Feeding rate and survival ofPlutella xylostella(L.) larvae (Lepidopteria: Plutellidae) after intoxication byBacillus thuringiensisberliner var. kurstaki and var.Aizawai

Laboratory tests were conducted to determine the effect of two varieties of Bacillus thuringiensis (Bt) on food consumption and survival of diamondback larvae, Plutella xylostella L. Third instar larvae were allowed to feed for 48 h on cabbage disks treated with a series of concentrations (0.006 to 100 μg AI/ml water) of the formulations Dipel 2X (6.4% AI; Bt var kurstaki) and Xentari (10.3% AI; Bt var aizawai). Surviving larvae were transferred to untreated leaves to complete their life cycle until pupation or death. After two days of feeding on treated leaves, Bt subsp. aizawai with LC50 (0.82 μg/ml) was less efficacious than Bt subsp. kurstaki with LC50 (0.45 μg/ml). Moreover, mortality of larvae on treated leaves increased, whereas food consumption was reduced with increasing concentrations of delta‐endoxin. Consumption of untreated leaves by surviving larvae and duration of larval stage were reduced significantly in both cases (expected 0.006 μg AI/ml) in comparison to control. Reduction in duration of larval stage by Bt var. kurstaki was more effective compared to Bt var. aizawai.