Persistent Toxicity of Some Microbial Pesticides against Bombyx mori (L.) Feeding on Mulberry Leave in Terai region of West Bengal

The persistent toxicity of two Bacillus thuringiensis var. Kurstaki formulations (B.t.k 55000 SU mg⁻¹ and B.t.k 32000 IU mg⁻¹), one from Avermectin-1.8%w/v (Vertimec), and Beauvaria bassiana-1x10⁻¹ spore mll(Bassina) including Chlorpyrrphos @ 0.1%, 0.1%, 0.1%, 0.2% and 0.05% respectively was evaluated against 3^rd instar larvae of Bombyx mori (L) on mulberry leaves. Avermectin, a microbial toxin was found most toxic compound causing 100% mortality on first day of spraying followed by B.t.k 55000 SU mg⁻¹ (96.49%), chlorpyriphos (85.95%) and B.t.k 32000 IU mg⁻¹ (80.67%). However all the pesticides lost their toxicity with the progression of days. The respective PT and LT₅₀ values were recorded as 298.32 and 3.16 (avermectin), 278.82 and 2.74 (chlorpyriphos), 247.92 and 2.64 (B.t.k 55000 SU mg⁻¹) and 203.55 and 2.11 (B.t.k 32000 IU mg⁻¹). The order of relative toxicity based on PT and LT₅₀ values were remained the same.