What Is Black Soldier Fly Frass, and How Do You Use It?

Frass is insect excrement mixed with shed exoskeletons and the leftovers of whatever the larvae were eating. In a black soldier fly system it is what remains in the bin once the larvae have gone.

It is not a waste product. It is the second product.

What It Is Used For

Frass is used as a soil amendment and fertiliser. It carries nitrogen, phosphorus and potassium, and unlike raw manure it comes out of the bin already substantially broken down, because the larvae did the breaking down.

The Chitin Angle

Frass contains chitin from the shed larval skins. This is the part that gets the most research interest, because chitin in soil is associated with plant defence responses — plants appear to react to it as a signal that fungal attack may be coming, and prime their defences accordingly. Research on this is active rather than settled, and results vary by crop and soil.

Using It Sensibly

• Let it finish. Fresh frass straight from an active bin still contains undigested food. Give it time to stabilise before putting it on plants.
• Use it as an amendment, not a growing medium. It is concentrated. Mixed into soil rather than used neat.
• Know what went in. Frass carries the chemistry of the input. A bin fed on unknown material produces an amendment of unknown composition.

The larger point: a black soldier fly system converts organic waste into two outputs — animal protein and a soil input — with nothing needing to go to landfill in between.