Gnats feed on a wide range of organic material depending on the species and life stage, from soil fungi and decaying plant matter to nectar, algae, and even blood. The tiny flies buzzing around your houseplants have a completely different diet than the ones biting you outdoors. Understanding what each type eats helps explain why they show up and how to get rid of them.
What Fungus Gnats Eat
Fungus gnats are the ones you’ll most often encounter indoors, hovering around potted plants and windowsills. Their larvae do almost all the eating. Living in the top layer of moist soil, the worm-like larvae feed on fungi, decaying organic matter, root hairs, leaf mold, and compost. Fungi make up a major part of their diet, and studies show that larvae develop faster and survive at higher rates when fungal food is abundant.
When fungi aren’t available, fungus gnat larvae will feed on healthy plant tissue instead. They chew through roots and can tunnel into the crowns and stems of plants, which is why a heavy infestation can stunt or kill seedlings and small houseplants. Organic mulch, grass clippings, manure-based fertilizers, and incompletely composted material all serve as food for the larvae.
Adult fungus gnats are mostly a nuisance. They don’t bite and they barely eat, surviving on small amounts of liquid from moist surfaces. Their short adult lives are focused almost entirely on reproducing. Females lay eggs in damp organic debris or potting soil, starting the cycle over.
What Biting Midges Eat
Biting midges (sometimes called no-see-ums) are a different story. The larvae live in mud, wet soil, or intertidal sand near water, where they consume decaying organic matter. Adults feed on liquid substances including plant juices, but females of many species also require blood meals to produce eggs. They’ll bite mammals, including people, and typically take several blood meals over their lifetime to fuel multiple batches of eggs. Males stick to plant juices and don’t bite.
What Black Flies and Buffalo Gnats Eat
Black flies, also called buffalo gnats or turkey gnats, have a diet that shifts dramatically between life stages. The larvae attach to submerged rocks and other objects in flowing streams, using fan-like structures near their mouths to filter algae and tiny bits of organic matter from the water. They need moving, oxygenated water to feed this way, which is why they’re associated with rivers and creeks rather than stagnant ponds.
Adult black flies of both sexes feed on pollen and nectar from flowers. But after mating, females need a blood meal for their eggs to mature. They’ll travel more than ten miles to find one, targeting horses, cattle, sheep, goats, poultry, pets, and humans. Their bites are painful because they slice the skin rather than piercing it, and large swarms can be a serious problem for livestock.
Larvae vs. Adults: Two Different Diets
A pattern holds across nearly all gnat species: larvae are the heavy feeders, consuming organic material to fuel their growth into adults. Fungus gnat larvae eat soil fungi and roots. Biting midge larvae eat decaying matter in wet soil. Black fly larvae filter algae from streams. In every case, the larval stage is where most of the feeding happens.
Adults tend to eat lightly if at all. Many survive on nectar, plant juices, or nothing. The exception is blood-feeding females, which need the protein from a blood meal specifically to develop their eggs. Male gnats across all species never bite.
Why Gnats Show Up in Your Home
If you’re seeing gnats indoors, they’re almost certainly fungus gnats, and they’re there because something is feeding their larvae. The most common culprit is overwatered houseplant soil. Moist potting mix rich in organic matter creates ideal conditions for fungi to grow, which is exactly what the larvae eat. Decomposing roots in a soggy pot are a buffet.
Other indoor attractants include organic fertilizers (especially manure or blood meal), incompletely composted potting amendments, and any container with standing moisture and decaying plant material. Fruit and vegetable scraps left on counters attract a different insect, the fruit fly, which is often confused with gnats but belongs to a separate family entirely.
Cutting Off Their Food Supply
Since fungus gnat larvae need moisture and organic decay to survive, the simplest control strategy is letting your soil dry out between waterings. The top inch or two of potting mix should be dry before you water again. This kills larvae and makes the soil inhospitable for egg-laying females.
A few other steps target their food sources directly:
- Use fully composted potting media. Incompletely composted organic matter often comes pre-infested with fungus gnat eggs or larvae. Pasteurized mixes are safer.
- Go easy on organic fertilizers. Heavy applications of manure, blood meal, or similar amendments create a rich food source for larvae in the soil.
- Clean up decaying plant material. Dead leaves sitting on top of the soil, decomposing mulch, and old grass clippings all feed larvae and attract egg-laying adults.
- Improve drainage. Pots without drainage holes or saucers that hold standing water keep soil wet enough for larvae to thrive.
For outdoor gnats like black flies and biting midges, removing food sources isn’t practical since their larvae feed in natural waterways and wet soil. Personal protection and timing outdoor activities around peak swarming periods are more realistic approaches for those species.