Differential Toxicity of Some Insecticides Against Egg and Larval Stages of Cotton Leafworm and Role of Two Detoxification Enzymes

Toxicity of some insecticides against egg masses and two larval instars of cotton leafworm (CLW), Spodoptera littoralis, field and laboratory strains was determined. The activities of glutathione S-transferases (GST) and alkaline phosphatases (ALP) in the two strains also were compared. Results revealed that, spinosad, spinetoram, emamectin benzoate and lufenuron have low ovicidal activity. Chlorpyrifos and methomyl at 10 ppm achieved 80.4 and 83.6% mortality of treated egg masses, respectively. On the other hand, spinosad, spinetoram and emamectin benzoate at the same concentration caused 18.9, 19.4 and 28.1% mortality of treated egg masses, respectively. While lufenuron at 25 ppm caused 54.9% mortality of treated egg masses, lufenuron at 100 ppm achieved 100% mortality of treated egg masses at 100 ppm. Although spinosad, spinetoram and emamectin benzoate have low ovicidal activity compared with methomyl and chlorpyrifos, they have a comparable residual toxicity with methomyl and chlorpyrifos against the neonates. The residual toxicity of lufenuron against neonates is low at all tested concentrations. The 2nd and 4th instar larvae of the field strain exerts high resistance levels towards methomyl, chlorpyrifos and esfenvalerate. Regarding the 2nd instar, resistance ratios in 2008 and 2009 cotton seasons were 43.9 and 50.8 towards methomyl, 27.6 and 24.7 towards chlorpyrifos and 76.4 and 79.2 towards esfenvalerate. For the 4th instar resistance ratios in 2008 and 2009 cotton seasons were 62.8 and 63.6 towards methomyl, 43.9 and 49.2 towards chlorpyrifos and 112.4 and 114.8 towards esfenvalerate. On the other hand, field strain shows low or no levels of tolerance to spinosad, spinetoram, emamectin benzoate, lufenuron and methoxyfenozide. Activities of GST and ALP in the field strain were higher compared with that in the laboratory strain. In conclusion, the alternation between these insecticides can avoid increasing selection pressure of CLW populations to these insecticides.