A RAPID AND SIMPLE FLOUR-DISK BIOASSAY FOR TESTING SUBSTANCES ACTIVE AGAINST STORED-PRODUCT INSECTS

A rapid and simple flour-disk bioassay was developed to assay biologic ally active substances on several species of stored-products insects. The speed, simplicity, and parsimony of the bioassay derive from a sin gle-step mixing of the test substance in aqueous solution with flour. Aliquots (100 mu L) of the stirred suspension are then pipetted onto a polystyrene Petri dish using an Eppendorf pipettor and allowed to dry at room temperature overnight to produce uniform flour disks containi ng the test substance. After equilibration at 30+/-1 degrees C and 70/-5% relative humidity for 24 h, the disks are individually weighed an d transferred to Petri dishes with weighed stored-products insects. Af ter 3 days, the remainder of the disk and the living insects are weigh ed again for calculations of food consumption, utilization, growth, an d mortality. Based on the flour-disk bioassay, the neem-based insectic ide, Margosan-O(R), significantly reduced consumption, growth, feeding , and dietary utilization in Cryptolestes ferrugineus (Stephens), Sito philus oryzae (L.), and Tribolium castaneum (Herbst) in a dose-depende nt manner. Margosan-O(R) also caused mortality, but the species differ ed widely in sensitivity, C. ferrugineus being the most sensitive and T. castaneum the least. The mortality of C. ferrugineus and S. oryzae was a consequence of both toxic and antifeedant (starvation) effects, but mortality of T. castaneum was caused entirely by its toxic action. Using a whole-kernel bioassay, it was found that Margosan-O(R) caused a dramatic reduction in the F-1 progeny of all three species.