Fungus gnats

About

Tiny, dark, mosquito-like flies about 1/16 to 1/8 inch long that flit around seed trays, houseplants and greenhouses. The adults are mostly a nuisance, but large numbers of larvae can damage the roots of seedlings.

Spotting it

Damage

Larvae feed on fungi and decaying matter in the top 2 to 3 inches of potting mix, but in large numbers they chew roots and stunt seedlings and young plants. They also nibble leaves resting on the soil surface.

How to find it early

Put yellow sticky cards at soil level to catch adults, or push chunks of raw potato into the mix for a few days and check underneath for clear larvae with shiny black heads.

When

Year-round indoors; worst during seed starting and in overwatered pots.

Life cycle

At about 75 degrees F the whole life cycle takes about 17 days, and a female can lay up to 200 eggs in the potting mix. Indoors they can breed all year wherever the mix stays moist.

What to do, gentlest first

Start at the top. Sprays are a last step; always read and follow the product label.

1. Let the mix dry out Prevent

   Avoid overwatering and let the top 1 to 2 inches of mix dry between waterings; make sure pots drain well.

2. Avoid half-finished compost Prevent

   Do not use incompletely composted material in seed-starting or potting mix, since it feeds the larvae.

3. Yellow sticky traps Trap

   Yellow sticky cards catch many adults and show whether your other steps are working.

4. Bti drench Natural enemies

   Water the mix with Bti (Bacillus thuringiensis israelensis), a bacterium that kills the larvae.

   Colorado State notes some Bti products are hard to find in stores. Follow the label.

5. Beneficial nematodes Natural enemies

   Steinernema feltiae nematodes applied as a drench kill larvae within a few days and can give season-long control after a few applications.

Plants it goes after

Basil, Broccoli, Cabbage, Christmas cactus, Dieffenbachia (dumb cane), Eggplant, Hoya (wax plant), Lettuce, Money tree, Monstera (Swiss cheese plant), Peace lily, Peperomia, Pepper (sweet and chili), Pilea (Chinese money plant), Strawflower, String of pearls, Succulents (echeveria, haworthia, sedum), Tomato

Pressure by region

A pest of seed-starting trays, houseplants and greenhouses, so it can show up indoors in any region. These ratings are estimates, not regional survey data.

Ratings come from extension sources where available; some are estimates, as noted above.