Spring is a time for lovers and the reemergence of fungus gnats from their winter hiding places. Even if you didn't know what they were at the time, you've likely seen these little flies that hover around plants looking very much like tiny mosquitoes. Both indoor and outdoor gardeners are regularly accosted by these common pests; gardeners will try just about anything to stop them from spreading disease throughout plantings, even freezing infested dirt.
Fungus gnats are tiny members of the order Diptera and close relatives of the common house and fruit flies. Unlike their cousins, fungus gnats only reach about 1/8 inch long and have long, thin legs that they let dangle beneath them as they fly, causing many people to mistake them for tiny mosquitoes. Fungus gnats live only about a month, but during that time each female may lay up to 300 eggs capable of hatching in a week. Larvae feed heavily for about two weeks before pupating and emerging to lay their own broods.
Cold Weather and Gnats
The short lifecycle of fungus gnats make it difficult to control them once their populations are large, since new larvae seem to be constantly emerging. Gardeners may reason that freezing the infested soil is the only way to end their torment at the hand of these bugs, but it could take a lot of ice cubes to kill fungus gnats. An Alaskan researcher discovered that the fungus gnat Exechia nugatoria is easily revived when frozen to minus 25 degrees Fahrenheit, and some members of the species may even come back when frozen to temperatures approaching minus 60 degrees. Fortunately, there are simpler ways to destroy these pests than leaving them exposed in the Alaskan wilderness.
Fungus gnats are primarily attracted to chronically wet soil. They are common in houseplants because of over-watering or poorly draining pots. Outdoor gardens can also have drainage issues that attract fungus gnats, but it's an uncommon situation. If the soil where your houseplants are growing is infested with fungus gnat larvae, repot your plants, making sure to knock as much soil from the roots as possible. Choose a pot that will drain quickly and a medium that will keep your plant from drying out, but not constantly soaked. Eliminating excess water is good for your plants and will deny fungus gnats the moist environment their larvae require.
Fungus gnats are also susceptible to chemical control, though it is generally reserved for large potted plants or outdoor gardens. When attempting to control fungus gnats, targeting larvae is the most effective method -- a microbial pesticide like Bacillus thuringiensis (Bt) subspecies Israelensis gets the job done with little effort. When mixed at a rate of 1 to 2 teaspoons per gallon for a light infestation or up to 4 to 8 teaspoons per gallon for a very severe infestation, this product can be applied during regular waterings using a watering can. Don't apply Bt at the same time that you fertilize or apply a fungicide; always read label directions before using all garden chemicals.
- University of Alaska-Fairbanks Institude of Arctic Biology: Scientist Finds Fungus Gnats Survive Winter Half-Frozen
- University of California Statewide Integrated Pest Management Program: Fungus Gnats, Shore Flies, Moth Flies and March Flies
- University of California Statewide Integrated Pest Management Program: Fungus Gnats
- Arizona Cooperative Extension: Fungus Gnat Integrated Pest Management
- Purdue Extension: Fungus Gnats and Shore Flies
- Kansas State University Cooperative Extension Service: Fungus Gnat
- University of Minnesota Department of Entomology: Gnatrol Label