Deb Babcock: Fungus gnats - an indoor plague

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Deb Babcock

Deb Babcock's gardening column appears Thursdays in Steamboat Today.

Find more gardening columns here.

Uh, oh. You brought plants inside from the garden, and now you have annoying bugs flying around the foliage. A number of houseplant insects tend to breed in garden soil, especially when they experience the nice warmth of the house and we start watering our houseplants on a regular basis.

Fungus gnats lay their eggs in very moist, acidic soils. Indoor gardeners inadvertently tend to nurture fungus gnats by over-watering and by not clearing away dead and decaying leaves from houseplants. The white, black-headed maggots feed on the soil fungi and sometimes eat roots, weakening growth.

If the problem is really bad, you first should let the top few inches of soil dry out completely. Then, drench the soil with an insecticide that contains Bacillus thuringiensis H-14 (Bt), also known as Gnatrol, which is safe for humans and mammals. It specifically attacks fungus gnat larvae and takes about three weeks to be effective. You might also consider re-potting the plant using fresh potting mix. Shake away all the old soil, making sure all maggots near the roots are discarded.

There also are several houseplant insect sprays, containing Pyrethrin, that kill fungus gnats at all stages of development. Thoroughly wet all parts of the plant with this spray. Treatment may need to be repeated in one to two weeks, if needed. What I like about this treatment is that it may be used on edible plants, such as kitchen herbs, up until and including the day you harvest them. (Of course, use common sense and rinse the edible portions thoroughly before placing them in your food.)

Be sure to check the other plants in your home, because once these insects begin flying, they'll move beyond their initial host plant.

Outside of vacation time, I rely on an inexpensive moisture tester that I poke into the plant soil about three-quarters of the way toward the bottom of the pot. Then, we don't water until it registers toward the dry end of the meter.

When you water, water until it seeps out the hole in the bottom of the pot, and discard the excess water right away.

Be sure to clear away any dead leaves that have fallen to the soil surface. This will help lessen the growth of fungus and provide fewer hiding places for fungus gnats.

Master Gardener Barb Sanders, who has a wonderful indoor garden, has found that placing a small bowl of sweetened water or mouthwash near the plants will attract the gnats, causing them to drown in the liquid. Barb also suggests using yellow sticky traps for monitoring the size of the population. These traps may be found in garden shops or made at home by using yellow construction paper strips coated with Tanglefoot®.

Maybe I'll learn to give my plants a little tough love next time I vacation.

Deb Babcock is a master gardener through the Colorado State University Cooperative Extension office in Routt County. Products mentioned in this article are not endorsed by the Master Gardener program but are provided for informational purposes. Call 879-0825.

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