Effects of Beauveria bassiana on immune responses in the silkworm, Bombyx mori: focus on haemocyte dynamics

The immune responses of insects are crucial for their defense against invasive foreign pathogens. This study investigates the interactions between entomopathogenic fungi and insect immune responses, specifically examining the effects of Beauveria bassiana on the circulating blood cells of silkworms (Bombyx mori). Utilizing haemocytometry, electron microscopy, and trypan blue staining, we assessed total blood cell counts along with granulocytes (GR) and plasmatocytes (PL). Significant reductions in total circulating blood cells of silkworms were observed across all treatment groups compared to the control (P < 0.05) at concentrations of 1 × 10⁵, 1 × 10⁶, and 1 × 10⁷ spores/mL. Notably, the 1 × 10⁷ spores/mL treatment resulted in a marked decrease in blood cell numbers at 3 h post-infection, indicating a robust cellular immune response. The GR count initially increased but decreased significantly at 3 h post-treatment in the highest concentration group, likely due to their rapid consumption during heightened immune activity. In contrast, the PL count decreased significantly at 0 h post-treatment and continued to decrease, reaching a minimum at 6 h. Scanning and transmission electron microscopy revealed morphological alterations in blood cells, including membrane damage and cytoplasmic leakage, which correlated with the observed declines in cell viability. The survival rate of blood cells significantly decreased, reaching a low of 21.24 % at 36 h post-treatment in the 1 × 10⁷ spores/mL group. Additionally, an increase in nodule formation was observed, peaking at 6 h post-infection in the high-concentration treatment, indicating an active immune response. These findings illustrate the pathogenic effects of B. bassiana on silkworm blood cells and enhance our understanding of immune system dynamics in response to fungal infection, providing insights into the potential applications of entomopathogenic fungi for biological control strategies in pest management.