Acute, sublethal and combination effects of azadirachtin and Bacillus thuringiensis toxins on Helicoverpa armigera (Lepidoptera: Noctuidae) larvae.

The efficacy of neem (1500 ppm azadirachtin (AI)), Delfin ® WG™, a biological insecticide based on selected strain of Bacillus thuringiensis Berliner (Bt) subspecies kurstaki , and Cry1Ac protein, either individually or in combination, were examined against first to fourth instar Helicoverpa armigera (Hubner) 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 EC 50 or EC 95 levels not only enhanced the toxicity, but also reduced the duration of action when used in a mixture. The LC 20 and LC 50 values for Cry1Ac toxin were 0.06 and 0.22 μg ml −1 , respectively. Bt–azadirachtin combinations of LC 50 +EC 20 and LC 50 +EC 50 result in 100% mortality. The mortality also was significant in LC 20 +EC 20 and LC 20 +EC 50 mixtures. 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.