous studies have found that many insects and microorganisms have developed resistance genes to and degradation enzymes of imidacloprid. Furthermore, the metabolites and degradation pathways of imidacloprid have been reported. However, reviews of the toxicity and degradation mechanisms of imidacloprid are rare. In this review, the toxicity and degradation mechanisms of imidacloprid are summarized in order to provide a theoretical and practical basis for the remediation of imidacloprid-contaminated environments.

CRISPR-mediated gene knockout reveals nicotinic acetylcholine receptor (nAChR) subunit α6 as a target of spinosyns in Helicoverpa armigera

BACKGROUND

The spinosyn insecticides (spinosad and spinetoram) have been intensively used to control a wide range of agricultural pests. However, resistance to spinosyns has evolved in several agricultural pests. Disruption of the nicotinic acetylcholine receptor subunit α6 (nAChRα6) has been associated with high levels of resistance to spinosyns in both field and laboratory-selected strains of several insect pests. Among the 12 nAChR subunits of Helicoverpa armigera, Haα6 has the closest sequence similarity (66.02%) to Haα7. Here we used CRISPR-mediated knockouts to evaluate the role of two nAChR subunits (Haα6 and Haα7) of H. armigera in toxicity of spinosyns.

RESULTS

Individual knockouts of Haα6 and Haα7 were created utilizing CRISPR/Cas9 system in H. armigera. The Haα6 knockout (Haα6-KO) strain exhibited high levels of resistance to spinosad (531-fold) and spinetoram (1105-fold) compared with the wild-type parent SCD strain, whereas the Haα7 knockout (Haα7-KO) strain showed no significant susceptibility changes to both spinosyns. Genetic analyses demonstrated that resistance to spinosad conferred by knockout of Haα6 was autosomal, incompletely recessive and tightly linked to the disruption mutation of Haα6. Both Haα6-KO and Haα7-KO strains had no significant effects on susceptibility to other four insecticides including emamectin benzoate, beta-cypermethrin, chlorantraniliprole and indoxacarb.

CONCLUSION

Our results provide in vivo functional evidence for Haα6 as a target of spinosyns in H. armigera, and little or no role of Haα7 in mediating toxicity of spinosyns. The results are valuable to the development of resistance monitoring and management methods for spinosyn resistance in H. armigera. © 2020 Society of Chemical Industry.

Tomato spotted wilt orthotospovirus influences the reproduction of its insect vector, western flower thrips, Frankliniella occidentalis, to facilitate transmission

BACKGROUND

Tomato spotted wilt orthotospovirus (TSWV), one of the most devastating viruses of ornamental plants and vegetable crops worldwide, is transmitted by the western flower thrips, Frankliniella occidentalis (Pergande), in a persistent-propagative manner. How TSWV influences the reproduction of its vector to enhance transmission and whether infection with TSWV changes the mating behavior of F. occidentalis are not fully understood.

RESULTS

TSWV-exposed thrips had a significantly longer developmental time than non-exposed individuals. More importantly, increased developmental time was predominantly associated with adults, a stage critical for dispersal and virus transmission. In addition, TSWV-exposed F. occidentalis produced substantially more progeny than did non-exposed thrips. Interestingly, most of the increase in progeny came from an increase in males, a sex with a greater dispersal and virus transmission capability. Specifically, the female/male ratio of progeny shifted from 1.3-7.0/1 to 0.6-1.1/1. As for mating behavior, copulation time was significantly longer in TSWV-exposed thrips. Finally, females tended to re-mate less when exposed to the virus. Resistance to re-mating may lead to reduced sperm availability in females, which translates to a larger number of male progeny under a haplodiploid system.

CONCLUSION

These combined results suggest that TSWV can influence the developmental time, mating behavior, fecundity, and offspring sex allocation of its vector F. occidentalis to facilitate virus transmission. As such, a monitoring program capable of the earlier detection of the virus in host plants and/or its insect vector, thrips, using double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), real time quantitative polymerase chain reaction (RT-qPCR) or virus detection strips might be beneficial for long-term, sustainable management. © 2020 Society of Chemical Industry.

Evolutionary trade-offs of insecticide resistance - The fitness costs associated with target-site mutations in the nAChR of Drosophila melanogaster

The evolution of resistance to drugs and pesticides poses a major threat to human health and food security. Neonicotinoids are highly effective insecticides used to control agricultural pests. They target the insect nicotinic acetylcholine receptor and mutations of the receptor that confer resistance have been slow to develop, with only one field‐evolved mutation being reported to date. This is an arginine‐to‐threonine substitution at position 81 of the nAChR_β1 subunit in neonicotinoid‐resistant aphids. To validate the role of R81T in neonicotinoid resistance and to test whether it may confer any significant fitness costs to insects, CRISPR/Cas9 was used to introduce an analogous mutation in the genome of Drosophila melanogaster. Flies carrying R81T showed an increased tolerance (resistance) to neonicotinoid insecticides, accompanied by a significant reduction in fitness. In comparison, flies carrying a deletion of the whole nAChR_α6 subunit, the target site of spinosyns, showed an increased tolerance to this class of insecticides but presented almost no fitness deficits.

