Gnat Facts, Identification and Control
Scientific Order: Diptera
Common Family: Six of the seven families within the superfamily Sciaroidea, particularly the family Sciaridae
Common Species in New York: Tens of thousands of species are commonly referred to as gnats, sometimes erroneously. Gnats are distributed all over the world, and thousands of species live in New York.
Size: Varies based on species, but always very small. Adults of different species may be as small as 2 millimeters, or as large as ½ an inch (12.7 millimeters).
Color: Black, grey, or even dark orange-yellow with translucent grey or clear wings. The identifiable mark on most species is the Y-shaped, vein-like pattern in their wings.
Behavior and Diet
Fungus gnats are attracted to fungi such as mold, decaying matter, and chronic sources of moisture. They’re commonly encountered in large numbers in greenhouses, because they’re attracted to light and they infest algal growth, mulch, leaf mold, compost, and wet soil. Fungus gnats are sometimes also called manure or mushroom flies.
Most adult fungus gnats don’t feed on anything except for trace amounts of water or flower nectar, and are only considered a nuisance. Fungus gnats don’t bite humans, but horseflies and other biting nuisance pests are often mistaken for fungus gnats.
Many species of fungus gnat larvae feed on root systems in soil. This feeding may damage the developing root systems of a plant, interfering with that plant’s ability to absorb water and nutrients. Damage inflicted on root systems by fungus larvae may stunt plant growth or even kill the plant, provided the damage is severe enough. Immature plants and/or seedlings are particularly vulnerable to feeding damage.
Adult fungus gnats are attracted to light. You may see them swarming around indoor light sources. They’re especially active during warm, moist autumn weather. Unlike many other types of fly, fungus gnats are poor fliers and typically don’t move much, especially when indoors. You’re more likely to find a fungus gnat resting on the leaf of a house plant than flying around your head.
Reproduction and Life Cycle
Fungus gnats have short life spans and grow quickly. Their entire life cycle process can complete in 20 to 28 days depending on the temperature of the growth environment. The fungus gnat’s life cycle is broken up into four distinct stages: egg, larvae, pupa, and adult.
Fungus gnat eggs are so small and semi-transparent that they can be quite difficult to see. The oval, round eggs are milk-white and feel smooth to the touch. They may resemble very small gelatin beads. Mother fungus gnats lay up to 300 eggs in batches, or “strings,” of as few as 2 or as many as 30 at a time. A mother can lay eggs multiple times, and could lay as many as 1000 eggs in her lifetime.
After laying her eggs, the mother will usually bury them under wet soil, decaying matter, leaves, or other plant products. Eggs need warmth and moisture to hatch, and the more they have, the faster they hatch. At about 75 degrees Fahrenheit, the eggs could hatch in as few as three days. At lower temperatures, the incubation process could take up to 10 days.
Like their eggs, fungus gnat larvae are clear-white and so semi-transparent that it’s possible to see their food channels inside their bodies. The larvae develop in four larval stages, or instars, during which they grow continuously. When fully grown and ready to pupae, larvae may be up to ¼ inch long. They have shiny black heads and long, legless bodies. They may resemble maggots.
Larvae eat continuously from the time they hatch, and require considerable moisture and energy to grow and pupae. They feed on decaying organic material, mulch, leaf mold, grass clippings, compost, algae and fungi, and other decay. They’ll generally only feed on root systems if their preferred food sources aren’t available, or if the root systems are inundated with moisture from overwatering. Larvae often burrow into soil to get at food, so don’t be surprised to find them fully or partially buried in potted plant soil. It takes around 10 days for larvae to grow through all four instars and finally pupate, though the process may take more or less time depending on the environment.
When a larva is fully grown, it stops feeding, nestles into the soil, and spins itself a thin silken cocoon. Inside the cocoon, the larva sheds its skin and transforms into a pupa. At this stage, pupae do nothing except develop into adults. They do not feed or move until finished. The pupal stage can last between 3 and 7 days. When the adult fungus gnat is fully formed, it breaks out of the cocoon. Fully formed adults are ready to fly and mate about an hour after emerging.
Adult fungus gnats live for around 8 days after emerging from their cocoons. During this final period of their lives, the adults eat only trace amounts of water or flower nectar. Their primary interest is mating and laying eggs. Adult females lay eggs in topsoil or decaying material after mating and either die shortly thereafter or mate again continuously.
Outdoor fungus gnats’ life cycles loosely link up with the seasons, dying off in winter and becoming active in spring and fall. Indoors, however, fungus gnat’s life cycles are short enough that generations cycle continuously, with different steps in the cycles overlapping for subsequent generations.
Signs of Infestation
The most obvious sign of infestation is to notice adult fungus gnats in proximity to your indoor plants. Look for small black gnats resting on leaves, in or near soil, on window sills, walls, or in flight. Gnats multiply quickly, so if you have a problem, chances are you’ll see quite a few adults.
Adult gnats are attracted to light sources, especially at night. Look for swarms of small black gnats around your indoor and outdoor lights at night, especially in warm or humid parts of the house.
Check potted soil for larvae, pupae, or eggs, especially if you see adults nearby. Remember, larvae burrow under soil, so you may need to wipe the top layer of your potted plant’s dirt away to find them effectively.
You may also notice poor growth conditions or plant damage in your house plants if larvae have been feeding on them.
Treatment and Prevention
Preventing fungal growth is the most important way to prevent fungus gnat infestation in your home. No fungi can grow without moisture and humidity, so control your home’s humidity level, especially in areas where you grow plants. Watch for plumbing leaks, condensation, puddling, or other sources of water. Do not overwater plants or leave watering cans sitting out. Allow dirt to dry between waterings. Consider investing in a dehumidifier. Thoroughly scrub away any fungal growth you find and wash the area where you found it.
Gnats are attracted to wilting flowers, leaves, and weakened root systems. Tend to your plants regularly. Dispose of dead or dying parts of your plant, trim overgrown or unruly branches or leaves, and make sure the plant gets enough water and sun. Dispose of any decomposing plant material quickly. If the gnat problem is isolated to one plant, consider disposing of that plant, re-potting it, or isolating it.
Fertilizer may attract fungus gnats as an egg-laying site. Don’t over-fertilize your plants, especially inside. Don’t leave loose dirt or fertilizer out in areas where you keep your plants, especially if those areas are also dark, humid, or warm.
Look for ways that fungus gnats could infiltrate your home and seal them up. Replace damaged window and door screens. Patch up or seal gaps or cracks in door and window frames. Look for cracks in walls or on siding or thresholds and fill them in with caulk or another sealant.
University of California Agriculture & Natural Resources Integrated Pest Management Program’s How to Manage Pests in Gardens and Landscapes: Fungus Gnats page
Colorado State University Extension “Fungus Gnats as Houseplant and Indoor Pests” fact sheet
North Carolina State University Cooperative Extension’s Residential, Structural and Community Pests Insect Notes “Fungus Gnats Indoors” by Michael Waldvogel
University of Florida Department of Entomology & Nematology darkwinged fungus gnats “Featured Creatures” page
American Orchid Society’s Orchid Pests and Diseases: Fungus Gnats page, by Paul J. Johnson, Ph. D.
Missouri Botanical Garden Home Garden Pests: Fungus Gnats page (this page incorrectly refers to the insect order Diptera as a family)
University of Vermont Extension Department of Plant and Soil Science’s “Fungus Gnats” page by G.R. Nielsen
Kansas State University Research and Extension “Fungus Gnat Management in Greenhouses and Nurseries” by Raymond A. Cloyd