Fungus gnats, members of the Sciaridae family, are small, dark, flying insects frequently mistaken for fruit flies or tiny mosquitoes. These common indoor pests typically measure about one-eighth of an inch long and are often seen hovering near the soil of houseplants. Fungus gnats do not bite humans or animals.
The Reason for the Confusion: Why They Don’t Bite
Fungus gnats lack the necessary anatomy to pierce skin and draw blood, meaning they cannot bite, and possess simple mouthparts designed only for consuming liquids like water droplets or flower nectar. They do not feed on blood or solid food, posing no direct health risk to people or pets. Confusion about their biting ability stems from their size and flight behavior, which is similar to other pests that do bite, such as midges or black flies that have specialized mouthparts for cutting skin. Fungus gnats are weak fliers that tend to hover near faces and eyes, especially when humidity is high.
The Real Danger: Damage to Plants and Roots
While the flying adults are harmless nuisances, the fungus gnat’s larval stage presents a genuine threat to plant health. Adult females lay tiny, whitish-yellow eggs in moist soil, and these eggs hatch into the larvae within a few days. The entire life cycle, from egg to adult, can be completed in as little as three to four weeks, allowing rapid population multiplication. The larvae are translucent, legless maggots with distinct, shiny black heads, residing primarily in the top inch or two of the soil.
Larvae feed on decaying organic matter and fungus, but when these food sources are depleted, they turn their attention to plant roots. They chew on root hairs and tender root tissue, and in severe infestations, may tunnel into the stems of young plants. This root damage prevents the plant from absorbing water and nutrients, leading to noticeable symptoms above the soil line, such as stunted growth, yellowing leaves, and sudden wilting. Furthermore, the feeding wounds created by the larvae can serve as entry points for various soil-borne plant pathogens. This potentially spreads fungal diseases like Pythium and Fusarium.
Simple Steps to Eliminate an Infestation
The most effective strategy against fungus gnats involves a two-pronged approach that targets both the flying adults and the soil-dwelling larvae. Since larvae require moist conditions to survive, adjusting watering practices is the most important cultural control. Allowing the top one to two inches of soil to dry out completely between waterings eliminates the moist environment where females lay eggs and larvae thrive. Yellow sticky traps are highly effective for controlling adult gnats.
These bright cards attract and trap the weak-flying adults, serving as both a monitoring tool and a way to reduce the population. Placing traps horizontally near the soil surface captures adults as they emerge or attempt to land to lay eggs.
To eliminate larvae, biological control agents offer an effective solution. One common treatment involves applying Bacillus thuringiensis israelensis (Bti), a naturally occurring bacterium mixed with water and applied as a soil drench. Bti is ingested by the larvae, effectively killing them without harming plants, pets, or humans.
Another option for larval control is the application of beneficial nematodes (Steinernema feltiae). These microscopic worms are natural parasites that actively seek out and kill the larvae in the soil. Nematodes are applied as a soil drench and provide longer-term suppression, continuing to reproduce and patrol the soil for new larvae.