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If you notice little gnats flying around your house plants, there’s a good chance those plants are being overwatered. Fungus gnats are insects commonly associated with overwatered houseplants. They can become a nuisance when they are present in large numbers and fly around inside a home. While fungus gnats are mostly a cosmetic problem, fungus gnat larvae can cause plant damage.


You can monitor for fungus gnat adults using yellow sticky traps placed near a plant’s leaves. Check with your local garden store or hardware store. These sticky traps are inexpensive and often include small stakes making them easy to use with potted plants. They not only help capture adult fungus gnats (and other insect pests), but they can help plant lovers keep track of the number of fungus gnats over time.


Some of you may have used vinegar traps or other methods commonly used to monitor for fruit flies, but they don’t work when monitoring for fungus gnats.


It may sound odd, but potato slices can help monitor for fungus gnat larvae. Place potato slices on the soil surface of potted plants. If fungus gnat larvae are present in the soil, they’ll come to the surface to feed on the potato tissue. Check the slices for maggots after three to four days.


As noted above, sticky traps can help control fungal gnats. However, altering environmental conditions of houseplants is the single most important step in managing this insect in a non- chemical manner. Keep the soil surface dry to eliminate favorable egg-laying sites for the insect by allowing the top inch of the soil to dry out before watering. Alternatively, watering from the bottom provides moisture for the roots while keeping the soil surface dry. Another option is to cover the soil with about an inch layer of coarse sand or fine gravel, which will help keep the surface drier and make the soil less attractive for egg-laying.


For biological control, products containing Bacillus thuringiensis subsp. israelensis (Bti), are available to homeowners and can be used to control fungus gnat larvae in soil. These treatments don’t affect eggs, pupae or adult fungus gnats. Apply these products with enough water to help the Bti filter through the soil to reach the larvae. Use several applications spaced five to seven days apart to control newly hatched larvae until the infestation is under control.


Unless a fungus gnat infestation is severe, chemical controls are not warranted. If adult numbers are excessive however, insecticides containing pyrethrins or synthetic pyrethroids can provide temporary control. If this is the route you choose, be sure to select a product that is labeled for indoor use on houseplants and read and follow all product label instructions.


Apply insecticides to plants and to the surface of potting soil where adult fungus gnats typically rest. Don’t spray the air with these products as such treatments are ineffective. Even in those situations where insecticide use may be warranted, keep in mind that chemical treatments should not be your sole management approach. Insecticides should always be used in combination with other non- chemical practices.


While fungus gnats are often more of a problem in the fall when houseplants that have been outdoors for the summer have become infested and are brought back indoors, these pests can be a problem anytime during the year. It may take three to four weeks of modified watering and use of sand/gravel to get fungus gnats in check.

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