What Is a Fungus Gnat?
The tiny black fly that drifts up from houseplant soil. The adults are harmless. The larvae in the wet soil eat roots.
A small, slender, dark fly about 1/8 inch long that breeds in damp potting soil, where its larvae feed on fungus and fine roots. Adults are weak fliers that hover near houseplants and rise when a pot is disturbed, which separates them from fruit flies (tan, red-eyed, quick, near fruit) and drain flies (fuzzy, moth-shaped, near drains). One generation takes about three weeks, and a single overwatered pot can hold hundreds of larvae.
Control works on every life stage at once: let the top two inches of soil dry between waterings to stop egg-laying, push yellow sticky cards into the pots to catch adults, and water with a BTI drench (mosquito bits soaked in water) to kill the larvae. Keep it up for a full three weeks. Vinegar traps catch fruit flies, not gnats, so if those traps fill up the problem is in the kitchen, not the plants.
Fixes that use this
Pests
Drain Flies Keep Coming Back? Kill the Biofilm
Drain flies breed in the biofilm coating a slow drain, not the water. Scrub the pipe, treat it, and repeat for about a week to actually break the cycle.
Time30 min + a week Cost$5–$20 easy
Pests
How to Get Rid of Fruit Flies Overnight
Fruit flies breed in fermenting sugar, not just ripe fruit. Find the real source, set a vinegar trap tonight, and most kitchens are clear by morning.
Time15 min + overnight Cost$0–$10 easy
Pests
How to Get Rid of Gnats in the House for Good
Gnats over houseplants are fungus gnats breeding in wet soil. Dry the pots, trap the adults, and kill the larvae with a $10 BTI drench. Gone in 3 weeks.
Time20 min, then 3 weeks of follow-through Cost$10–$25 easy