What Temperature Silkworms Actually Need

Heat-Sensitive by Design

Bombyx mori is a notably heat-sensitive insect, and the reason is domestication. Intensive selection appears to have deprived it of the chance to acquire thermotolerance — the animal was never asked to survive a hot summer unaided, so it never developed the capacity to.

Where the Ceiling Is

Viability and cocoon traits decline sharply above 28 °C (about 82 °F). That is not a gentle tail-off; it is the point at which the important qualitative characters start to fail.

Research comparing conditions gives a clear picture of the two regimes: control rearing at around 26 °C with 85% relative humidity, against heat-stress conditions at 34 °C.

Humidity Makes It Worse, But Only When Hot

A useful nuance: high humidity is harmful mainly in combination with high temperature. Damp alone is not the problem — damp and hot together is. Which is exactly the condition a sealed plastic cup on a warm shelf produces.

This is the same mechanism behind touffée, the non-infectious form of flacherie brought on by heat stress rather than a pathogen.

What the Animal Does About It

What defence it has runs on heat shock proteins — HSPs, a conserved emergency system that helps other proteins keep their shape under stress. Silkworms upregulate them under heat, and strains differ in how well they manage it, which is why thermotolerance is something breeders screen for.

The Practical Summary

Keep them at ordinary room temperature and ventilate. The two ways keepers destroy a cup are opposite ends of the same axis: refrigerating them, and leaving them somewhere warm and closed. Cold kills outright; hot and humid kills through stress and then disease.

Related: do silkworms need water? · the four silkworm diseases