Press-Republican

March 4, 2013

Fungus gnats a nuisance houseplant pest

By JOLENE WALLACE, Cornell Cooperative Extension
Press-Republican

---- — Two weeks ago, I saw a millipede and a slug on the steps going down to our patio. 

I was surprised to see them but not as surprised as they probably were when the temperatures dropped to below zero again. I suspect that they were wintering between the steps and the foundation of the house and were drawn out by the milder temperatures we were experiencing. They beat a hasty retreat that evening and are now back where they belong.

We don’t get calls about a wide variety of insects during the winter, but one that we commonly do get is about fungus gnats. To the naked eye, fungus gnats look much like fruit flies and may be found swarming around houseplants in homes or offices. 

Fungus gnats can be a nuisance year-round, but they commonly become worse in the fall and winter. If you put your houseplants outside during the summer, they may be infested when you bring them back in, and the warmer temperatures of your home encourage their reproduction. The main factor for the fungus gnat becoming a pest, however, is the moisture level of your houseplant’s soil.

COMBATING GNATS

During the fall and winter, your houseplants do not need as much water as they do during the spring and summer. The shorter day length and cooler temperatures slow plant growth and water usage. 

The extra moisture in your growing medium encourages the development of the fungus gnat. Adults are about 1/8 inch long and are a nuisance but do not bite or feed on your plants. They are weak, erratic flyers. The female fungus gnat can lay up to 200 eggs in the soil of houseplants. The larvae are wormlike, translucent and usually found in the top 2 to 3 inches of soil. 

They feed on decaying matter, such as leaves fallen from the plant, fungi and algae. They may also feed on the roots of your houseplant. They pupate for a week in the soil and emerge fully grown in two to three weeks, at which time the cycle begins again.

Yellow sticky traps, either purchased or homemade, will catch some of the adult fungus gnats. Using slices of raw potato will help you to determine whether larvae are present in your planting mix. Insert 1/4-inch thick slices of potato into the soil. Check the underside of the potato every few days to see if larvae have migrated to it and begun feeding.

If you have fungus gnats, you should repot your plant and discard the old potting medium. Shake the soil off the roots of your plant, and repot with a good potting mix. Potting mix should be used, not potting soil. If your plant needs to go into a larger pot, choose one that is just one size larger than the one it has been in to avoid excess potting mix holding more water than the roots need. Keep track of how often you water, and remember to allow the top inch or 2 of soil to dry before watering again.

Jolene Wallace is the horticulture program assistant for Cornell Cooperative Extension in Clinton County. Contact her at 561-7450 or jmw442@cornell.edu.