An electron microscope study of the toxic action of Bacillus sphaericus in Culex quinquefasciatus larvae

Culex quinquefasciatus treated with Bacillus sphaericus undergo progressive poisoning which is characterized by cessation of feeding by 4 hr and general body paralysis by 36 hr. A variety of histopathological lesions were observed in the midgut, central nervous system, and the skeletal muscle system. The midgut had cellular damage as early as 4 hr, whereas the central nervous system and skeletal muscles were impaired later. Midgut lesions were characterized by the occurrence of lysosomes in the cytoplasm of epithelial cells. With time, the lysosomes increased in number and size which resulted in complete destruction of epithelial cells by 36 hr. At this stage, damaged mitochondria and lamellar bodies were detected in the neural tissue, and skeletal muscles contained impaired myofibrils and swollen mitochondria. The myoepidermal cells, which serve as a structural link between skeletal muscles and the cuticle, were also damaged. These results suggest that besides the midgut disruption, central nervous system and skeletal muscle lesions may be crucial for onset of paralysis in B. sphaericus-treated mosquito larvae.