Silkworm Frass as Fertiliser

The Other By-Product

Rearing silkworms produces two things in bulk that are not silk: pupae, and frass — the droppings. A tray of feeding fifth-instar larvae generates a startling amount of it, and it is not waste either.

What the Research Reports

Studies applying silkworm frass to crops report that it:

• increases crop yield
• improves plant growth and nutritional quality
• raises the nutrient and organic matter content of the soil
• increases soil enzyme activity — a measure of how biologically alive a soil is

Work on frass as a nitrogen fertiliser for crops such as tomato has looked at the economics as well as the agronomy, and frass from edible insects generally is now studied as a fertiliser class in its own right.

Why It Works

The logic is simple once stated. A silkworm eats mulberry leaf and very little of it is retained — the animal is a conversion step, not a sink. What comes out is largely processed plant matter, partly broken down, in a form soil organisms can work with immediately.

For a Home Culture

If you keep silkworms in any quantity, the frass at the bottom of the container is a soil amendment rather than something to bin. Two practical notes: it should be composted or aged rather than applied heavily to a plant directly, as with any fresh manure, and frass from worms fed on unknown wild-picked leaf carries whatever those leaves carried.

Frass from chow-fed worms has a known input, which is one more quiet argument for artificial diet.

Related: raising silkworms at home · do silkworms need water?