Combinations of entomopathogenic nematodes with neonicotinoid insecticides have been studied extensively. Generally, the interactions appear to be stronger for imidacloprid than for thiamethoxam. Synergistic effects of nematode-imidacloprid combinations on third-instar mortality and speed of kill have been shown with six nematode species and in five white grub species. Imidacloprid disrupts the normal nerve function of the grub, resulting in decreased behavioral avoidance and grooming, and therefore increased nematode attachment and penetration. However, the degree of interaction appears to vary with nematode and white grub species. Strong and consistent synergism occurs with Heterorhabditis bacteriophora and especially Steinernema glaseri. S. feltiae, H. megidis, and H. marelatus were only tested in one experiment against Exomala orientalis where they showed strong synergism. In contrast, S. kushidai and S. scarabaei, two scarab-specific nematode species, generally did not interact with imidacloprid. From the perspective of white grub species, synergistic interaction was the strongest in the more nematode-resistant species Cyclocephala pasadenae, C. hirta, C. borealis, and E. orientalis, and weaker in the more nematode-susceptible Popillia japonica. However, in the more nematode-resistant Rhizotrogus majalis and Maladera castanea there was generally no nematode-imidacloprid interaction. For M. castanea this may be because this species is resistant to imidacloprid. Because imidacloprid does not compromise nematode recycling in grubs, this combination is not only promising for curative white grub control but could also play a role in augmentative and inoculative approaches to white grub management, especially of scarab species that are not very susceptible to nematodes.
Species 1: Coleoptera Scarabaeidae Popillia japonica (Japanese beetle)
Species 2: Coleoptera Scarabaeidae Exomala orientalis (Oriental beetle)
Species 3: Coleoptera Scarabaeidae Cyclocephala borealis (Northern masked chafer)
Keywords: Entomopathogenic nematodes, neonicotinoid insecticides
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