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 treatments during or immediately after egg laying. Their efficacy sharply declines when the grubs reach late-instar stage. As both imidacloprid and thiamethoxam 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 early third-instar Popillia japonica and Cyclocephala borealis grubs in turfgrass. Imidacloprid provided control of P japonica equivalent to the most widely used curative organophosphate, trichlorfon, by 14 days after treatment, but thiamethoxam had no affect. In contrast, both imidacloprid and thiamethoxam caused significant reductions in C borealis populations. Both insecticides altered the overwintering behavior of P japonica by significantly reducing the normal downward movement of grubs in October. Halofenozide, an ecdysone agonist, also caused rapid mortality of the late second-instar C borealis equivalent to trichlorfon, but had no affect on P japonica. In another experiment on a site naturally infested with entomopathogenic nematodes, the exclusive treatment of third-instar P japonica with imidacloprid resulted 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 these differences were significant only for trichlorfon. These results indicate that imidacloprid can effectively control late second-instars of both P japonca and C borealis and can significantly reduce survival of overwintered third-instar P japonica by altering their normal overwintering behavior.