Variability in resistance pattern of various groups of insecticides evaluated against Spodoptera litura (Fabricius) during a period spanning over three decades

Among the eighteen insecticides tested against the third instar larvae of Spodoptera litura (Fab.), deltamethrin was found to be the most toxic insecticide on the basis of LC₅₀ values, followed by lambdacyhalothrin, cypermethrin, fenvalerate, fenpropathrin, methyl parathion, chlorpyriphos, phosphamidon, endosulfan, quinalphos, monocrotophos, malathion, pyrethrum, fenitrothion, lindane, fenthion and dimethoate. The former fourteen insecticides were found to be more toxic than lindane, being 575.92,340.00, 215.42, 193.95, 142.53, 39.30, 12.24, 7.69, 6.58, 5.85, 5.59, 3.73, 1.15 and 1.09 times, respectively as toxic as lindane. Fenthion and dimethoate were less toxic than lindane, being 0.76 and 0.75 times as toxic as lindane. Carbaryl (2%) gave only 10% larval mortality, while nicotine sulphate (2%) gave nil mortality. The resistance built-up in S. litura to various in.secticides of different groups during the last three decades was evident. An increase in the LC₅₀ value of pyrethrum against S. litura during the period 1961–1995 indicated development of resistance to the extent of 20.62-fold. S. litura developed 14.71 and 23.05-fold resistance to endosulfan and lindane, respectively during 1961–1995. S. litura developed resistance to all the organophosphates during the last quarter century with the degree of resistance development varying from 1.82-fold in monocrotophos to 39.14-fold in fenitrothion. Dimethoate, malathion, fenthion and methyl parathion recorded 9.39, 4.37, 4.01 and 3.59-fold resistance respectively, during 1970–1995. In the case of synthetic pyrethroids, S. litura developed 9.03, 1.45 and 98.00-fold resistance to cypermethrin, fenvalerate and deltamethrin within a short span of eight years (1987–1995).