CORVALLIS, Oregon – Most people shudder when they see maggots in their compost bin or compost pile. They may be surprising in their appearance and movements, but they won’t hurt you or the compost. In fact, they play a role in the breakdown of plant and animal tissues.
Often these maggots are the larval life stage of compost-dwelling soldier flies, according to Linda Brewer, a soil scientist with the Oregon State University Extension Service. This European insect (Hermetia illucens) is found throughout North America. They are especially common where flies access moist, decaying food sources or garbage.
Young soldier fly larvae are gray-white in color, segmented, about 1 inch in length, and very active. As they mature, the larvae take on a dark brown color, are torpedo-shaped and flattened, with tough-looking skin. The head is small and narrower than the body. The body has only hair and spines – no legs. The back of the body is blunt and harbors respiratory pores or spiracles.
When the larvae pupate, or if they die before they mature, the chitin – a component of their tough skin – will break down and contribute nitrogen and glucose to the compost. Decomposer bacteria benefit from both nutrients, Brewer said.
Adult soldier flies are black and about 5/8 inch long. Their smoky black wings are held over their backs when at rest, and the first abdominal segment has light areas. Adults feed and lay eggs on food scraps and other moist types of organic matter.
Adult soldier flies emerge, mate, and die within two days. The dark-colored adults are often mistaken for black wasps, but they don’t sting or carry disease, Brewer said.
Soldier fly larvae are voracious consumers of high nitrogen content materials such as kitchen food scraps and manure as they decompose. They break down organic matter in manure or compost heaps. These materials are then decomposed by smaller members of the decomposer community. Adult flies can inoculate compost with beneficial decomposer bacteria and fungi from other sources.
Soldier flies rarely invade homes unless a compost pile is close to the structure, Brewer said. They are almost exclusively found in compost bins, or juicy mulches and manure piles. In the southeastern United States, they are used to reduce the volume of hog manure.
In addition to reducing manure volume, soldier fly larvae also reduce moisture and stabilize plant nutrients in manure within their own bodies.
Female soldier flies lay their eggs on the surface of exposed nitrogen-rich material. Incorporating enough leaves, dry grass, shredded paper and other “brown” organic matter into the pile to cover any nitrogenous food sources is a surefire way to avoid attracting egg-laying females. Kitchen scraps incorporated into any household compost pile should be well buried in the pile and covered with two to four inches of dry, absorbent material, according to Brewer.
If your food-rich compost contains many soldier fly larvae, you can reduce their numbers by turning the compost and layering dry materials. The presence of soldier fly larvae indicates that the compost is very moist. Water has an enormous ability to absorb heat without changing its temperature. The extra dry materials and turning will encourage the compost pile to warm up. Soldier fly larvae cannot tolerate temperatures above 113 degrees Fahrenheit – fairly easy to achieve in a well-balanced compost pile.
You can further discourage these flies by covering any holes in the compost bin with a window screen taped with exterior caulk or other waterproof caulk to exclude adult flies looking for good sites to lay their eggs.
Soldier fly larvae can also thrive in worm bins, where they can compete with worms for food sources.
In a worm bin, bury food scraps deep in the worm bedding and cover well. Soldier flies can eat anything on the surface, but they won’t eat worms or their eggs.
Birds love protein-rich soldier fly larvae, Brewer said. The larvae can be removed from the compost heap and fed to the chickens or simply thrown on the ground where the birds can find them.