Colorado Master Gardeners: What are “fruit flies” doing in my houseplants? |

Colorado Master Gardeners: What are “fruit flies” doing in my houseplants?

The fungus gnat is considered a nuisance pest in our homes during winter. In large numbers, however, the larvae can cause significant root damage, and they may have entered your home with outdoor plants that have been brought inside for the winter.

These flies will never be 100 percent eradicated, but some control is possible.

First, hold back on the watering. Fungus gnats love wet soil. Let the top 1 to 2 inches of the potting medium dry out between waterings. That will decrease the survival rate of the eggs and hatched larvae, and your pots will not become a fun place to lay eggs.

Second, it is always a good idea to pick up the dead leaves and blossoms of the plants on top of the soil. No more hiding places.

Third, you have several options for more aggressive control. These insects are attracted to those yellow sticky traps, which may slow down the egg-laying females. They are available at garden centers, nurseries and You can ven make your own traps with yellow construction paper and Tanglefoot.

There is a parasitic nematode (microscopic roundworms which infect the larvae in an unspeakable way, causing death), which provides a long-term control and might be more appropriate for large commercial greenhouses. Home Depot has Fungus Gnat Control Nematodes.

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I have found the larvae are very effectively stopped by a product containing Bacillus thuringiensis subspecies, israelensis, or Bti. If you are starting seeds, the young roots are particularly vulnerable to the chewing larvae. Products containing this strain of Bti are available at garden centers, nurseries and

This is a very safe biological control method. Be sure you are purchasing a product with the Bti israelensis listed as the active ingredient. Products on the market include Gnatrol, Gardens Alive Knock Out Gnats and Bonide Mosquito Beater WSP.

Before using any of these products, be sure to carefully read and follow the label instructions. The granules are mixed with water, and the plants are then drenched. Bti only works on the larvae of the gnats, so several applications through the course of a few weeks should do the trick to get rid of the flies.


More information about this problem can be found at the following websites and resources.

Barbara Sanders and her husband, Bill, moved to Steamboat Springs from Hawaii in 1997. She learned about Colorado plants and trees through the Master Gardener program and while volunteering at the Yampa River Botanic Park. Sanders finds native plants most interesting as "they are adapted to our crazy, changeable climate and to our different soils" and her vegetable garden the most fun, which she tends with her husband.

Barbara Sanders

The CSU Master Gardeners are available to answer questions from 10 a.m. to 1 p.m. each Thursday at the Extension Office. Stop by 136 Sixth Street, call 970-870-5241 or email to

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