Contact and stomach toxicity of four combination insecticides against Spodoptera litura

The contact and stomach toxicity of four combination insecticides viz., indoxacarb+cypermethrin, indoxacarb+fipronil, chlorpyrifos+fipronil and chlorpyrifos+cypermethrin each at 0.09, 0.07, 0.05, 0.02, 0.009, 0.006 and 0.003% were determined against 7d old larvae of Spodoptera litura by leaf dip bioassay method. Indoxacarb in combination with cypermethrin was more toxic (RT= 2.55 at LC₉₀) than its combination with fipronil (RT=1.00 at LC₉₀). The dosage mortality response at 24HAE, further indicated that indoxacarb in combination with cypermethrin was more toxic than indoxacarb+fipronil and chlorpyrifos+fipronil, the LC₃₀, LC₅₀ and LC₉₀ values being 0.004, 0.01 and 0.074%; 0.006, 0.017 and 0.183%; and 0.023, 0.056 and 0.049%, respectively. The same trend was followed at 48HAE too. Cypermethrin combination with chlorpyrifos proved least toxic while its combination with indoxacarb made it most toxic out of the four combinations. The LT₅₀ of indoxacarb+cypermethrin and indoxacarb+fipronil was 5.20 and 12.94h and the LT₉₀ was 59.91 and 194.20h, respectively at a concentration of 0.05%. The same trend in the duration mortality response between the two combination insecticides has been observed at other concentrations viz., 0.02, 0.009 and 0.006%. More time was required to affect 90% mortality than 30 and 50%. A comparison of duration mortality response of chlorpyrifos based insecticides, involving fipronil and cypermethrin indicated that fipronil based combination was quicker in action than the one containing cypermethrin.