It might be hard to tell but fungus gnats are tiny, 1/8” (3.2mm) flies with seemingly long legs, long antennae, and one pair of transparent wings. Look for a Y-shaped vein in the center of each wing. Fungus gnats are similar to fruit flies, but are more likely to be flying around houseplants, screens from electronic devices, near sunny windows or, like their outdoor counterparts, flying in your face. You’d likely need a magnifying glass to see gnats, but there are clues in your houseplants as well. Look for larvae in your houseplants’ soil. Plant damage and wilting may occur in severe infestations.
Adult females lay eggs just under the surface of damp potting soil. Once hatched, the white, legless larvae at 1/8” can be seen in the top inch of the soil.
Still not sure? Insert slices of potato (approx. 1 sq. in.) far enough into the soil surface to cover the cut edges. If larvae are present, they should be found feeding on the underside after a few days. (Then trash the tater.) This ‘trap’ method confirms presence but will not stop the problem.
While they don’t cause any human health problems, they can be annoying. If the population of fungus gnats becomes large enough, they can affect potted plants by spreading fungal diseases and damaging the roots and stems.
Gnats feed on microbes, organic matter, and fungi in damp potting soil mixes, which is why they are often a problem in greenhouses. Feeding can include tender roots and shoots—damage that can kill seedlings directly or through fungal infection.
Warm indoor temperatures and average or high humidity can keep fungus gnat life cycles going over and over, with overlapping generations.
Fungus gnats can be transported on plants, plant pots and soil, and can infest other plants indoors when the females look for damp growing medium. Examine plants you buy at the store, bring home as gifts, or bring home from an office setting.
Damp soil will allow these pests to survive and thrive, so avoid overwatering plants. Keep in mind that fungus gnats occur outdoors, and can also infest indoor plants if they enter through open doors and windows.
Changing their habitat is key. Let soil dry out on top, and water plants by bottom-watering, or use a large funnel that directs water into the soil without saturating the top. You can purchase slow drip watering funnels for this use.
Adding a 1” layer of sand to the soil surface—and keeping it dry—will eventually stop reproduction.
A quicker, and possibly more effective method is to replace the potting soil completely, and rinse both the plant roots and the pot well. Let the pot dry. Don’t let the roots dry! (Rinse soil off of roots without damaging them. Keep them in water while you quickly repot plant in new, clean soil).
To maintain gnat-free houseplants, be fussy where you get your plants, and change your watering methods. It’s likely you’ll have to repot all houseplants in the room, but not all plants in the house because fungus gnats are weak flyers.