Fungus flies are widespread indoor pests that look like mosquitoes but don’t bite and are frequently found around plants. The larvae feed on roots, root hairs, fungus, and other organic material in wet, rich soil. Fungus flies are so minute that you might not see them unless they swarm. They are sometimes mistaken for fruit flies due to their small size. Adult flies measure approximately 1/8″ long, have lengthy legs, antennae, and one pair of wings, and are grey to black in appearance. Each larva is 1/4 inch long, with a pale or translucent body and a gleaming blackhead.
Adults may lay up to 300 eggs in rich, wet soils and survive for approximately a week. During their two-week lifespan, small larvae emerge within 4-6 days and begin feasting on plant roots. Before young adults leave the earth and begin the next generation, the pupal stage lasts 3-4 days. Depending on the climate, the full life cycle from egg to adult can be accomplished in as little as 3-4 weeks. Potted plants may host each stage — egg, larvae, pupae, and adult — in numerous generations at once because of their propensity and relatively brief gestation.
Fungus gnats spend most of their time on the soil surface of potted plants, although they can also be spotted flying around the pot’s outside edge or near drainage holes. Because they are not strong fliers, they prefer to wander along the ground and only fly in brief spurts. Their flight pattern is irregular, and they are much slower than fruit flies, resembling mosquitoes in flight.
Because adult fungus flies do not bite or feed, they pose no hazard to humans or plants. However, if they are not regulated and allowed to breed in great numbers, they may become a nuisance. Fungus flies’ larvae are a minor hazard to young plants. Larvae usually feed on organic materials present in the soil, as well as the roots of indoor plants on occasion. Because the roots of seedlings and other less-established houseplants are so sensitive, they are particularly vulnerable to injury.
Fungus gnats lay their eggs on decaying organic materials, which are abundant in plant detritus. As a result, it’s critical to keep detritus like fallen leaves, blooms, and fruit out of the soil around your plants. If you have fungus gnats surrounding your houseplants, use a potting mix without composted materials such as processed forest products, bark fines, or real compost.