They can feed on organic waste, from food scraps to kitchen waste. They also feed on coffee grounds, meat, and fish scraps. The larvae turn all the organic and leftover food waste into animal feed. They can eat twice their body weight daily and convert and store the food they eat quickly. This makes them an extremely nutritious feed for chickens.

Unlike houseflies, adult black soldier flies have greatly reduced sponging mouthparts, so can only consume liquids such as flower nectar or not eat at all. They do not regurgitate food along with digestive enzymes as do houseflies, thus do not spread diseases.

They feed on a wide range of organic materials, including manure, spent grains from brewing, household food waste, grocery or restaurant waste, paper mill sludge, and carrion (dead animals).

Black soldier fly larvae are voracious eaters and are primarily fed with organic waste materials. They can consume a wide range of organic matter, including fruits, vegetables, food scraps, and agricultural by-products.

The most common species of soldier fly, Hermetia illucens, is commonly considered a filth fly because of its habit of breeding in manure and garbage. The feeding habits of the larva, however, are not easy to classify. The larval stage of the soldier fly feeds on decaying organic material including manure and very moist, rotting vegetable matter.