Fungus Gnats

By: Lyn Chimera

Not sure about you, but about a month or so after I bring my plants in for the winter there are tiny pesky flies flying about. They often go right in front of my face which is annoying because they are impossible to catch between your hands.

These tiny flies may well be fungus gnats (Bradysia spp.). They are small dark flies about 1/8” long, blackish grey with long dangling legs. Their antennae have many segments and are longer than their heads. These gnats and have a single pair of greyish transparent wings with a Y-shaped vein near each wing tip. You definitely need a hand lens or microscopes to see these details.

Fungus gnats thrive on plenty of moisture and decaying plant matter. The adults can be a nuisance, but it’s the larvae that are the damaging stage of this insect. Larvae are usually concentrated in the top 1-2 inches of the so. The 1/4 inch larvae are translucent, legless, with a black head. Larvae prefer to feed on fungi rather than healthy plant tissue, as they need fungi for optimal survival. Larvae feeding below the soil surface on root hairs, small feeder roots and fungus can cause stunted growth, off-color leaves or even leaf drop.

To control the gnats, avoid overwatering and water from the bottom if possible. Fungus gnats breed in the moist top 2 inches of the soil so keeping that dry will interrupt that process. Using a sterile potting mix and removing plant debris from the soil surface is helpful. For monitoring adults, yellow sticky traps are effective.

An interesting fact is a female’s offspring will either be all male or all female. Female fungus gnats lay about 200 eggs which hatch in 3-6 days. The four larval instars then feed for about 2 weeks and usually pupate near the soil surface within a thread chamber. After 3-7 days in the pupal stage, adults emerge and live for up to 8 days. They can develop from egg to adult in 3-4 weeks. A generation of fungus gnats (from female to female) can be produced in about 17 days depending upon temperature. The warmer it is, the faster they will develop and the more generations will be produced in a year.

I battle these pests every year and have had success just keeping the plants on the dry side for the first month or so after they are brought in.