Fungus Gnats - The Old Farmer's Almanac

How to Identify a Fungus Gnat

• Size: Adult fungus gnats are tiny. Their size ranges from about 1/16 to 1/8 inch in length (1.5 to 3mm), which is about the same size as a fruit fly. Fungus gnat larvae may be up to 1/8 inch in length.
• Appearance: Adult fungus gnats are grayish-black and have gray or see-through wings. Their long legs and long antennae give them a mosquito-like appearance, though they are much smaller. Compared to a fruit fly, fungus gnats have a thinner body with longer legs and antennae. Larvae have a small, black head and a thin, white, or see-through body.
• Activity: Fungus gnats tend to spend most of their time on the soil surface of potted plants, but they may be seen flying around the outer edge of the pot or near drainage holes as well. They are not strong fliers, so they walk along the soil and fly only in short bursts. Their flight is erratic and much slower than that of fruit flies, acting more like mosquitoes while flying.Annoyingly, fungus gnats tend to fly into people’s faces and drinks, though they are completely harmless, and a few well-placed swats will show them what’s what.

Fungus Gnat Damage

In small numbers, fungus gnats are more of an annoyance than anything. In fact, adult gnats don’t actively harm plants or people. However, if their population gets out of hand, the larvae may start feeding on plant roots, causing notable damage. This is especially bad for young plants, such as seedlings with only a few delicate roots. Fungus gnats can also spread the plant pathogen that causes damping off and the eventual death of seedlings.

Fungus gnat damage will appear similar to any other root-related issue, such as root rot. Lower leaves may turn yellow and drop, and the plant’s growth may slow down or stop altogether. In particularly bad cases, wilting of the entire plant could occur, followed by the death of the plant if the roots are extremely damaged.