Insecticide resistance triggers a reduction of virulence to host-plant defenses in the brown planthopper

Plants produce various phytochemicals against herbivory, but phytophagous insects have encountered phytochemicals in the co-evolutionary history between plants and insects. Selection pressure exerted by phytochemicals triggers a preadaptation to insecticide resistance in insect pests that is due to detoxification systems that are common to phytochemicals and insecticides in the insect's body. We investigated the interaction between the host plant resistance of rice cultivars and the brown planthopper, Nilaparvata lugens (Stål) (Hemiptera: Delphacidae). Forty brown planthopper resistance gene loci (BPHs) that are related to phytochemicals in various cultivars of rice inhibit feeding by N. lugens, but this planthopper has developed virulence to resistant rice cultivars possessing some genes. Nilaparvata lugens has also developed resistance to some insecticides as a rapid adaptation to human-driven selection. We tested the hypothesis that the evolution of insects' resistance to an insecticide is promoted by their encounters with phytochemicals. We compared the virulence of imidacloprid-resistant and control N. lugens strains toward seven rice cultivars that possess different genes, and we observed that the imidacloprid-resistant N. lugens had reduced virulence to three rice cultivars, Rathu Heenati (BPH3, BPH17), Babawee (BPH4), and Balamawee (BPH27, three quantitative trait loci), meaning that the development of imidacloprid resistance has a negative impact on the virulence of N. lugens to three cultivars. Our results indicate trade-offs rather than a co-relationship between the evolution of insecticide resistance and encounters with host plant defense.