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

Abstract

Abstract A rapid and simple flour-disk bioassay was developed to assay biologically active substances on several species of stored-products insects. The speed, simplicity, and parsimony of the bioassay derive from a single-step mixing of the test substance in aqueous solution with flour. Aliquots (100 μL) 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 containing the test substance. After equilibration at 30 ± 1°C and 70 ± 5% relative humidity for 24 h, the disks are individually weighed and transferred to Petri dishes with weighed stored-products insects. After 3 days, the remainder of the disk and the living insects are weighed again for calculations of food consumption, utilization, growth, and mortality. Based on the flour-disk bioassay, the neem-based insecticide, Margosan-O ® , significantly reduced consumption, growth, feeding, and dietary utilization in Cryptolestes ferrugineus (Stephens), Sitophilus oryzae (L.), and Tribolium castaneum (Herbst) in a dose-dependent manner. Margosan-O ® also caused mortality, but the species differed 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 ® caused a dramatic reduction in the F 1 progeny of all three species.