Neonicotinoid insecticides alter diapause behavior and survival of overwintering white grubs (Coleoptera : Scarabaeidae)

The chloronicotinyl, imidacloprid, and the thianicotinyl, thiamethoxam, are effective insecticides against white grubs when applied as preventative tr eatments during or immediately after egg laying. Their efficacy sharply dec lines when the grubs reach late-instar stage. As both imidacloprid and thia methoxam act on post-synaptic nicotinic acetylcholine receptors and modify insect behavior, we hypothesized that the two compounds will interfere with overwintering behavior of scarabs, thus reducing their ecological fitness and exposing them to increased winter mortality. We tested this hypothesis by applying the two compounds curatively against late second-instar and ear ly third-instar Popillia japonica and Cyclocephala borealis grubs in turfgr ass. Imidacloprid provided control of P japonica equivalent to the most wid ely used curative organophosphate, trichlorfon, by 14 days after treatment, but thiamethoxam had no affect. In contrast, both imidacloprid and thiamet hoxam caused significant reductions in C borealis populations. Both insecti cides altered the overwintering behavior of P japonica by significantly red ucing the normal downward movement of grubs in October. Halofenozide, an ec dysone agonist, also caused rapid mortality of the late second-instar C bor ealis equivalent to trichlorfon, but had no affect on P japonica. In anothe r experiment on a site naturally infested with entomopathogenic nematodes, the exclusive treatment of third-instar P japonica with imidacloprid result ed in no significant mortality in the autumn (up to 15 days after treatment ), but caused a significant reduction in the survival of overwintered grubs . There was an increase in the numbers of grubs infected with nematodes in the imidacloprid and trichlorfon treatments compared with control, but thes e differences were significant only for trichlorfon. These results indicate that imidacloprid can effectively control late second-instars of both P ja ponca and C borealis and can significantly reduce survival of overwintered third-instar P japonica by altering their normal overwintering behavior. (C ) 2001 Society of Chemical Industry.