Insecticide-Induced Resurgence of the Brown Planthopper, Nilaparvata lugens,1 on Rice Varieties with Different Levels of Resistance

Three rice varieties, IR29, IR40, and IR42, which are, respectively, susceptible, moderately resistant, and resistant, to Nilaparvata lugens Stal in the Philippines, were treated in the field with decamethrin, an insecticide known to cause resurgence. N. lugens populations increased to a significantly higher level in the treated plots than in untreated checks, but the degree of resurgence varied among varieties. the maximum population increases in the treated plots compared with the checks were ca. 74-, 50-, and 5-fold, respectively, for IR29, IR40, and IR42. Decamethrin was toxic to predators, and this reduction of natural enemies of N. lugens may have contributed to the pest resurgence. However, the differences between populations of N. lugens in treated and nontreated plots within varieties were larger than differences in predator numbers, suggesting that other factors were also involved.