Fungas Gnats

Fungus Gnats

by Don Janssen, Extension Educator
Fungus Gnat

Fungus Gnat
Photo Credit: UNL Entomology

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Fruit flies are common around the house during late summer and early fall. All it takes is an overripe banana, tomatoes ripening on the counter or melon rinds in the wastebasket -- and like magic, fruit flies appear.

But in winter? Winter fruit flies often are not fruit flies at all but rather fungus gnats, which look enough like fruit flies to a casual observer to be confused with them.

Both fruit flies and fungus gnats speed the process of decay of plant matter. Adult fruit flies home in on potential food sources such as overripe fruit or winter squash past its prime, and lay their eggs on it. Maggots hatch in a day or two and commence feeding. As the plant matter breaks down, various fungi start to grow in it. These fungi -- and others growing in houseplant pots, drains and other chronically damp places -- are attractive to fungus gnats, which are every bit as quick as fruit flies to zero in on potential breeding sites. Numbers can increase quickly.

During warm months, both of these insects can enter homes from outdoors. They can easily pass through ordinary window screen. In winter, management can be more successful because you don't have a continuous supply of new recruits to deal with.

Focus control efforts on five areas: fruit and vegetable storage areas that are open to air (bowls, crocks, bags on floors or in pantries), open trash containers, potted indoor plants, drains, and damp rugs, carpets and paper products in the basement or garage.

Leave out on the counter only as much fruit as you will consume in a day or two. Wash containers frequently (every other day or so) in hot, soapy water to dislodge and kill any eggs they might contain.

Line trash containers with disposable plastic bags. Containers with no bag or a leaking bag should be washed frequently, especially if the bottom tends to remain damp or wet.

If you suspect houseplant pots of harboring fungus gnats or fruit flies, you can treat the soil with insecticidal soap or incorporate diatomaceous earth in the soil to kill the maggots. Another approach is to cut back on watering so the soil dries out between waterings. Any maggots present in the soil will dry out, and lack of water will reduce fungal growth and so reduce the food supply for adult fungus gnats. A third option is a combination of letting the soil dry between waterings and then watering with a solution of water and insecticidal soap.

Basements can get extremely damp during the summer. Fabric and paper will absorb moisture and can become home to populations of fungus gnats and a host of other creatures, many of which may carry over into the winter. Use a dehumidifier in the summer to reduce the dampness, thus eliminating breeding sites for these flies and reducing the growth of mold.

Drains should be cleaned once a month year round. This will kill fungus gnats and minimize fungal growth.

Household insecticides for flying insects will control fruit flies and fungus gnats, but this is like treating the symptom rather than the cause of an infestation. Eliminate potential breeding habitat instead.

Measures that dry or clean out breeding habitat for these insects will also reduce the fungal spore load inside your home and help freshen the air. Both of these can enhance health and comfort while greatly reducing the flying nuisance numbers -- at least until warm weather comes back and activates outdoor populations.

(This resource was last updated January 2005 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement)

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University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office