Fungus gnats (family Sciaridae) are small, dark flies commonly observed near houseplants and in greenhouses. These insects are generally harmless to people and pets, as they do not bite or transmit human diseases, though they can be a nuisance. The primary concern lies in their interaction with plants, where the soil-dwelling larval stage can be detrimental, particularly to seedlings, cuttings, and stressed plants. The presence of flying adult gnats often signals overwatering, as the larvae thrive in consistently moist, organically rich soil.
Distinguishing Characteristics and Life Cycle
Adult fungus gnats are slender, mosquito-like insects, typically measuring about one-eighth of an inch in length with long legs and antennae. They are weak fliers and are often seen walking rapidly across the soil surface or resting on plant leaves and nearby windows. The larval stage, which is responsible for plant damage, is a translucent, worm-like maggot with a distinct, shiny black head. These larvae live within the top two to four inches of the potting medium, where moisture is highest.
Fungus gnats undergo a four-stage life cycle: egg, larva, pupa, and adult. Females lay up to 300 eggs in moist soil, which hatch into larvae within about four days. The larval stage lasts 10 to 14 days before entering a four-day pupal stage, from which adults emerge. This rapid life cycle can complete a generation in as little as 17 days, allowing populations to increase quickly in favorable, consistently damp conditions.
The Specific Damage Caused by Larvae
Larvae feed beneath the soil surface. While they primarily consume decaying organic matter and fungi, they will turn to living plant tissue when resources are scarce or populations are high. They cause damage by chewing on fine root hairs, which are the primary structures for water and nutrient absorption, and may tunnel into main roots. This root injury impairs the plant’s ability to take up water, leading to symptoms such as wilting, yellowing foliage, and stunted growth, even if the soil moisture is adequate.
Damage is most severe for young plants, fresh cuttings, and seedlings, where root loss can be fatal, a condition often called “damping-off.” Beyond direct feeding damage, both the larvae and the adults can mechanically spread spores of plant pathogens like Pythium and Fusarium between plants. Adults are merely a persistent nuisance as they fly erratically near faces and lights.
Management and Prevention Strategies
Preventing fungus gnat infestations requires eliminating the consistently moist soil environment that their larvae need for survival. The most effective cultural control is allowing the top one to two inches of potting mix to dry out completely between waterings, which kills eggs and newly hatched larvae. Using a well-draining potting mix and ensuring containers have adequate drainage holes also discourages the waterlogging that attracts these pests.
For active infestations, an integrated approach combining physical and biological controls is effective. Yellow sticky traps placed near the plants capture flying adults, which helps monitor population size and reduces the number of females laying new eggs. To target the damaging larval stage directly, apply a soil drench containing Bacillus thuringiensis israelensis (Bti). This naturally occurring soil bacterium is consumed by the larvae and produces a protein toxic only to fly larvae, including fungus gnats and mosquitoes, yet is harmless to plants, humans, and pets.
A physical barrier can also be created by spreading a quarter-inch layer of fine sand, pea gravel, or diatomaceous earth over the soil surface. This layer dries out quickly and prevents the adults from accessing the moist soil to lay eggs, while diatomaceous earth also acts as a sharp, desiccating agent against any emerging larvae. Repeated application of Bti every seven to fourteen days is necessary to break the continuous life cycle, as the initial treatment will not affect the pupae or eggs.