Insecticide-mediated changes in the population and toxicity of the thrips species, Frankliniella occidentalis (Pergande) and Thrips flavus (Schrank) (Thysanoptera: Thripidae)

Frankliniella occidentalis (Pergande) and Thrips flavus (Schrank) (Thysanoptera: Thripidae) cause considerable damage to agricultural crops. In this study, we investigated the variations in the population density of these 2 thrips species in 2 sites (Dabai and Heilongtan) with different insecticide application levels. Bioassays were performed to determine the susceptibility of both thrips species to imidacloprid, abamectin, and high-bromine cyhalothrin in summer. The results showed that the F. occidentalis species were more abundant in Dabai than in Heilongtan during both winter and summer. The proportions of F. occidentalis in winter and summer were 28.32-43.35% and 61.79-76.02%, respectively. Moreover, F. occidentalis resistance against the 3 insecticides was significantly higher than that of T. flavus in both 2017 and 2019. Compared with 2017, the LC50 values of F. occidentalis populations in Dabai to imidacloprid, abamectin, and lambda-cyhalothrin increased to 100.076, 16.52, and 130.44 mg/liter, respectively. The number of F. occidentalis, reaching the proportion of 91.63% in 90 days, was significantly higher than that of the control after imidacloprid treatment. In conclusion, thrips interspecies competition is affected by the irrational use of insecticides, which may cause the replacement of native species by invasive species, thereby leading to an outbreak.