Submitted to: Biopesticides International
Publication Type: Research Notes
Publication Acceptance Date: 2/19/2010
Publication Date: 6/1/2010
Publication URL: http://ddr.nal.usda.gov/handle/10113/49728
Citation: Oppert, B.S. 2010. Rapid bioassay to screen potential biopesticides in Tenebrio molitor larvae. Biopesticides International. 6(1):67-73. Interpretive Summary: Bioassays are used to test the response of insects to products that might kill them, such as insecticides. Bioassays for beetle pests of stored products are challenging because of the preparation time and tedious procedures to evaluate individual insect response. An improved bioassay was developed based on punched disks of whole grain bread in 96-well plates. The bread diet bioassay was found to be satisfactory for evaluating the response of yellow mealworm larvae to a microbial toxin over weekly intervals. The bioassay offers improvements over previous methods in shorter preparation time, less expensive materials, and the ability to screen larger numbers of larvae. The method will be useful for testing new insecticide products.
Technical Abstract: 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 LC50 values were obtained at different intoxication time intervals. The LC50 decreased as the interval of toxin exposure increased. At day 35, the LC50 for Cry3Aa protoxin was 5.73 micrograms/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 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.