Nighttime Application of Talc-based Bacillus thuringiensis Enhances Caterpillar Control in Zero-Tillage Paddy-Straw-Mulched Potato System: A Sustainable Alternative to Synthetic Practice

Abstract

Lepidopteran caterpillars are important defoliating and boring pests in potato cultivation because their feeding reduces photosynthetic leaf area, weakens crop recovery and may indirectly lower tuber yield. In conventional vegetable and potato-based farming systems, caterpillar suppression is commonly attempted with synthetic insecticides such as chlorantraniliprole, emamectin benzoate, spinosad, indoxacarb, flubendiamide, lambda-cyhalothrin and profenophos. Although these molecules may provide rapid knock-down or feeding inhibition, repeated use can increase production cost and may raise concerns related to pesticide residue, farmer and labour exposure, non-target toxicity, natural-enemy suppression, aquatic toxicity and resistance development. The present study was designed from an organic and low-cost farming perspective to evaluate whether a talc-based Bacillus thuringiensis var. kurstaki formulation can act as an effective biological replacement for toxic synthetic caterpillar-control insecticides in potato cultivation rather than being mixed with them. A talc-based Bt formulation containing approximately 108 CFU g-1 and costing about Rs. 150 kg-1 was evaluated during the 2021-22 potato season under zero-tillage paddy-straw-mulched cultivation using store-bought Kufri Jyoti potatoes as low-cost seed tubers. The study followed three linked steps: first, the harmful implications of conventional caterpillar pesticides were considered; second, Bt was selected as a low-residue biological alternative intended to reduce chemical exposure for tubers, farmers and labourers; and third, a spray-timing experiment was conducted to identify a more effective timing for caterpillar control. Twenty-four independent 15 ft x 5 ft beds were analysed across four Bt timing treatments. Mean yield per 75 sq ft was highest under preventive night spray before visible caterpillar attack (16.50 kg), followed closely by night spray after three days of attack (16.00 kg), whereas morning spray after attack produced the lowest mean yield (7.08 kg). One-way ANOVA showed a highly significant treatment effect on yield, F(3,20) = 28.08, p = 2.27 x 10^-7. Welch ANOVA confirmed significance under unequal variance, F(3,8.61) = 29.76, p = 6.99 x 10^-5. The result suggests that the Bacillus thuringiensis application may be serve as alternative of toxic synthetic caterpillar control insecticides if the bacterial formulation spray is used in proper time to give enough time for stabilization on the leaf surface. Once the bacterial inoculant gets attached to the leaf surface, immediate caterpillar threats have been reduced.