Bioassay Rating System for Predicting Mortality of Moribund Corn Rootworm (Coleoptera: Chrysomelidae) Larvae Following Exposure to Insecticide-treated Soil

We developed a rating system to predict the mortality of Diabrotica undecimpunctata howardi Barber larvae used to bioassay soil insecticides. Larvae were categorized, following 48 h exposure to insecticide-treated soil, as living, dead, or in one of five intermediate categories based on movement when exposed to bright light. The eventual mortality of larvae within a rating category was determined by returning the larvae to the treated soil for an additional 5 days and establishing the probability of mortality corresponding to each category. Percentage of dead larvae in the five behavioral rating categories was independent of the insecticide used. Compared to the bioassay method in which the test organisms are considered either alive or dead, the rating system was more consistent and more precise with respect to the assumptions underlying conventional parametric statistics. The rating-system approach is a sensitive and rapid bioassay technique that might be adapted to other test organisms and situations where the subject is under continuous exposure to the toxin being tested.