Dominant Inheritance of Field-Evolved Resistance to Fipronil in Plutella xylostella (Lepidoptera: Plutellidae)

A comprehensive assessment of insecticide resistance mutations in source and immigrant populations of the diamondback moth Plutella xylostella (L.)

BACKGROUND

The diamondback moth (DBM) Plutella xylostella has developed resistance to almost all insecticides used to control it. Populations of DBM in temperate regions mainly migrate from annual breeding areas. However, the distribution pattern of insecticide resistance of DBM within the context of long-distance migration remains unclear.

RESULTS

In this study, we examined the frequency of 14 resistance mutations for 52 populations of DBM collected in 2010, 2011, 2017 and 2018 across China using a high-throughput KASP genotyping method. Mutations L1041F and T929I conferring pyrethroid resistance, and mutations G4946E and E1338D conferring chlorantraniliprole resistance were near fixation in most populations, whereas resistant alleles of F1020S, M918I, A309V and F1845Y were uncommon or absent in most populations. Resistance allele frequencies were relatively stable among different years, although the frequency of two mutations decreased. Principal component analysis based on resistant allele frequencies separated a southern population as an outlier, whereas the immigrants clustered with other populations, congruent with the migration pattern of northern immigrants coming from the Sichuan area of southwestern China. Most resistant mutations deviated from Hardy-Weinberg equilibrium due to a lower than expected frequency of heterozygotes. The deviation index of heterozygosity for resistant alleles was significantly higher than the index obtained from single nucleotide polymorphisms across the genome. These findings suggest heterogeneous selection pressures on resistant mutations.

CONCLUSION

Our results provide a picture of resistant mutation patterns in DBM shaped by insecticide usage and migration of this pest, and highlight the widespread distribution of resistance alleles in DBM. © 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.

Distinct roles of two RDL GABA receptors in fipronil action in the diamondback moth (Plutella xylostella)

The phenylpyrazole insecticide fipronil blocks resistance to dieldrin (RDL) γ-aminobutyric acid (GABA) receptors in insects, thereby impairing inhibitory neurotransmission. Some insect species, such as the diamondback moth (Plutella xylostella), possess more than one Rdl gene. The involvement of multiple Rdls in fipronil toxicity and resistance remains largely unknown. In this study, we investigated the roles of two Rdl genes, PxRdl1 and PxRdl2, in P. xylostella fipronil action. In Xenopus oocytes, PxRDL2 receptors were 40 times less sensitive to fipronil than PxRDL1. PxRDL2 receptors were also less sensitive to GABA compared with PxRDL1. Knockout of the fipronil-sensitive PxRdl1 reduced the fipronil potency 10-fold, whereas knockout of the fipronil-resistant PxRdl2 enhanced the fipronil potency 4.4-fold. Furthermore, in two fipronil-resistant diamondback moth field populations, PxRdl2 expression was elevated 3.7- and 4.1-fold compared with a susceptible strain, whereas PxRdl1 expression was comparable among the resistant and susceptible strains. Collectively, our results indicate antagonistic effects of PxRDL1 and PxRDL2 on fipronil action in vivo and suggest that enhanced expression of fipronil-resistant PxRdl2 is potentially a new mechanism of fipronil resistance in insects.

Genetic basis and realized heritability of laboratory selected spirotetramat resistance for insecticide resistance management in Oxycarenus hyalinipennis Costa (Hemiptera: Lygaeidae)

Dusky cotton bug, Oxycarenus hyalinipennis Costa (Hemiptera: Lygaeidae) is become a major pest of cotton. It causes damage to cotton by sucking the cell sap or by staining the cotton seed. Insect pests in Pakistan are mainly managed by use of insecticides, applying this practice leads to resistance development. In this study, O. hyalinipennis was selected with spirotetramat under laboratory conditions to investigate genetic mode of resistance to spirotetramat. Selection with spirotetramat for eleven generations resulted in a 727-fold resistance compared to the susceptible strain. The LC₅₀ values of spirotetramat in both reciprocal crosses were significantly different from each other and degree of dominanace values were 0.25 for cross-1 and 0.01 for cross-2. Monogenic model proved the contribution of more than one gene in controlling the spirotetramat resistance. Moreover, the value for realized heritability of spirotetramat resistance was 0.13. It can be concluded that spirotetramat resistance was sex linked, polygenic and incompletely dominant. These findings could be helpful in management of spirotetramat resistance in O. hyalinipennis as incompletely dominant and polygenic resistance tend to develop slowly and is manageable.

