Susceptibility of Chilo partellus (Swinhoe) populations to insecticides

The experiment was conducted to generate the information on the relative susceptibility of 3^rd instar larvae of three populations of Chilo partellus (Swinhoe), reared for 8 (P₁), 3 (P₂) and 1 (P₃) generations under laboratory conditions to the recommended insecticide i.e. Decis 2.8 EC (deltamethrin) and the Bt formulation, Dipel 8L (Bacillus thuringiensis var. kurstaki strain HD-1). On the basis of LC₅₀ values, the order of susceptibility of all the populations to Decis 2.8 EC and Dipel 8L was P₁> P₂> P₃. The LC₅₀ for Decis 2.8 EC and Dipel 8L after an exposure of 48 hours, was highest in P₃ (0.00047% and 0.00796%) while for P₂, it was comparatively lower (0.00031% and 0.00605%) and for P₁ lowest (0.00005% and 0.00181%). The increased susceptibility of continuously laboratory reared populations reported herein might perhaps be due to deleterious genetic effects or inbreeding depression that occurred over generations.