Fungus Gnats

— Written By and last updated by Nicole Vernon
en Español

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It is annoying to come in from a long day and sit down for a moment and along comes a pesky gnat! Once they find you, it is not likely they will leave very soon. More than likely, these gnats are fungus gnats that quickly become a nuisance in homes. Adult fungus gnats are small, ranging from 1/10 to 1/8 inch long. They are slender with long legs and have dark colored antennae. Their color ranges from gray to black or orange to yellow. Details are hard to distinguish without a hand lens due to their size.

Photo of adult fungus gnat.

Adult fungus gnat.

Fungus gnats are attracted to moisture. Potted house plants and sink drains are often the culprits that attract these flying insects. To help determine if the gnats are coming from a plant or drain, cover the plant or drain with a screen or cheesecloth and check the next day for evidence of the gnats. The plant’s soil may accumulate excess moisture. Check the soil regularly and if there is excess water, drain the water off. Moving the plant outside or to a sunny location can help the soil dry out. When the soil dries out, any larvae present in the soil will die. Do not get carried away and leave the soil to dry too long. The plant still needs water to survive.

Adult gnats are easily controlled with pyrethrin spray or aerosols labeled for use against “gnats” or “flying insects.” These may not be the best option for control because they are very short-term and give a temporary solution. Unless you can access and treat the source, more flies may appear once the chemical dissipates. Another less hazardous option is using Bacillus thuringiensis var. Israelensis (Bti). Some common trade names for the product are Gnatrol® and VectoBac® and can be found in garden supply centers. The active ingredient is a naturally occurring bacterium found in soils. These products can be applied as soil drenches to potted plants indoors. Regardless of how “safe” you consider any pesticide or insect control product, always read the label and follow directions and safety precautions.

If gnats are breeding in a drain, use a commercial drain cleaner and/or clean out the trap to eliminate the breeding site. Other areas these gnats are drawn to are utility rooms that may have a leak in the waterline to a clothes washer and crawlspaces that hold moisture. There may be other leaks around the home as a result of storm damage or poor maintenance. Yellow paper sticky traps can also be used to monitor the insect’s activity.

Remember, the key to solving indoor fungus gnat problems is to find and eliminate the source of excess moisture.