Acute toxicity and sub-lethal effects of a pyrethroid (cypermethrin) on survival, development and fitness ofHelicoverpa armigera

The chickpea pod borer, Helicoverpa armigera (Hübner) is one of the most destructive pests of India and most of the countries. In this experiment, sub-lethal effects of cypermethrin were evaluated against sixth instar larvae of this pest that had ingested insecticide-treated chickpea pods. The LC50, LC30 and LC10 values of cypermethrin were 80.38, 26.65 and 5.41 mg a.i. L−1, respectively, against sixth instar. The sub-lethal effects on fitness, eggs, larvae, pupae, adult longevity and reproduction were observed in H. armigera population that survived exposure to the sub-lethal doses of LC50, LC30 and LC10 of cypermethrin. Survivorship was reduced to 36 days if H. armigera exposed to the sub-lethal dose of LC50, whereas it was 42 days in unexposed groups. Larvae treated with LC50 dose exhibit lowest intrinsic rate of increase (rm) (0.0413 females/female/day) and highest (0.0517) with the unexposed group. Time taken by H. armigera to complete one generation (Tc) was 40.09 days, while this time significantly lowered to 32.85 days when exposed to the sub-lethal dose of LC30. Hatching eggs were greatly reduced in the larvae exposed to cypermethrin at all the sub-lethal doses. About 430 eggs/female/generation were recorded after treatment with 80.38 mg/L as compared with untreated females (1390). Larvae that were exposed to sub-lethal concentrations of cypermethrin exhibited lower pupal weight and prolonged pupal developmental times compared control larvae. The overall fitness performance was better in the unexposed population as compared to the population exposed to sub-lethal doses.