Fungus gnats are tiny flying insects commonly found hovering around houseplants. While they resemble other biting insects, fungus gnats are primarily a nuisance. This article clarifies their nature and offers practical advice for managing them.
Understanding Fungus Gnats
Fungus gnats are small, delicate flies, typically 1/16 to 1/8 inch long. They resemble mosquitoes but cannot bite humans or animals. Adults are harmless, feeding on liquids like water or flower nectar. Their primary role is as decomposers, converting dead vegetation into nutrients.
The fungus gnat life cycle involves four stages: egg, larva, pupa, and adult. Females lay up to 200 eggs in moist organic debris or potting soil, preferring areas with fungi. Larvae hatch in 3 to 6 days and cause plant damage. They feed on decaying organic matter, fungi, algae, and sometimes plant roots, particularly in overly moist soil.
Larvae feed for about two weeks before pupating near the soil surface. After 3 to 7 days, adults emerge, living about 8 days. The entire life cycle completes in 3 to 4 weeks at typical room temperatures, leading to continuous reproduction on indoor plants. Their presence often indicates overwatering, creating an ideal breeding environment.
Identifying Other Biting Insects
While bites are often attributed to fungus gnats, other insects are responsible. Mosquitoes, for instance, are known for their itchy, raised, red welts. Female mosquitoes require blood meals to produce eggs and possess specialized mouthparts for piercing skin.
Biting midges, sometimes called “no-see-ums” or “sand flies,” are tiny flies that deliver painful bites. These small insects cause intensely itchy, red welts. Fleas also cause itchy bites, usually appearing as small, red clusters or lines, particularly around ankles and legs. Fleas are wingless parasites that feed on the blood of humans and pets.
Black flies, or “buffalo gnats,” are stout flies found near running water. Their bites can cause swelling and may initially be painless but lead to persistent itching and bleeding. These biting insects possess mouthparts designed to puncture skin and feed on blood, which causes irritation and welts.
Managing Fungus Gnat Infestations
Managing fungus gnat infestations primarily involves addressing the moist conditions they favor. Allowing the top 1 to 2 inches of potting soil to dry out completely between waterings disrupts their life cycle and makes the environment less attractive for egg-laying. Bottom-watering plants can also help keep the soil surface dry, deterring gnats.
Physical controls can reduce adult populations. Yellow sticky traps effectively capture adult fungus gnats, as these pests are attracted to yellow. Placing traps near affected plants significantly decreases the number of adults laying eggs. Covering the soil surface with a thin layer of sand or diatomaceous earth creates a barrier, preventing adults from laying eggs and emerging larvae from reaching the surface.
Biological controls offer another approach to managing larvae. Bacillus thuringiensis subspecies israelensis (Bti) is a naturally occurring bacterium that targets and kills fungus gnat larvae when ingested. Bti can be applied by soaking “mosquito bits” or similar products in water and using the infused water to drench the soil. This method is safe for plants, pets, and humans. Additionally, hydrogen peroxide diluted with water (1 part hydrogen peroxide to 4 parts water) can be used to water plants, killing larvae on contact without harming them.