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Insecticide resistance in Plutella xylostella(Linnaeus) (Lepidoptera: Plutellidae) is a major constraint on the global production of cruciferous crops. For effective management of insecticide resistance, it is necessary to develop a molecular detection tool for predicting insecticide resistance levels based on the mutation frequency of target sites. In this study, a susceptible strain (SHggt) of P. xylostellawas subjected to chlorantraniliprole and tetraniliprole selection under laboratory conditions to obtain the CHLSeland TETSelstrains, respectively, to determine their resistance development, cross-resistance and mutation frequencies of the P. xylostellaryanodine receptor (PxRyR). In addition, the tetraniliprole resistance and the mutation frequencies of the PxRyR from 7 field populations were evaluated. Continuous selection over 30 generations resulted in resistance ratios (RRs) of 7,073.2-fold and 6,971.0-fold for the CHLSeland TETSelstrains, respectively, and thousandfold increases in cross-resistance to unexposed diamides, e.g., cyantraniliprole and flubendiamide, were observed. For the field populations, three out of seven populations have developed more than thousandfold resistance to tetraniliprole. Among the three investigated target site mutations in PxRyR, only I4790K was detected in both laboratory-selected strains. However, 2 mutations, I4790K and G4946E, were detected in field populations. A positive correlation between RRs and K allele frequencies was observed in the laboratory-selected/relaxed strains and field populations of P. xylostella. These results suggest a possible link between the development of anthranilic diamide resistance and the frequency of the PxRyR I4790K mutation, which can be used to develop effective strategies for diamide resistance management in P. xylostella.Graphical AbstractThe I4790K mutation plays a key role in anthranilic diamide resistance in P. xylostella. The K allele frequency increased when the resistance ratio (RR) increased but decreased when the selection pressure was removed and when the RR decreased. A positive correlation between resistance ratios and K allele frequencies was observed in the laboratory-selected and relaxed strains and field populations. This correlation could be used for the rapid prediction of diamide resistance levels in field populations of P. xylostella.

The status of insecticide resistance levels is important for applying suitable pest management approaches. The present study investigated the insecticide resistance of Plutella xylostella(Linnaeus) (Lepidoptera: Plutellidae) populations from five major cruciferous growing areas in Taiwan. The surveyed locations were distributed from central to southern Taiwan and included Taichung, Changhua, Yunlin, Chiayi, and Kaohsiung. High levels of resistance to spinosad, indoxacarb, metaflumizone, and chlorantraniliprole were recorded among the surveyed populations except in Taichung. The resistance ratios ranged from 2.376 to 1,236-fold for spinosad and 24.63–1,511-fold for indoxacarb. Similarly, those for metaflumizone and chlorantraniliprole, were 2.563–76.21- and 4.457–683.0-fold, respectively. However, Bacillus thuringiensissubsp. aizawai, Bacillus thuringiensissubsp. kurstaki, emamectin benzoate, and diafenthiuron were still relatively effective against most field populations of P. xylostella. After approximately 10 generations of being maintained in the laboratory without exposure to insecticides, the resistance ratios of the Kaohsiung and Changhua populations declined to approximately 1.4–10-fold, and insecticides with control failure likelihood also began to show a negligible risk of control failure. Although spinosad, indoxacarb, metaflumizone, and chlorantraniliprole have lost their effectiveness in most field populations of P. xylostellain Taiwan, their effectiveness may be recovered in the absence of insecticide-selection pressure for approximately 10 generations. Therefore, we suggest that a constant survey of insecticide resistance and well-designed insecticide rotation based on the survey results are necessary for the effective control and insecticide resistance management of P. xylostella.Graphical Abstract