CORVALLIS, Ore. — Most people shudder when they see maggots in a bin composter or compost pile. They may look startling, but they won’t harm you or the compost. In fact, they help break down plant and animal tissues.
Often, these “maggots” are the larval stage of soldier flies (Hermetia illucens), according to Linda Brewer, Oregon State University Extension Service soil scientist. This European insect is now widespread across North America. Soldier flies are especially common where flies have access to wet, rotting food or garbage.
Identifying soldier fly larvae
Young larvae are gray-white, segmented, about 1 inch long and very active. As they mature, they turn dark brown, become flattened and torpedo-shaped, and develop tough-looking skin. Their head is small and narrower than the body, which features only hairs and spines — no legs. At the rear are breathing pores called spiracles.
When larvae pupate or die, the chitin in their skin decomposes, contributing nitrogen and glucose to the compost. Decomposer bacteria thrive on both nutrients, Brewer said.
Adult soldier flies are about 5/8 inch long, black with smoky wings held flat over the back, and marked with clear areas on the first abdominal segment. They live only two days, during which they mate and lay eggs on moist organic matter. Adults are often mistaken for black wasps, but they don’t bite or carry disease.
Role in compost and manure
Larvae are voracious consumers of nitrogen-rich materials such as food scraps and manure. They accelerate decomposition and inoculate compost with beneficial bacteria and fungi. In the southeastern United States, they are used to reduce the volume of hog manure.
Soldier fly larvae also:
- Break down organic material in manure or compost piles
- Reduce manure moisture and volume
- Stabilize plant nutrients in their own bodies, making them available to the soil when decomposed
Soldier flies are rarely found indoors, Brewer said, unless a compost pile is located close to a house.
Managing soldier fly populations
Female flies lay eggs on exposed nitrogen-rich material. To discourage them, cover food scraps with carbon-rich “brown” material such as:
- Leaves
- Dry grass
- Shredded paper
Bury kitchen waste well in the compost pile and cover it with 2 to 4 inches of dry, absorbent material.
If larvae become abundant:
- Turn the compost and mix in dry material
- Allow the pile to heat up — soldier fly larvae can’t survive above 113°F
- Cover holes in compost bins with window screen attached using waterproof caulk to exclude egg-laying adults
In worm bins, bury food scraps deeply in bedding to reduce larvae competition. Soldier flies may eat surface scraps but won’t harm worms or their eggs.
Nature’s recyclers — and bird food
Soldier fly larvae play a useful role in composting. Birds also benefit, since larvae are high in protein. They can be removed from compost piles and offered to chickens or left on the ground for wild birds to find.
Previously titled Don’t worry, maggots help break down compost pile
