Fungus gnats are tiny flying insects commonly seen hovering around indoor plants, often mistaken for mosquitoes or fruit flies. These small pests can become a noticeable presence in homes and offices, typically near potted plants.
Do Fungus Gnats Bite?
Fungus gnats do not bite humans, pets, or other animals. Their mouthparts are designed for feeding on liquids like water or flower nectar, not for piercing skin. While some gnat species, such as biting midges, can deliver painful bites, fungus gnats lack the necessary mouthparts. Therefore, any itchy welts attributed to “gnats” are likely from a different type of insect.
Why Fungus Gnats Are a Nuisance
While adult fungus gnats do not bite, they can be a nuisance, particularly when large numbers are flying around lights, windows, and potted plants. They are weak fliers, tending to remain near infested plants but can disperse short distances throughout a home. The primary concern with fungus gnats lies not with the adults but with their larvae.
Fungus gnat larvae live in moist soil and feed on fungi, algae, and decaying organic matter in the soil. However, they can also feed on plant roots, especially in overwatered or unhealthy plants. Root damage can lead to plant stress, stunted growth, yellowing foliage, or even plant death, especially for vulnerable seedlings or young plants. Furthermore, both larvae and adults can potentially spread plant pathogens, contributing to plant diseases.
Identifying and Controlling Fungus Gnats
Identifying fungus gnats involves observing adult flies and signs of larval activity. Adult fungus gnats are small, typically 1/8 inch long, with slender bodies, long legs, and antennae, often appearing dark greyish-brown. They are weak fliers, often seen running across the soil surface or resting on plant foliage. Unlike fruit flies, which are typically rounder and attracted to fermenting produce, fungus gnats are more slender and found near plant soil.
Signs of larval activity include wilting plants or visible larvae in the soil, appearing as translucent, worm-like creatures with shiny black heads. These larvae can sometimes leave thin, silvery slime trails on the soil surface, similar to small snails or slugs. Since fungus gnats thrive in damp conditions, control methods focus on managing soil moisture. Allowing the top few inches of soil to dry out between waterings can significantly reduce their breeding environment.
For active infestations, several control methods can be employed:
Yellow sticky traps are effective for monitoring and capturing adult fungus gnats, reducing the number of egg-laying females.
Applying a layer of diatomaceous earth to the soil surface can physically abrade and dehydrate emerging larvae and adults.
A diluted hydrogen peroxide solution (typically one part 3% hydrogen peroxide to four parts water) can be poured through the soil to kill larvae on contact.
Mosquito dunks, which contain the bacterium Bacillus thuringiensis israelensis (BTI), can be steeped in water and used to water plants; BTI specifically targets and kills gnat larvae without harming plants or other organisms.
Removing any decaying plant matter from the soil surface also eliminates potential food sources and breeding sites for the larvae.