Fungus Gnats

Fungus gnats are small dark flies whose larvae live in the top layer of wet medium and feed on fungus, decaying matter and fine root hairs. The adults are a nuisance. The larvae are the actual problem, and they hit seedlings hardest.

What you are seeing#

Adults first. Dark gray or black flies about an eighth of an inch long, with long legs and clear wings, walking on the soil surface and making short erratic flights when you move the pot. They are weak fliers and stay low, near the medium.

Look at the soil surface, not the leaves. Scrape the top half inch with a finger and look for larvae: translucent white worms up to a quarter inch long with a shiny black head. A phone macro or 30x loupe makes the black head obvious. Also check the undersides of the lowest leaves and the pot rim, where adults rest.

On a mature plant in veg you may see nothing above ground at all. On seedlings and clones you get stunted growth, wilting that does not respond to watering, and yellowing lower leaves, because larvae have eaten the small amount of root the plant had.

Yellow sticky cards laid flat on the soil are your monitoring tool. A card that fills in 2 days means a large population.

Why it happens#

A constantly wet medium surface. Gnats lay eggs in the top inch of damp soil and will not lay in a dry one. If you water a little every day, or the surface never dries between waterings, you have built them a nursery.

They usually arrive in a bag of soil or on a nursery plant, and cheap or badly stored potting soil is a common source. Peat-heavy mixes hold surface moisture longer and are more attractive to them. Standing water in saucers, spilled runoff and open nutrient containers all give them somewhere else to breed.

How to fix it#

1. Let the top 1 to 2 inches dry completely between waterings. This alone breaks most infestations. Water deeply and less often rather than a little every day.
2. Cover the surface. Half an inch of coarse sand, horticultural grit or diatomaceous earth on top of the medium stops adults from laying and stops new adults from getting out. This is the single most effective mechanical control.
3. Put yellow sticky cards flat on every pot. They catch adults and tell you whether you are winning.
4. Drench with Bti (Bacillus thuringiensis subspecies israelensis, sold as mosquito bits or granules). Steep per the label and water it in as your regular watering. Bti kills larvae and is harmless to the plant and to you.
5. Repeat the Bti drench every 5 to 7 days for 3 weeks. Eggs hatch in 4 to 6 days and the larval stage runs about 2 weeks, so a single drench only kills the larvae present that day and the next hatch walks right past it. Three rounds covers a full generation.
6. Clean up water sources. Empty saucers, mop spilled runoff, cap nutrient containers.
7. Add 20 to 30 percent perlite to your next mix so the surface dries faster in future.

How to tell it apart#

Versus overwatering. These two travel together and you often have both. Overwatering alone gives firm, heavy, downward curling leaves on a heavy pot with no insects. Gnats give you flies on the surface and larvae in the top inch. If you find larvae, treat both: the wet surface is what invited them.

Versus root rot. Rotted roots are brown, slimy and sour smelling. Gnat-damaged roots are pale but ragged and chewed, with no slime and no smell. Pull a seedling and look before deciding.

Versus whiteflies. Whiteflies are white, not dark, they live on leaf undersides rather than the soil, and a shaken plant sends up a visible white cloud. Fungus gnats stay near the medium and rarely settle on foliage.

How long until it looks better#

Adult numbers drop noticeably within 5 to 7 days of covering the surface and starting Bti, and the cards should be much emptier by day 10. Expect the full 3 weeks to clear the last generation.

A mature plant shrugs this off and needs no recovery time, since the adults do no real damage. Seedlings that lost root mass sit stalled for a week or two and some do not make it. Yellowed leaves stay yellow. Judge recovery on new growth and on an empty sticky card.