Disruption of nicotinic acetylcholine receptor α6 mediated by CRISPR/Cas9 confers resistance to spinosyns in Plutella xylostella

BACKGROUND

The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system provides some advantages over other reverse genetic techniques to investigate the causal relationship between insecticide resistance phenotype and candidate gene. Several studies published to date point to the nicotinic acetylcholine receptor (nAChR) α6 subunit strongly associated with spinosyns resistance in insects, including Plutella xylostella. However, reverse genetic verification of the P. xylostella nAChRα6 has not yet been achieved via an in vivo approach.

RESULTS

Here, we successfully constructed a homozygous strain (Pxα6-KO) with a 2-nt deletion mutation of nAChRα6 by CRISPR/Cas9 coupled with non-homologous end joining approach in P. xylostella. The manipulated mutation results in a frame shift in the open reading frame of transcripts, which produces a predicted protein truncated in the TM3-TM4 loop region. When compared to the background strain IPP-S, the knockout strain Pxα6-KO exhibited 229- and 1462-fold resistance to spinosad and spinetoram, respectively, but no or limited (resistance ratios <3-fold) effects on the toxicities of imidacloprid, abamectin, β-cypermethrin, indoxacarb, metaflumizone and chlorantraniliprole. Furthermore, the mode of inheritance of the acquired spinetoram resistance was autosomal recessive and significantly linked with the 2-nt deletion mutation of nAChRα6 in the Pxα6-KO strain.

CONCLUSION

In vivo functional investigation demonstrates the causality of the Pxα6 truncating mutation with high levels of resistance to spinosyns in P. xylostella. Our results suggest the Pxα6-KO strain underlies an autosomal, recessive mode of inheritance for spinetoram resistance, and reinforces the association of this gene to the mode of action of spinosyns. © 2019 Society of Chemical Industry.

Invasion biology, ecology, and management of Frankliniella occidentalis in China

Frankliniella occidentalis is an economically important invasive pest worldwide, which can damage various horticultural crops and ornamental plants. F. occidentalis was first intercepted in Kunming, Yunnan province in 2000, and first reported to establish a population in Beijing, China in 2003. Since then, this pest is currently distributed across tens of provinces in mainland China and cause increasingly serious damage and loss. To control this pest, invasion biology, monitoring, and integrated pest management have been generally and intensively studied for 15 years in China. Furthermore, western flower thrips (WFT) as an important invasive insect pest, the research achievements on WFT has contributed to the promotion of technological innovation and development for invasive alien species management strategies and techniques in China. This review provides an overview for research on the biology, ecology, prevention, and management of this pest during 15 years in China. Meanwhile, China's "4E action" strategy on F. occidentalis is also discussed in this review.

The evolution of multiple-insecticide resistance in UK populations of tomato leafminer, Tuta absoluta

BACKGROUND

The tomato leafminer, Tuta absoluta, is an economically important pest of tomatoes in Europe, Africa, Asia and South America. In the UK this species is controlled using an integrated pest management (IPM) programme which incorporates the insecticides spinosad and chlorantraniliprole. In response to UK grower concerns of loss of efficacy of these compounds at certain sites, insecticide bioassays were performed on five populations collected from four commercial glasshouses and potential mechanisms of resistance investigated.

RESULTS

We observed high levels of resistance to spinosad in four of the strains, and in two of these tolerance to chlorantraniliprole. Selection of one of these strains with chlorantraniliprole rapidly resulted in a line exhibiting potent resistance to this compound. Sequencing of messenger RNA encoding the nicotinic acetylcholine receptor (nAChR) α6 subunit, target of spinosad, revealed Taα6 transcripts in the spinosad-resistant strains that lack exon 4 and encode a highly truncated protein, or contain a triplet deletion in the predicted first transmembrane domain resulting in the loss of a highly conserved amino acid. Sequencing of the ryanodine receptor gene, encoding the target of diamide insecticides, of the chlorantraniliprole-selected line revealed an amino acid substitution (G4903V) that has been previously linked to diamide resistance in populations of T. absoluta in the Mediterranean and South America.

CONCLUSION

Taken together our results reveal emerging resistance in UK populations of T. absoluta to two of the most important insecticides used as part of IPM, with significant implications for the control of this species in the UK. © 2019 Society of Chemical Industry.