Characterization of pyridalyl resistance in a laboratory-selected strain of Frankliniella occidentalis.

Frankliniella occidentalis, the western flower thrips, is one of most notorious pests on a variety of crops worldwide and many populations have high resistance to different types of insecticides. In order to determine the susceptibility of F. occidentalis to the insecticide pyridalyl in the field and to understand the potential mechanism of resistance, we conducted field monitoring of resistance and investigated cross-resistance, fitness cost, and synergism for the 75.8-fold pyridalyl-resistant strain (PY-R) established with a long-term successive laboratory selection. The results showed that all field populations of F. occidentalis had high susceptibility to pyridalyl from 2016 to 2017. The PY-R strain showed no cross-resistance to spinosad, spinetoram, abamectin, imidacloprid, or pyriproxyfen. Tests of synergism demonstrated that piperonyl butoxide (PBO) and diethyl maleate (DEM) significantly inhibited pyridalyl resistance in the PY-R strain. Moreover, PY-R showed lower pupation and emergence rates than the susceptible strain (XY-S), and female fecundity was also significantly lower in PY-R than in XY-S. By calculating the net replacement rate (R) of XY-S, the fitness of PY-R was established as 0.52. In conclusion, the susceptibility of F. occidentalis to pyridalyl is still high in China, but resistance could be developed by successive-generation selections. Enhanced metabolic detoxification is possibly involved in resistance in PY-R, which caused significant fitness costs. Thus, pyridalyl will likely prove useful in the field for controlling F. occidentalis in China, and development of resistance could be delayed by rotating pyridalyl with other chemical agents lacking cross-resistance.