Synergism of imidacloprid and entomopathogenic nematodes against white grubs: the mechanism

Entomopathogenic nematodes and the chloronicotinyl insecticide, imidaclopri d, interact synergistically on the mortality of third-instar white grubs (C oleoptera: Scarabaeidae). The degree of interaction, however, varies with n ematode species, being synergistic for Steinernema glaseri (Steiner) and He terorhabditis bacteriophora Poinar, but only additive for Steinernema kushi dai Mamiya. The mechanism of the interaction between imidacloprid and these three entomopathogenic nematodes was studied in the laboratory. In vials w ith soil and grass, mortality, speed of kill, and nematode establishment we re negatively affected by imidacloprid with S. kushidai but positively affe cted with S. glaseri and H. bacteriophora. In all other experiments, imidac loprid had a similar effect for all three nematode species on various facto rs important for the successful nematode infection in white grubs. Nematode attraction to grubs was not affected by imidacloprid treatment of the grub s. Establishment of intra-hemocoelically injected nematodes was always high er in imidacloprid-treated grubs but the differences were small and in most cases not significant. The major factor responsible for synergistic intera ctions between imidacloprid and entomopathogenic nematodes appears to be th e general disruption of normal nerve function due to imidacloprid resulting in drastically reduced activity of the grubs. This sluggishness facilitate s host attachment of infective juvenile nematodes. Grooming and evasive beh avior in response to nematode attack was also reduced in imidacloprid-treat ed grubs. The degree to which different white grub species responded to ent omopathogenic nematode attack varied considerably. Untreated Popillia japon ica Newman (Coleoptera: Scarabaeidae) grubs were the most responsive to nem atode attack among the species tested. Untreated Cyclocephala borealis Arro w (Coleoptera: Scarabaeidae) grubs showed a weaker grooming and no evasion response, and untreated C. hirta LeConte (Coleoptera: Scarabaeidae) grubs s howed no significant response. Chewing/biting behavior was significantly in creased in the presence of nematodes in untreated P. japonica and C. boreal is but not in C. hirta and imidacloprid-treated P. japonica and C. borealis . Our observations, however, did not provide an explanation for the lack of synergism between imidacloprid and S. kushidai.