Fungus Gnats

Tips for controlling these pesky plant pests.

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Q. Tiny gnats that we think may be coming from our indoor potted plants are driving us bonkers. How can we get rid of them without losing our plants and spraying harmful chemicals?

A. These extremely annoying critters — probably fungus gnats — thrive on the moist soil and agreeable temperatures. Because you seem to have a fairly severe infestation you may need to take your plants outside (provided temperatures are well above freezing) and treat them with soil-drench insecticides. Or, you can leave your plants inside and apply a drench of Bti (Bacillus thuringiensis subsp. israelensis) to the soil surface (mix one to two teaspoons of Bti per gallon of water). Make sure that the soil isn't already too wet before you apply the drench. As you know, Bt is typically used to kill mosquito larvae but is harmless to people, pets and birds. Bti, a highly selective form of Bt, will get rid of the fungus-gnat larvae. You'll have to keep swatting and hanging pest strips to cut down the adult population.

Fungus gnats love wet soil, so be sure not to overwater your plants. Try letting the top inch or two of soil in your containers dry before watering again. When shopping for new plants or when bringing your plants in for the winter, carefully inspect them to make sure you're not introducing these intruders into your home.

One way to tell if a plant is infested with fungus gnats is to place a slice of raw potato on the soil surface. The potato will draw the larvae, which have clear to milky bodies about 1/4-inch long and shiny black heads. You can also lay sticky pest strips across the rim of the pot to check (and trap) the adults.

Good luck.

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