Interactions of insecticides, entomopathogenic fungi, and earthworms as they relate to white grub IPM in turfgrass systems

White grubs (Coleoptera: Scarabaeidae) are important turfgrass pests in Virginia. Insecticides such as the neonicotinoid imidacloprid are commonly applied to turfgrass in order to control these pests. As an alternative to synthetic insecticides, entomopathogenic fungi (EPF), including Metarhizium brunneum (Petch) and Beauveria bassiana (Balsamo) Vuillemin may also be used for white grub control. The interaction of combining these two control tactics for white grubs in Virginia merits further investigation as does their effects on other soil organisms such as earthworms, which cohabitate with white grubs in turfgrass soil ecosystems. Herein, I investigate the following: 1) the efficacy of combined applications of the EPF, M. brunneum and B. bassiana with lower rates of imidacloprid or the diamide insecticide, chlorantraniliprole against white grubs; 2) interactions of earthworms with white grubs and EPF; and 3) the effect of white grub control products on earthworms. In the laboratory, a combined application of one half the recommended rate of chlorantraniliprole plus the full recommended rate of B. bassiana caused significantly higher mortality of third instar Cyclocephala spp. grubs than the untreated control. In the field, imidacloprid applied at lower rates as a single treatment or as part of a combined treatment with EPF resulted in significantly fewer grubs when applications were made in June. In the greenhouse, Japanese beetle, Popillia japonica Newman females laid a significantly reduced number of eggs in turf treated with lower rate of imidacloprid either applied as a single treatment or as part of a combined treatment compared with untreated control. In an earthworm-white grub interaction study, the earthworms Eisenia fetida (Savingy) and E. hortenis (Michaelsen) were shown to transfer B. bassiana spores from fungus-infected soil to uninfected soil in the laboratory. However, the presence of earthworms in fungal infected soil did not enhance the mortality of Cyclocephala spp. grubs. In bioassays conducted in the laboratory, only two neonicotinoids, dinotefuran and imidacloprid, caused significantly higher mortality to adult Lumbricus terrestris L. earthworms than untreated control consistently. When applied as a drench to turfgrass in spring, summer, and fall, none of the insecticides significantly reduced the earthworm densities compared with a water control.