Host crop influence on the susceptibility of the American bollworm, Helicoverpa armigera, to Bacillus thuringiensis ssp kurstaki HD-73

Studies on the susceptibility of F-1 neonates of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) collected from chickpea in Delhi and cotton in Punjab, Haryana and Rajasthan in northern India, to Bacillus thuringiensis ssp. kurstaki HD-73, and the impact of host crop diets on insect susceptibility, were carried out by diet incorporation bioassays. The susceptibility of F-1 neonates of H. armigera to Bacillus thuringiensis ssp. kurstaki HD-73 ranged from twofold (LC50 96 h, 84.5-164.2 mug (ai) l(-1)) for chickpea to about fivefold (LC50 96 h, 51.1-247.7 mug (ai) l(-1)) for cotton. The F-1 neonates of insects collected from pearl millet were twice as tolerant as those collected from cotton and sunflower at Sirsa to B. thuringiensis ssp. kurstaki HD-73, suggesting that there was an influence of host crops on insect susceptibility. Insects originally collected from cotton fields at Bhatinda and reared for four generations on a chickpea-based meridic diet were used to initiate host-specific colonies of H. armigera. These host-specific colonies were allowed to complete one generation on meridic diets prepared with different hosts, viz., cabbage, cauliflower, chickpea, green pea, pearl millet, and pigeon pea. Larvae of H. armigera were heaviest on the 15th day, and had a higher growth rate on a pigeon pea-based diet than all other host diets. The larval period was shorter on chickpea and pigeon pea, with higher percentage pupation than all other host-diets. The pupal weight of H. armigera was greater on chickpea and pigeon pea diets than on other host diets. The growth and development of larvae was significantly poorer on pearl millet diet than on other host diets. The F-1 neonates of H. armigera belonging to cabbage, cauliflower, and pearl millet host-specific colonies were more susceptible than those belonging to chickpea, green pea, and pigeon pea host-specific colonies to B. thuringiensis ssp. kurstaki HD-73, suggesting the importance of proteinaceous nutrients in tolerance. The F-1 neonates of the pearl millet colony of H. armigera grown on a chickpea-diet for 4 days were significantly more tolerant to B. thuringiensis ssp. kurstaki HD-73 than those reared on the pearl millet-based diet. These studies show the impact of the host diet of H. armigera on tolerance to B. thuringiensis.