Association Between Susceptibility of Thrips palmi to Spinetoram and Frequency of G275E Mutation Provides Basis for Molecular Quantification of Field-Evolved Resistance

Abstract

Putative mechanisms underlying spinosyn resistance have been identified in controlled studies on many species; however, mechanisms underlying field-evolved resistance and the development of a molecular diagnostic method for monitoring field resistance have lagged behind. Here, we examined levels of resistance of melon thrips, Thrips palmi Karny (Thysanoptera:Thripidae), to spinetoram as well as target site mutations in field populations across China to identify potential mechanisms and useful molecular markers for diagnostic and quantifying purposes. In resistant populations, we identified the G275E mutation, which has previously been linked to spinosyns resistance, and F314V mutation, both located in the α6 subunit of the nicotinic acetylcholine receptor. There was a strong correlation between levels of spinetoram resistance and allele frequency of G275E mutation in field-collected populations (r2 = 0.84) and those reared under laboratory conditions for two to five generations (r2 = 0.91). LC50 ranged from 0.12 to 0.66 mg/liter in populations without G275E mutation, whereas it ranged from 33.12 to 39.91 mg/liter in most populations with a G275E mutation frequency more than 90%. Our results indicate that the field-evolved resistance of T. palmi to spinetoram in China is mainly conferred by the G275E mutation. The frequency of the G275E mutation provides a useful diagnostic for quantifying resistance levels in field populations of T. palmi.

Variation in the toxicity of a novel meta‐diamide insecticide, broflanilide, among thrips pest species and developmental stages

“…We compared the toxicity of broflanilide to T. palmi with that of five other insecticides. Spinosyns are the most frequently used insecticides in controlling thrips pests, 4,10,12 but field populations of thrips have developed resistance to the spinosyns in many areas 10,22,34 . In the two populations of T. palmi tested, one population had an LC 50 of 0.3385 while the other had a much higher value >47 mg L −1 , indicating the reduced susceptibility of this population to spinetoram.…”

“…Spinosyns are the most frequently used insecticides in controlling thrips pests, 4,10,12 but field populations of thrips have developed resistance to the spinosyns in many areas. 10,22,34 In the two populations of T. palmi tested, one population had an LC 50 of 0.3385 while the other had a much higher value >47 mg L −1 , indicating the reduced susceptibility of this population to spinetoram. However, previous studies showed that the baseline susceptibility of spinosyns in this thrips species is only about 0.1 mg L −1 .…”

“…Tang, et al 33 used a leaf-dipping method to test the toxicity of broflanilide to lepidopteran larvae, while in our study, a spraying tower was used. We also tested the field-collected populations directly to avoid any effects of laboratory rearing on susceptibility, as found in T. palmi when tested for responses to spinetoram, 34 whereas other researchers have often tested laboratory lines.…”

KASP genotyping and semi‐quantitation of G275E mutation in the α6 subunit of Thrips palminAChR gene conferring spinetoram resistance

“…The resistance level of field populations of Thrips palmi to spinetoram is linearly related to the frequency of the G275E mutation. 17 This system provides an ideal model to test not only the utility of KASP for genotyping the resistance locus but also for testing whether the method can be used to semi-quantify mutation frequency at the population level. Our study highlights the usefulness of this method in both contexts.…”

“…The susceptibility of each population to spinetoram was tested using the bioassay method in the laboratory, and the G275E mutation was examined by PCR amplification and Sanger sequencing of target DNA, including this site. The resistance level of field populations of Thrips palmi to spinetoram is linearly related to the frequency of the G275E mutation 17 . This system provides an ideal model to test not only the utility of KASP for genotyping the resistance locus but also for testing whether the method can be used to semi‐quantify mutation frequency at the population level.…”