Toxicity of the insect growth regulators, buprofezin and pyriproxyfen, to the glassy-winged sharpshooter, Homalodisca coagulata say (Homoptera: Cicadellidae)

Abstract

In laboratory studies we evaluated lethal effects of two insect growth regulators (IGRs) on the egg stage and five larval instars of the glassy-winged sharpshooter (GWSS). The effect of both IGRs was dose-dependent and varied according to the age of the life stage. The activity of buprofezin, a chitin synthesis inhibitor, was significantly greater when topical, residual treatment was applied to the 1st (LC 50=30.3 μg/ml) or 2nd (LC 50=39 μg/ml) instars compared to later instars. No significant differences in LC 50's were observed between older instars and adults. Toxicity responses of GWSS immature stages to buprofezin were compared over 2 years with higher susceptibility observed in the second year. Buprofezin did not delay developmental times from the 1st to the next two instars upon treatment. Pyriproxyfen, a juvenile hormone mimic, had a lethal effect on 1–2 d old GWSS eggs but did not deter late-aged eggs from hatching to 1st instars indicating that it's action was age and stage specific. Tests conducted in the second year showed GWSS eggs to be more sensitive to pyriproxyfen compared to the previous year. No toxicity was observed when older immatures or adults were treated with pyriproxyfen. Results from this study cannot only serve as baselines for buprofezin and pyriproxyfen in resistance-monitoring programmes for GWSS, but can also have significant implications for development of management programs, including accurate timing of the applications of the two IGRs that target the most susceptible stages, eggs and early instars.