Effect of sublethal doses of some synthetic pyrethroids on biological parameters of tobacco caterpillar, Spodoptera litura (Fab.)1

The 8 day old larvae of tobacco caterpillar, S. litura were fed with the leaf discs treated with different insecticides viz., alphamethrin, cypermethrin, deltamethrin, lambda cyhalothrin and dimethoate (0.3 and 0.01% conc. each) under laboratory conditions continuously for two days (day 1 & day 2), and then with fresh untreated leaves until they entered pupal stage. The foliar treatment with sublethal doses of insecticides caused a significant reduction in feeding and larval weight at 2DAF over control. Synthetic pyrethroids at higher concentration resulted in a significant negative weight gain (= wt. loss) whereas with lower concentration nominal weight gain was observed for example 1 and 4 mg/larva in alphamethrin and deltamethrin respectively as against 94 and 127 mg/larva in dimethoate and control, respectively. The ingestibility per cent at 0.01 was better than 0.3% lambda cyhalothrin and dimethoate being at par at 0.01%. Larval weight loss at 0.3% of SPs was so prominent that it significantly extended the larval period by approx. 2.5d. Mean pupal weight was significantly reduced in both of the concentrations of SPs and dimethoate. Alphamethrin, cypermethrin and deltamethrin at 0.3% reduced the pupal weight by approx. 50 to 60%, whereas lambdacyhalothrin and dimethoate by 38 and 15%, respectively. Alphamethrin at 0.3% reduced the pupation by 57.1%. The adult emergence was significantly reduced (50 to 60 at 0.3% and 22 to 30% at 0.01%) by SPs in comparison of untreated control. Dimethoate (0.01 and 0.3%) was though lower but statistically at par with. No mortality was caused by SPs and dimethoate at 0.01%, however at 0.3% the maximum terminal mortality was caused by alphamethrin (21.42%) and minimum in cypermethrin (7.4%). No mortality was observed in dimethoate at 0.3% and in control.