Lethal and sublethal effects of azadirachtin on Riptortus pedestris (F.) and its egg parasitoids, Gryon japonicum (Ashmead) and Ooencyrtus nezarae Ishii

Abstract

Azadirachtin may cause acute toxicity and alter behavior of natural enemies. We evaluate the lethal and sublethal effect of five concentrations of azadirachtin through topical application and residual exposure on Riptortus pedestris (F.). Its effects were also assessed against egg parasitoids, Gryon japonicum (Ashmead) and Ooencyrtus nezarae Ishii, by pre- and post-parasitization treatment of host eggs. The toxicity of azadirachtin to R. pedestris increased significantly with higher concentrations. A morphological deformity in nymphs was also observed in topical application after molting into the next instar. Azadirachtin significantly reduced egg parasitism by G. japonicum and O. nezarae at concentrations of 15 and 10 mL/L. Interestingly, the male ratio of G. japonicum progeny was increased significantly when host eggs were treated with azadirachtin before parasitization. However, the risk levels of azadirachtin to G. japonicum and O. nezarae were classified as harmless and slightly harmful, respectively, at a concentration of 10 mL/L. Thus, this concentration could be a potential alternative to broad-spectrum chemical insecticides for managing R. pedestris.