While studying muscardine in silkworms in the 19th century, Agostino Bassi found that the causal agent was a fungus. This was the first demonstration of the germ theory of disease, the first time a microorganism was recognized as an animal pathogen.
Metarhizium species such as M. anisopliae can cause fatal disease in over 200 species of insect.
Green muscardine is caused by Nomuraea rileyi. Keepers of silkworms recognize symptoms such as dark brownish lesions with lighter centers on the sides and back of the larva. At death the larva turns white and within a few days it is covered in a bright green fungal coating.
When suffering from white muscardine, an insect larva may become inactive and stop eating. The elasticity of its cuticle is lost and it may experience vomiting and diarrhea. As it dies it hardens. The fungus leaves the body of its host covered in powdery white conidia. The fungal layer is tough due to oxalate crystals, and this slows the decay of the body. When a pupa is infected, it often mummifies. It shrinks and wrinkles before growing a fungal coating. In an adult moth, the body hardens and the wings drop off.
During infection, the fungus absorbs water and nutrients from the host. The hemolymph of the insect crystallizes and thickens. The fungus usually produces toxins, as well. After it kills the host, the fungus continues to absorb water from the body, causing it to harden further.
Yellow red muscardine
Fungicidal agents such as azadirachtin and phytoallexin have been used against some muscardine pathogens. Silkworm breeders dust their cages with slaked lime to discourage fungal growth. In India a dust of chaff soaked in formalin is applied to the larvae.
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