Fungus gnat larvae feed on fungi, decaying organic matter, algae, and plant roots in soil. Adult fungus gnats, by contrast, do not feed at all. This distinction matters because it means almost all the damage these insects cause happens underground, during the roughly two-week larval stage before they become the small flying gnats you actually notice.
What Larvae Eat in Soil
Fungus gnat larvae are small, translucent worms (about a quarter inch long) that live in the top few inches of moist soil. Their primary diet consists of fungi and decomposing organic material, which is exactly how they got their name. In a typical houseplant pot, that means they’re feeding on the fungal networks and rotting plant matter naturally present in potting mix. Peat-based soils, compost-rich blends, and any mix that stays consistently damp provide an ideal buffet.
When fungal food runs low or larval populations get high, they shift to feeding on living plant tissue. They start with root hairs, the finest, most delicate parts of the root system, and can tunnel into larger roots if the infestation is severe enough. This root damage shows up as wilting, yellowing leaves, stunted growth, or a plant that looks underwatered even though the soil is moist.
Why Algae Is a Major Attractant
In hydroponic setups and indoor growing systems, algae is the primary food source that draws fungus gnats in the first place. Any surface that stays wet and receives light will grow a thin green film of algae, whether that’s the top of a rockwool cube, damp coco coir, or the rim of a self-watering pot. Larvae thrive on this algae, and adult gnats are attracted to these conditions when choosing where to lay eggs. When the algae runs out, larvae move on to fine root hairs, which is when plants start showing real damage.
This is why fungus gnats are such a persistent problem in greenhouses and indoor gardens. They don’t need soil specifically. They need moisture, organic material, and something growing in it. Algae-covered surfaces in soilless systems check all those boxes.
Adult Gnats Don’t Eat
The small dark flies hovering around your plants are adult fungus gnats, and they do not bite or feed. Their mouthparts aren’t built for it. Adults exist to mate and lay eggs, and they typically live only about a week. So while they’re annoying, the adults themselves cause no direct harm to your plants or to you.
That said, adults do contribute to plant problems in an indirect way. As they move between pots and growing areas, they can carry fungal pathogens on their bodies. Research from Penn State found that fungus gnat larvae can ingest and excrete viable spores of Pythium, a common root rot organism, effectively spreading disease from one plant to another. The larvae act as vectors, picking up harmful fungi while feeding and depositing them in new root zones.
The Feeding Timeline
Fungus gnats go through four larval stages before pupating, and all of the feeding happens during this period. At around 72°F (a typical indoor temperature), larvae take approximately two weeks to develop from egg to pupa. That’s a narrow window, but female gnats can lay up to 200 eggs at a time in moist soil, and generations overlap continuously indoors. So even though any individual larva feeds for only about two weeks, a pot with ongoing moisture problems can support a nonstop cycle of feeding larvae.
Eggs are laid in the top layer of soil, and newly hatched larvae stay near the surface where organic matter and fungi are most concentrated. As they grow through their four stages, they venture deeper and are more likely to encounter and damage roots. The largest, most mature larvae cause the most root feeding.
What This Means for Your Plants
A small number of fungus gnat larvae in a houseplant pot is normal and usually harmless. They’re eating decomposing matter and fungi that are already breaking down, essentially recycling nutrients. The problem starts when populations explode, which happens when soil stays consistently wet. Overwatering creates the perfect environment: abundant fungi, soft organic material, and damp conditions that keep eggs and larvae from drying out.
Healthy, established plants can tolerate moderate larval feeding without visible symptoms. Seedlings, cuttings, and young transplants are far more vulnerable because their root systems are small and delicate. Heavy infestations in seed trays or propagation setups can kill young plants outright by destroying their limited root tissue before they have a chance to establish.
The most effective way to cut off the food supply is to let the top inch or two of soil dry between waterings. This kills eggs and young larvae, reduces fungal growth in the surface layer, and makes the environment far less attractive to egg-laying adults. Covering exposed soil surfaces also helps by eliminating the algae growth that serves as an initial food source.