Rapid Bioassay to Screen Potential Biopesticides in Tenebrio molitor Larvae

A simplified assay was devised to evaluate the response of Tenebrio molitor larvae to potential insect control products. The assay incorporates punched disks of flattened whole-grain bread placed in 96-well plates, with treatments applied topically, and neonate larvae added to each well. To evaluate the method, increasing doses of the Bacillus thuringiensis Cry3Aa protoxin were added as a suspension to diet disks. Larval mortality was recorded, and LD 50 values were obtained at different intoxication time intervals. The LD 50 decreased as the interval of toxin exposure increased. At day 35, the LD 50 for Cry3Aa protoxin was 8.06 mg/ disk, approximately an order of magnitude higher than values obtained for lepidopteran larvae exposed to Cry1A toxins in diet overlay assays at much shorter time intervals. The lower efficacy and slower response probably account for the lack of incorporation of Cry3Aa into integrated pest management of coleopteran storage pests. The slower response of T. molitor to Cry3Aa may be behavioral, or it may be an indication of differences in the mode of action of Cry3A and Cry1A toxins. The diet-disk assay offers improvements over previous assays, including shorter preparation time, relatively simple and inexpensive, and larger numbers of larvae screened.