Pest Profiles: Fungus Gnats

Oct 1, 2017

Fungus gnats have started making an appearance in my home. Not huge numbers, but enough to be noticeable. Fungus gnats are small flies that lay eggs in moist organic material such as potting soil or compost. They look sort of like mosquitoes and are usually between 1/8 and 1/16 of an inch long. You may mistake them for fruit flies but fruit flies look a little beefier and also are better fliers than fungus gnats. Fungus gnats are attracted to light, so you may see them near windows but they usually hang out on leaves of houseplants or on the soil in your houseplant pots. They lay eggs in the soil and the larvae burrow into the soil and feed on root hairs and other organic material in the soil. Usually they're just a nuisance in the house, but they can grow to such numbers that a plant's ability to take up water is compromised because fungus gnat larvae have destroyed the plant root hairs. Larvae may also spread plant pathogens, especially in commercial nurseries. A reproductive cycle from adult laying eggs to those eggs hatching larvae which grow to adulthood takes about 17 days and may happen more quickly when it's warm outside.

There are all kinds of folk remedies for these critters. Common treatment suggestions include covering plant soil with a layer of sand or other inorganic mulch, sprinkling cinnamon on the soil surface, or even treating soil with fabric softener. But it turns out that these remedies are pretty ineffective. While mulching with inorganic material does block the gnats' ability to lay eggs on the surface of the soil, if the soil dries out and pulls away from the sides of the pot, the gnats will enter at the sides. In fact, fungus gnats have even been found to enter and lay eggs at plant pot drainage holes. So cultural remedies are unlikely to completely eliminate the problem, although emptying excess water in plant saucers and not overwatering your plants is a good start. 

If you suspect you have fungus gnats, yellow sticky traps placed on the surface of the plant soil will trap the adults. Larvae can be monitored by placing chunks of raw potato in your plants, with the cut side down on the soil. The larvae will enter the potato, at which point you can throw it away and replace it with fresh chunks.

If you determine that you have a fungus gnat problem, there are a couple of biological control agents that will control them in pots and container soil mixes. These include Steinernema nematodes, Hypoaspis predatory mites, and the biological insecticide Bacillus thuringiensis subspecies israelensis (Bti). Bti is readily available in retail nurseries so it may be the easiest agent to acquire. Bti doesn't persist or reproduce indoors, so you may have to do repeated applications at about 5 day intervals to provide control. Nematodes and mites can be mail-ordered but are live and perishable, so must be applied immediately upon receipt. Nematodes can reproduce and provide longer term control of fungus gnats. If your fungus gnats are an outdoor problem, several natural predators will help manage the population, including predatory hunter flies. Avoid broad spectrum insecticides to conserve these and other natural enemies. 

For more information, take a look at the UCANR literature on fungus gnats here and here. Don't forget to subscribe to our blog so that you receive an email notification when a new post goes up. If you have questions, contact us online, by phone or in person to get answers to your gardening quandaries!