Press-Republican

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March 4, 2013

Fungus gnats a nuisance houseplant pest

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.

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