Silkworm Diseases: The Four That Matter

Four Diseases, Four Different Causes

Silkworm losses in sericulture come down overwhelmingly to four diseases — and they are caused by four entirely different kinds of organism: a parasite, a virus, bacteria, and a fungus. Telling them apart matters, because nothing that helps with one helps with another.

Pébrine — a microsporidian parasite

Caused by Nosema bombycis. Tiny dark pepper-like spots on the larva. Uniquely among the four it passes from mother moth to offspring through the egg, which is why control is by inspecting breeding stock rather than treating worms. See the full story of pébrine and Pasteur.

Grasserie — a virus

Caused by a nuclear polyhedrosis virus. The larvae go shiny and bloated, and the skin ruptures. It is the most visually distinctive of the four — a swollen, glossy caterpillar leaking fluid is grasserie until proven otherwise.

Flacherie — from what they ate

Caused by eating infected or contaminated mulberry leaves. There are two forms: an infectious viral flacherie, and a non-infectious form brought on by heat stress, known as touffée. Both are effectively a lethal diarrhoea. A limp, slack larva that has stopped feeding is the picture.

Muscardine — a fungus

White muscardine is caused by Beauveria bassiana, an insect-killing fungus. It attacks larvae and pupae, and infected animals eventually become hard and chalky as the fungus fruits through the body.

What This Means in a Cup of Feeders

These are diseases of dense commercial rearing, not usually of a cup on a shelf. But the shape of them is worth knowing: most silkworm death in a home is husbandry — cold, damp, or stale food — rather than disease. Chilling a cup kills far more silkworms in the reptile hobby than any pathogen does.

The exception is buying from an unreliable source. Pathogens travel with stock, and that is why commercial eggs are certified.

Related: the four silkworm diseases · raising silkworms at home