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New Mexico State University

Issue: November 28

Fruit flies or fungus gnats? You need to identify insects to properly manage problems

Q. I have fruit flies in my home. I have left no fruit out in a bowl (or vegetables). I have scrubbed my kitchen top to bottom, but still have hundreds of these insects. I used to have a compost bowl, but I have even cleaned that up and I have not had one for about 2 weeks now. I have no pets and only green house plants. What do you suggest to remedy this problem?


A. I wonder if what you have is an infestation of fungus gnats coming from the houseplants. They are smaller than fruit flies, but may perhaps be confused with them.

In either case (fungus gnats or fruit flies), you can reduce the number flying insects with a sticky yellow trap. The yellow traps are similar to old-fashioned fly-paper and can be purchased in some garden centers or home supply stores. They can also be made from various household items. Yellow surveyors tape, yellow butter/margarine tubs (empty), or other yellow plastic items smeared with cooking oil or tree tanglefoot (a beeswax and oil product available at some garden centers). If these traps are placed in a bright location (window with direct sunlight) they may effectively catch some of the adult insects that are flying around your home. When you have caught some, take samples to your local County Extension Service office to confirm which insect is present and the appropriate treatment.

There is also the possibility of drain flies (that develop in the traps in drains) and other insects, so identification is important.

If your problem does turn out to be fungus gnats, the trapping should be continued to reduce the adult gnat population. There is a Bacillus thuringiensis variety israelensis product that can be drenched through houseplant soil to control the fungus gnat larvae feeding on fungi and plant roots in the potting soil.

Potting soil is often the source of this nuisance pest so pasteurization of potting soil is helpful. Do not sterilize (over-heat); rather heat the potting soil to about 160 degrees for 30 minutes. This can create unpleasant odors, so if it can be done in the garage (portable electric oven) or with soil wrapped in aluminum foil over a barbecue, the odors can be kept from your house. Let the soil cool and air-out for a while before using it for potting.

For more gardening information, visit the NMSU Extension publications World Wide Web site at

Send your gardening questions to:

Yard and Garden, Attn: Dr. Curtis Smith
NMSU Agricultural Science Center
1036 Miller Rd. SW
Los Lunas, NM 87031

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. NMSU and the U.S. Department of Agriculture cooperating.