Frass is the mix of droppings, shed skins and leftover feed that remains after insect larvae are sieved out. From mealworm farms it is a nitrogen-rich, fine-grained fertiliser containing chitin.
1Composition
The nutrient content depends on the larvae's diet, but frass from grain-fed mealworms is typically richer in nitrogen than worm castings. The shed exoskeletons add chitin, a polysaccharide that soil microbes break down slowly.
2Chitin and plant health
Plants recognise chitin fragments as a signal of fungal or insect attack, and research suggests that chitin in the root zone can prime their defence responses. Chitin also feeds chitin-degrading microbes, some of which compete with plant pathogens.
3Regulation
In the EU insect frass is classed as an animal by-product. Before sale as a fertiliser it must be heat-treated at 70 °C for at least 60 minutes (Regulation (EU) 2021/1925).