Reverse genetics reveals contrary effects of two Rdl-homologous GABA receptors of Helicoverpa armigera on the toxicity of cyclodiene insecticides

The resistance to dieldrin gene (Rdl) encodes a subunit of the insect γ-amino butyric acid (GABA) receptor, and the encoded Rdl subunit is a major target site for cyclodiene and phenylpyrazole insecticides. Since the substitution of a single amino acid (Ala to Ser/Gly at position 302) of the Drosophila melanogaster Rdl gene was first identified to confer high level resistance to dieldrin, mutations at the equivalent positions have been reported to confer resistance to dieldrin and/or fipronil in a wide range of different insects. In the cotton bollworm Helicoverpa armigera, there are two Rdl homologs (HaRdl-1 and HaRdl-2) in close proximity on the Z chromosome, which as wild-type sequences, encode alanine and serine respectively at amino acid position 302. In the present study, we used the CRISPR/Cas9 gene editing approach to knock out HaRdl-1 and HaRdl-2 and establish two homozygous knockout strains (ΔRdl-1 and ΔRdl-2). The ΔRdl-1 strain showed low levels of resistance (8.0- to 9.3-fold) to three cyclodiene insecticides (endosulfan, aldrin and dieldrin) compared with the background SCD strain. In contrast, toxicity of the three cyclodiene insecticides to the ΔRdl-2 strain increased significantly (3.6- to 6.3-fold) when compared with the SCD strain. Genetic analysis indicated the obtained resistance to endosulfan and dieldrin in the ΔRdl-1 strain was sex-linked, which is consistent with the fact that HaRdl-1 locus is located on the Z chromosome. The above results demonstrate that both HaRdl-1 and HaRdl-2 are important determinants for the susceptibility of H. armigera SCD strain to the three cyclodiene insecticides, but have opposite effects. It was also found that HaRdl-1 and HaRdl-2 are involved, to some extent, in mediating sensitivity of H. armigera to avermectin and fipronil respectively. We speculate that the HaRdl-1 and HaRdl-2 subunits have different pharmacological properties, which contribute to the differential sensitivities of H. armigera to the tested cyclodienes and other insecticides.

RDL A301S alone does not confer high levels of resistance to cyclodiene organochlorine or phenyl pyrazole insecticides in Plutella xylostella

Mutations in the GABA-gated chloride channel are associated with resistance to cyclodiene organochlorine and phenyl pyrazole insecticides. The best characterised of these is A301S, which was initially identified in a Dieldrin resistant strain of Drosophila melanogaster. The orthologous mutation has been found in a variety of different crop pests including the diamond back moth Plutella xylostella. However, the contribution of this mutation to resistance in this species remains unclear. We have used the CRISPR/Cas9 system in order to edit Plutella xylostella PxGABARalpha1 to Serine at the 301 orthologous position (282 in PxGABARalpha1) in an insecticide sensitive strain isolated from Vero Beach (VB) USA. In this edited line, no high level of resistance is conferred to Dieldrin, Endosulfan or Fipronil, rather only a subtle shift in sensitivity which could not confer commercially important resistance. We conclude that the high level of commercial resistance to cyclodiene organochlorine and phenyl pyrazole insecticides observed in some field isolates of Plutella xylostella cannot arise from A282S in PxGABARalpha1 alone.

Long-term monitoring and characterization of resistance to chlorfenapyr in Plutella xylostella (Lepidoptera: Plutellidae) from China

BACKGROUND

Plutella xylostella (L.) is a worldwide pest of crucifer crops. Chlorfenapyr has been used extensively to manage several insect pests in vegetables, including P. xylostella. In this study, 48 field populations were collected from 11 geographic locations of China from 2008 to 2017 for resistance monitoring to chlorfenapyr. Inheritance and synergism of chlorfenapyr resistance were characterized in a field-derived population.

RESULTS

Compared with the susceptible SZ strain, 33 of 39 populations collected from Eastern China were susceptible or developed low levels of resistance to chlorfenapyr (< 10-fold), five populations developed mid-level resistance (11-33-fold), and only one population had high level resistance (208-fold). However, seven of nine populations sampled from Southern China evolved 136-334-fold resistance. Interestingly, the dynamic of the median lethal concentration (LC₅₀ ) values from four continuous sampling sites (NJ, HF, JN and KS) showed a sharp increase in 2009, which implies that adults may have immigrated from chlorfenapyr-resistant regions. A highly resistant field population (GZ15) was further studied. By crossing the GZ15 and susceptible SZ strains we showed that resistance to chlorfenapyr was autosomally inherited and incompletely dominant. Synergism bioassays revealed that metabolic detoxification might not be responsible for chlorfenapyr resistance in the GZ15 population.

