A bioassay to evaluate the efficacy of insecticides against the Asiatic garden beetle

The Asiatic garden beetle (AGB), Maladera formosae (Coleoptera: Scarabaeidae) is a pest of corn planted in sandy soils in northern Indiana, southern Michigan, and northwest Ohio. Asiatic garden beetle grubs overwinter in field soil and emerge from diapause in early spring, which often coincides with planting. The grubs then feed on the seedling corn seeds and roots which can result in substantial stand loss. Currently, there is little information on the efficacy of chemical products applied curatively against the AGB grub in field crops. Therefore, the objective of this research was to develop methods for a laboratory bioassay approximating field conditions to evaluate insecticides against AGB grubs. The bioassay is conducted in cylindrical containers of autoclaved field soil from a field with a history of infestation. We apply the test insecticide with a pipette in a line to mimic an in-furrow application, at a volume scaled down from field-recommended rates. Then we plant one corn seed per container, cover with soil, and add one field-collected grub to the surface, allowing it to burrow down. We monitor and maintain soil moisture, and measure plant growth characteristics until each arena is destructively sampled and grub mortality assessed. With this method we can either calculate the LC₅₀ of a given product, or test pre-determined product rates for efficacy. This information will be useful to make management recommendations to farmers struggling with this pest.