Toxicity of insecticides to Plutella xylostella (L.) as influenced by the sex of larvae

Diamond back moth, Plutella xylostella L., the most destructive pest of Brassica vegetables, has developed field resistance to all major classes of insecticides. In the present study, the known larval sexual dimorphic character of P. xylostella was exploited to evaluate variability in toxicity of insecticides. Amongst the conventional insecticides, cypermethrin's LC50 value (a.i.) was 19.22 ppm (in male) and 28.42 ppm (in female), respectively. Likewise with chlorpyriphos it was 14.69 and 28.81 ppm, respectively. Similar variations with sexes of larvae were observed with thiodicarb and indoxacarb. Amongst the newer insecticides, emamectin benzoate gave an LC50 of 0.84 ppm (male) and 0.90 ppm (female), while it was 0.29 ppm and 0.22 ppm, respectively with flubendiamide; with chlorantraniliprole it was lower (0.11 ppm in male), and higher of 0.15 ppm (in female). With spinosad it was an LC50 of 0.51 ppm and 0.40 ppm, in male and female, respectively. Cypermethrin, chlorpyriphos, thiodicarb, indoxacarb, emamectin benzoate and chlorantraniliprole registered lower LC50 values against male larvae, whereas, flubendiamide and spinosad registered higher LC50 values against female larvae. However, these LC50 values were statistically at par indicating no significance in sexes as regards toxicity of insecticides.