CONCLUSIONS

Some field populations of P. xylostella from China have evolved high levels of resistance to chlorfenapyr. Field-evolved resistance to chlorfenapyr is autosomal and dominant in the GZ15 population. These findings provide useful information for the design of effective resistance management strategies against P. xylostella in the field. © 2018 Society of Chemical Industry.

Monitoring, Cross-Resistance, Inheritance, and Synergism of Plutella xylostella (Lepidoptera: Plutellidae) Resistance to Pyridalyl in China

Pyridalyl is an insecticide that shows significant efficacy against Plutella xylostella, a notorious pest insect worldwide. In this study, we monitored resistance of P. xylostella to pyridalyl in China from 2016 to 2017, determined cross-resistance, inheritance, and synergism of pyridalyl resistance in two pyridalyl-resistant populations, one field-evolved resistant population (ZL-PR) and one laboratory-selected resistant population (XY-PR). We found that variation in susceptibility among 15 field populations in China from 2016 to 2017 was high, with mean LC50 values ranging from 1.839 to 1,652 mg/liter. The laboratory-selected XY-PR strain showed 31.3-fold resistance to pyridalyl and moderate cross-resistance to fipronil. The ZL-PR displayed 1,050.2-fold resistance to pyridalyl and high resistance to all tested insecticides. Genetic analysis illustrated that pyridalyl resistance in ZL-PR was autosomally inherited and incompletely recessive. However, pyridalyl resistance in the XY-PR strain was autosomally inherited but incompletely dominant. Moreover, piperonyl butoxide significantly inhibited pyridalyl resistance in the XY-PR strain. In conclusion, P. xylostella field populations from South China have high levels of resistance to pyridalyl and different modes of inheritance of resistance were found in XY-PR and ZL-PR. Moreover, enhanced oxidative metabolism is possibly involved in resistance of the XY-PR strain but not in the ZL-PR strain.

Function and pharmacology of glutamate-gated chloride channel exon 9 splice variants from the diamondback moth Plutella xylostella

Glutamate-gated chloride channels (GluCls) are found only in invertebrates and mediate fast inhibitory neurotransmission. The structural and functional diversity of GluCls are produced through assembly of multiple subunits and via posttranscriptional alternations. Alternative splicing is the most common way to achieve this in insect GluCls and splicing occurs primarily at exons 3 and 9. As expression pattern and pharmacological properties of exon 9 alternative splices in invertebrate GluCls remain poorly understood, the cDNAs encoding three alternative splice variants (9a, 9b and 9c) of the PxGluCl gene from the diamondback moth Plutella xylostella were constructed and their pharmacological characterizations were examined using electrophysiological studies. Alternative splicing of exon 9 had little to no impact on PxGluCl sensitivity towards the agonist glutamate when subunits were singly or co-expressed in Xenopus oocytes. In contrast, the allosteric modulator abamectin and the chloride channel blocker fipronil had differing effects on PxGluCl splice variants. PxGluCl9c channels were more resistant to abamectin and PxGluCl9b channels were more sensitive to fipronil than other homomeric channels. In addition, heteromeric channels containing different splice variants showed similar sensitivity to abamectin (except for 9c) and reduced sensitivity to fipronil than homomeric channels. These findings suggest that functionally indistinguishable but pharmacologically distinct GluCls could be formed in P. xylostella and that the upregulated constitutive expression of the specific variants may contribute to the evolution of insecticide resistance in P. xylostella and other arthropods.

The effects of insecticides on butterflies - A review

Pesticides, in particular insecticides, can be very beneficial but have also been found to have harmful side effects on non-target insects. Butterflies play an important role in ecosystems, are well monitored and are recognised as good indicators of environmental health. The amount of information already known about butterfly ecology and the increased availability of genomes make them a very valuable model for the study of non-target effects of pesticide usage. The effects of pesticides are not simply linear, but complex through their interactions with a large variety of biotic and abiotic factors. Furthermore, these effects manifest themselves at a variety of levels, from the molecular to metapopulation level. Research should therefore aim to dissect these complex effects at a number of levels, but as we discuss in this review, this is seldom if ever done in butterflies. We suggest that in order dissect the complex effects of pesticides on butterflies we need to integrate detailed molecular studies, including characterising sequence variability of relevant target genes, with more classical evolutionary ecology; from direct toxicity tests on individual larvae in the laboratory to field studies that consider the potentiation of pesticides by ecologically relevant environmental biotic and abiotic stressors. Such integration would better inform population-level responses across broad geographical scales and provide more in-depth information about the non-target impacts of pesticides.

