Development of some lepidopterous cotton pests as affected by exposure to sublethal levels of endotoxins of Bacillus thuringiensis for different periods

Abstract

Three spore-δ-endotoxin preparations from Bacillus thuringiensis vars. kurstaki and entomocidus including a commercial biological insecticide, Thuricide, were studied with respect to their biological interactions with the lepidopterous cotton pests, Spodoptera littoralis, Spodoptera exigua, and Heliothis armigera. The biological effects of sublethal doses and the length of exposure time to low endotoxin concentrations were assessed during and after removal of toxin from the diet. Retardation in larval development, reduction in egg production of the moths, and fertility of the egg were observed together with significant reduction in pupal weight and appearance of deformities in both pupae and moth populations. The influence of exposure to sublethal toxin concentrations was also manifested in term of decreasing the adult emergence, fecundity reduction, and prolonging the generation period. In the exposed insects, the precentage of larvae that survived and succeeded to pupate increased with the decrease in the toxin concentration and with the decrease in exposure time. The reduction of the pupal weight significantly increased with the increase in either toxin concentration or the duration of exposure. The longevity of the moths was not affected by larval treatment and the data showed no clear correlation with the concentration and time of larval exposure to the toxin.