Seventy-eight Bacillus thuringiensis isolates were selected for a screening against the Lepidoptera species Agrotis segetum to search the higher insecticidal activity. In a preliminary bioassay, the spore-crystal mixture of 78 B. thuringiensis isolates was tested against L1 larvae of A. segetum. Fifty-two isolates had more than 60% corrected mortality after 3 days. Seven isolates caused a corrected mortality of 100% on A. segetum. Twelve isolates were selected for a second bioassay investigating the effect of the vegetative insecticidal protein (Vip) against third-instar larvae. After 7 days, the weight gain and the larval stage of each larva were recorded. This bioassay showed an aberration in larval growth increases, morphology, and weight gain. After plasmid pattern analysis, the most active strains are most likely B. thuringiensis kurstaki strains expressing the Vip3A toxin. The absence of two proteinase activities observed in the case of Cry1Ac would be the consequence of the difference in susceptibility of A. segetum to the toxins used.
Keywords: Agrotis segetum; Bacillus thuringiensis; Bioassays; biochemical characterization; vegetative insecticidal proteins; zymogram.