Wax Removal and Diamondback Moth Performance in Collards Cultivars

The diamondback moth Plutella xylostella (Linnaeus, 1758) (Lepidoptera: Plutellidae) is an herbivorous specialist on Brassicaceae species. Brassicas spp. plants developed a range of defenses (chemical, physical, and morphological) to prevent herbivores attack. In this study, we reported the antixenotic and antibiotic effects of outermost layer of two species of epicuticular wax of Brassicaceae, Brassica oleracea L. var. "Santo Antônio," and Hybrid Kope F1 100MX, on larvae and adult of P. xylostella. In the choice experiment, P. xylostella adults showed an oviposition preference for collard cultivars Santo Antônio (control) and Hybrid Kope F1 100MX with wax removal. In the no-choice experiment, oviposition was 6.4 times higher in the Hybrid Kope F1 100MX with wax removal than without wax removal. There were significant differences among larvae feeding on leaf disks of Hybrid Kope F1 100MX in the treatments with (65.3 mg) and without wax removal (23.5 mg). The net reproduction rate (R ₀ ), and intrinsic (rm) and finite rates of increase (λ) of P. xylostella in the cv. Santo Antônio were bigger in the treatment without wax removal (R ₀  = 50.4, rm = 0.23 and λ = 1.26) than treatment with wax removal (R ₀  = 28.5, rm = 0.20 and λ = 1.22). However, only the R ₀ value was affected by mechanical wax removal in the Hybrid Kope F1 100MX (with wax removal R ₀  = 43.3 and without wax removal R ₀  = 30.8). In conclusion, the results indicate that collard's wax is important to accessibility and development of P. xylostella, and its removal changes the resistance of collard's varieties to P. xylostella.

Cross-resistance to three phenylpyrazole insecticides and A2'N mutation detection of GABA receptor subunit in fipronil-resistant Laodelphax striatellus (Hemiptera: Delphacidae)

BACKGROUND

Laodelphax striatellus (Fallén) is an important pest of crops in East Asia. Over the past decade, phenylpyrazole insecticides, which target the insect GABA receptor, have increasingly been used as alternatives against rice planthoppers.

RESULTS

Cross-resistance to ethiprole and butene-fipronil was detected in a laboratory-selected fipronil-resistant strain of L. striatellus (LsFR). Compared with a fipronil-susceptible strain (LsFS), LsFR had obtained a high-level resistance to fipronil (112.1-fold) and moderate resistance to ethiprole (24.5-fold) and butene-fipronil (14.7-fold). For the resistance of field populations, LC₅₀ values of ethiprole were remarkably higher than the other two analogues in Gaochun and Yancheng populations in 2016. Significant correlations were demonstrated between the LC₅₀ values of three phenylpyrazole insecticides (R = 0.944-0.998, P = 0.007-0.016). Additionally, an AS-PCR assay was developed to detect the A2'N mutant GABA receptor in L. striatellus strains or populations. It was noteworthy that mutation frequencies of 19.2 and 3.6% appeared in Lujiang and Gaochun populations in 2016, respectively. Furthermore, there was an extremely significant difference in genomic expression of Lsrdl between the LsFS and LsFR individuals (1.85-fold, F = 26.8, P = 0.0008).

CONCLUSIONS

This study could help us better understand the cross-resistance mechanisms in L. striatellus, and be beneficial for proposing effective pest management strategies of phenylpyrazole resistance. © 2016 Society of Chemical Industry.

Semisynthesis of Esters of Fraxinellone C4/10-Oxime and Their Pesticidal Activities

A total of 20 esters of fraxinellone C4/10-oxime were synthesized and determined by melting points, optical rotation, infrared spectra, proton nuclear magnetic resonance spectra, and high-resolution mass spectrometry spectra. Two steric configurations of compounds 7i and 8i were unambiguously confirmed by X-ray crystallography. Additionally, their pesticidal activities were assessed on two typical lepidopteran pests, Mythimna separata Walker and Plutella xylostella Linnaeus. Generally, all compounds exhibited less potent oral toxicity than toosendanin against third-instar larvae of P. xylostella. However, all compounds showed the growth inhibitory property against early third-instar larvae of M. separata. Notably, compounds 7m, 8b, 8k, 9, and 11 displayed more potent pesticidal activity than toosendanin. This demonstrated that introducing the C-4 carbonyl or oxime group on fraxinellone resulted in more promising derivatives than those bearing a C-10 carbonyl or oxime substituent.