Comparison of neonicotinoid insecticides as synergists for entomopathogenic nematodes

In previous greenhouse and field studies, the neonicotinoid insecticide imidacloprid and the entomopathogenic nematodes Heterorhabditis bacteriophora and Steinernema glaseri interacted synergistically against third-instars of the Japanese beetle, Popillia japonica, the oriental beetle, Exomala (=Anomala) orientalis, and three masked chafer species, Cyclocephala hirta, C. pasadenae, and C. borealis (Coleoptera: Scarabaeidae). We tested whether this interaction would also occur with other neonicotinoids, primarily thiamethoxam. In laboratory, greenhouse and field experiments, imidacloprid provided stronger and more consistent synergism with nematodes than thiamethoxam. White grub mortality resulting from nematode–neonicotinoid combinations was synergistic/additive/antagonistic in 75/25/0% of our observations with imidacloprid and 37/42/21% of our observations with thiamethoxam. Neonicotinoid–nematode interactions varied with white grub species. Imidacloprid always interacted synergistically with nematodes against E. orientalis and P. japonica, whereas no enhancement occurred against Rhizotrogus majalis and Maladera castanea. Against E. orientalis, imidacloprid interacted synergistically with five nematode species, H. bacteriophora, H. megidis, H. marelatus, S. glaseri, and S. feltiae. Synergistic combinations of nematodes and a neonicotinoid insecticide could be used for curative treatments of white grub infestations, especially against E. orientalis and P. japonica. This combination may allow spot-treatment of turf areas that exceed damage thresholds, thereby limiting the environmental impact of the insecticide application.