Dose-dependent Impact of Bacillus thuringiensis on Silkworm (Bombyx mori) Survival and Pathogenicity

The environmental impact of microbial pesticides, including Bacillus thuringiensis (Bt), is a growing concern. Although Bt is widely used for pest control due to its efficacy, understanding its potential toxicity to non-target organisms such as silkworms is essential for environmental safety. The current study was conducted to determine how different Bt doses affect larval mortality, growth, and development of silkworm (Bombyx mori) larvae. The study was conducted at Group of Pesticide environmental risk assessment and control, Institute of Plant Protection, Chinese Academy of Agricultural Sciences (IPPCAAS), between 2019 and 2020. This study investigated the dose-dependent toxicity of Bt to silkworms following the NY/T 3152.3-2017 guidelines for microbial pesticide risk assessment. A dose-response experiment was conducted using five Bt concentrations (3.16 × 10⁴, 1 × 10⁵, 3.16 × 10⁵, 1 × 10⁶, and 3.16 × 10⁶ CFU/mL), along with blank and inactivated controls. Each treatment consisted of 20 silkworms with four replicates, and the experiment lasted 23 days. The effects of Bt on silkworm survival and development were evaluated using Probit analysis and one-way ANOVA. The result revealed that the 23-day LC₅₀ was 4.48 × 10⁵ CFU/mL (95% confidence interval: 2.77 × 10⁵- 6.06 × 10⁵ CFU/mL). At 1.00 × 10⁶ CFU/mL, cocoon weight was significantly higher than in the blank control, though no significant differences were observed among other groups. Cocoon shell weight was significantly higher at 3.16 × 10⁴ and 1 × 10⁶ CFU/mL compared to the control. Pathogenicity tests confirmed that the isolated and tested strains were identical, and infected silkworms exhibited typical Bt-induced symptoms, including body decay and side-lying behavior. Overall, these findings provide important data for environmental risk assessment, indicating that careful management of Bt concentrations is critical to minimize unintended harm to beneficial organisms within agricultural ecosystems.