Status of bio-efficacy of synthetic pyrethroids against tobacco caterpillar, Spodoptera litura (Fabricius).

The results indicated that amongst synthetic pyrethroids deltamethrin was the most toxic insecticide and dimethoate was the least. At 0.01%, the earliest mortality response was observed at 2 hours after exposure (HAE) in deltamethrin (6.6%), at 3 HAE in each of the cypermethrin, lambda cyhalothrin and dimethoate (3.3 to 6.6%) and at 6 HAE in alphamethrin (3.3%). The LT50 value of deltamethrin, cypermethrin, alphamethrin, lambdacyhalothrin and dimethoate was 10.30, 10.93, 14.40, 22.54, 23.80h respectively at the concentration closer to LC50 at 24 HAE. Deltamethrin therefore was the fastest acting synthetic pyrethroid amongst SPs and lambda cyhalothrin was the slowest. Dimethoate was evidently slowest than SPs. At 24h exposure, dimethoate was almost 9 times less toxic than the most active synthetic pyrethroid deltamethrin and about 1.8 times less toxic than the least toxic alphamethrin at LC50 level. The order of toxicity was: deltamethrin (242) > cypermethrin (393) > lambda cyhalothrin (771) > alphamethrin (1166) > dimethoate (2200ppm). The order of toxicity of synthetic pyrethroids at LC50 level, at 48h exposure was: deltamethrin (143) > cypermethrin (222) > lambda cyhalothrin (307) > alphamethrin (398).