Acute, sublethal and combination effects of azadirachtin andBacillus thuringiensistoxins onHelicoverpa armigera(Lepidoptera: Noctuidae) larvae

Abstract

AbstractThe efficacy of neem (1500 ppm azadirachtin (AI)), Delfin®WG™, a biological insecticide based on selected strain ofBacillus thuringiensisBerliner (Bt) subspecieskurstaki, and Cry1Ac protein, either individually or in combination, were examined against first to fourth instarHelicoverpa armigera(Hübner) larvae. Using an oral administration method, various growth inhibitory concentrations (EC) and lethal concentrations (LC) were determined for each bioagent. Combinations of sublethal concentrations of Bt spray formulation with azadirachtin at EC50or EC95levels not only enhanced the toxicity, but also reduced the duration of action when used in a mixture. The LC20and LC50values for Cry1Ac toxin were 0.06 and 0.22 μg ml−1, respectively. Bt–azadirachtin combinations of LC50+EC20and LC50+EC50result in 100% mortality. The mortality also was significant in LC20+EC20and LC20+EC50mixtures. These studies imply that the combined action is not synergistic but complimentary, with azadirachtin particularly facilitating the action of Bt. The Bt spray–azadirachtin combination is more economical than combinations that involve isolating the toxic protein, as the Bt spray formulations can be combined in a spray mixture with neem. These combinations may be useful for controlling bollworm populations that have acquired resistance to Bt as they may not survive the effect of mixture. Azadirachtin may be useful as a means of reducing the endotoxin concentrations in a mixture, to promote increased economic savings and further reduce the probability of resistance development to either insect control agent.