Relative Efficacy of Some Biopesticides against Different Larval Instars of Pieris brassicae (Linnaeus)

Bioefficacy of different biopesticides viz. Bacillus thuringiensis var. kurstaki (Btk-55000 SU mg⁻¹) @ 0.1%, B. thuringiensis var. kurstaki (Btk - 32000 IU mg⁻¹) @ 0.1%, avermectin-1.8%w/v @ 0.1% and Beauvaria bassiana-1x107 spore mL⁻¹ @ 0.2% were tested against different larval stages of Pieris brassicae (Linnaeus) under laboratory conditions. Avermectin provided 100, 96.67, 88 33 and 73.22% mortality after 72 h of feeding to 2nd, 3rd, 4th and 5th instar, respectively, and closely followed by Btk- 55000 SU mg⁻¹. The study also revealed that potentiality of all the biopesticides sequentially decreased with the increase of larval age. Decrease in overall efficacy of avermectin, Btk-55000 SU mg⁻¹ and Btk-32000 IU mg⁻¹ was 32.78, 41155 and 61.27% from 2nd to 5th instar, respectively. Considering the LC₅₀ values, avermectin was found most toxic compound having the values against subsequent instars 131.89, 271.29, 411.18 and 538.25 ppm, respectively followed by Btk-55000 SU mg1. B. bassiana was found least toxic towards all the instars and the relative toxicity of avermectin, Btk.-55000 SU mg⁻¹ and Btk-32000 IU mg⁻¹ against 2nd instar larvae was recorded 25.97, 17.71 and 8.67 times more than that of B. bassiana respectively.