The legume pod borer, Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) has developed high levels of resistance to conventional insecticides, and therefore, efforts are being made to develop transgenic chickpea expressing toxin genes from the bacterium, Bacillus thuringiensis (Bt) for controlling this pest. However, there is an apprehension that acid exudates in chickpea might interfere with biological activity of Bt. Therefore, we studied the biological activity of Bt (Biolep R ) on four chickpea genotypes with different levels of resistance to H. armigera under field conditions, and by incorporating lyophilized lead and pod tissue into the artificial diet with and without Bt. The pH of the acid exudates varied from 2.1 to 2.90, and malic and oxalic acid were the major components of the acid exudates in different chickpea genotypes. There was no survival of H. armigera larvae in chickpea plants spayed with 0.1, 0.2 and 0.5% of Bt. There was a significant reduction in larval survival, larval and pupal weights and fecundity, and prolongation of larval and pupal periods in chickpea plots sprayed with Bt (0.05%) as compared to the unsprayed plants. Biological activity of Bt was lower on artificial diets with leaf or pod powder of chickpea genotypes, which might be because of a low intake of Bt toxins due to antifeedant effects of acid exudates in the chickpea or reduction in biological activity of Bt due to the interaction of biochemical constituents in chickpea with the Bt toxins. Larval survival, larval and pupal weights, pupation and adult emergence were significantly lower on diets with leaf or pod powder of the H.armigera-resistant genotypes than on the susceptible check. Chickpea genotypes with resistance to H.armigera acted in concert with Bt to cause adverse effects on the survival and development of this insect.The results suggested that development of transgenic chickpeas expressing toxin genes form Bt will be quite effective for controlling of the pod borer, H. armigera.

The gram pod borer, Helicoverpa armigera, is one of the most important constraints to chickpea production. High acidity of chickpea exudates is associated with resistance to pod borer, H. armigera; however, acidic exudates in chickpea might influence the biological activity of the bacterium, Bacillus thuringiensis (Bt), applied as a foliar spray or deployed in transgenic plants for controlling H. armigera. Therefore, studies were undertaken to evaluate the biological activity of Bt towards H. armigera on chickpea genotypes with different amounts of organic acids. Significantly lower leaf feeding, larval survival and larval weights were observed on ICC 506EB, followed by C 235, and ICCV 10 across Bt concentrations. Leaf feeding by the larvae and larval survival and weights decreased with an increase in Bt concentration. However, rate of decrease in leaf feeding and larval survival and weights with an increase in Bt concentration was greater on L 550 and ICCV 10 than on the resistant check, ICC 506EB, suggesting that factors in the resistant genotypes, particularly the acid exudates, resulted in lower levels of biological activity of Bt possibly because of antifeedant effects of the acid exudates. Antifeedant effects of acid exudates reduced food consumption and hence might reduce the efficacy of Bt sprays on insect-resistant chickpea genotypes or Bt-transgenic chickpeas, although the combined effect of plant resistance based on organic acids, and Bt had a greater effect on survival and development of H. armigera than Bt alone.

Please cite this article as: Surekha Devi, V., Sharma, H.C., Arjuna Rao, P., Influence of oxalic and malic acids in chickpea leaf exudates on the biological activity of CryIAc towards Helicoverpa armigera, Journal of Insect Physiology (2013), doi: http://dx