Fungus gnats in houseplants: start here.

1. Find the pot behind the flies.

Fungus gnats are small, delicate, dark flies often seen walking on potting mix or fluttering near a plant. The adults are mainly a nuisance; their larvae live in the mix. Flies near fruit or a drain may be something else.

Watch the soil surface of nearby pots, especially after watering. Check for constantly damp mix, standing water, and decaying plant debris. Finding flies on a window alone does not tell you which pot is producing them.

Compare the insect with Extension photographs below; our diagram is not an identification key. A few adults do not prove root rot.

2. Make the surface less welcoming.

1. Check before watering. Allow the surface to dry between waterings where the plant’s needs permit. Do not leave a moisture-loving plant dry throughout just to fight gnats.
2. Let excess water escape. Empty saucers and outer pots, and make sure drainage holes are open.
3. Remove decaying material. Clear fallen leaves and rotting scraps from the pot. Check nearby stored mix or other damp organic material if flies keep appearing.

Indoor plants often use less water as days shorten. Carrying the summer watering schedule into autumn can leave the mix wet for longer. Adjust to the actual pot, light, and plant, rather than a fixed number of days.

3. Catch adults and watch the trend.

Place a yellow sticky card near the affected plant, following its instructions. It catches adults, not larvae in the soil. Keep its position consistent when comparing catches.

Expect to monitor for several weeks. Different life stages can overlap, so new adults may still emerge after you make changes. Fruit-fly vinegar traps are not a substitute for fungus-gnat monitoring.

4. Reassess a persistent problem.

If numbers remain high, check whether another pot or damp source was missed. Replace broken-down, persistently wet potting mix when repotting is warranted. If the plant is wilting despite wet soil, use the root-check guide; unhealthy roots can have several causes.

For a continuing infestation, a product containing Bacillus thuringiensis subsp. israelensis (Bti) may be an option for larvae. Choose a product labeled for fungus gnats and your intended indoor use, and follow its current directions. It does not kill every life stage; do not substitute a caterpillar-control Bt product or improvise a dose.

Returning outdoor plants to the house? Use the fall move-in checklist. For routine moisture checks, return to your pothos or other plant’s care guide.