Q&A: Fungus Gnats Swarming Houseplants

I mistakenly introduced a houseplant from a less than desirable source into my home and it was tainted with fungus gnats. Now they are everywhere! What can I do to get rid of them?

—L. R.
Dillon, Colo.

There are numerous species of fungus gnats. These insects are not host specific, and may target many houseplants, including poinsettia, African violets, cyclamens and foliage plants. Bedding plants also are likely victims, especially young seedlings. Though the larvae are doing the damage, the adults are first seen—tiny mosquito-like insects that fly from plants as they are watered. These adults are 1/10 to 1/8 of an inch long (2.5 mm), dark and slender, with one pair of wings. They are members of the fly family, Diptera.

Fungus gnats prosper in damp, dark areas rich in decaying organic matter and fungus. Females lay up to 300 eggs on moist soil. The larvae hatch in about four days and begin to feed on organic matter in the soil, including small plant roots. Larval stage lasts about two weeks. After pupating for five to six days, they emerge as adults, capable of flight and mating. Adult life span is just over a week. All stages may be present at once, with overlapping broods. As long as adequate moisture and warm conditions exist, they will continue to reproduce—and so are often a problem in greenhouses, especially where seedlings are grown.

Symptoms: Plants show yellowing and loss of vigor, weak foliage and sometimes sudden wilting. Usually the onset is not dramatic, and it may be attributed to other causes. Tapping the plant to see if tiny adults dart away will aid in diagnosis. Sliced potatoes may also be placed on the surface of the soil and left for a time to attract the larvae.

Controls: Presence of fungus often indicates over watering. One of the best controls is simply to let the growing medium dry well between watering. Be mindful that in a greenhouse decaying matter and moist conditions may prevail under the benches; these areas must be cleaned up and allowed to dry as well. Media with good drainage is also important, since soggy, peat-heavy mixes are favored by fungus gnats. Biological controls for treating the potting soils include parasitic nematodes and predatory mites. The strain of Bacillus thuringiensis var. israelensis known as Gnatrol is recommended, and moderately easy to find. Insect growth regulators, which are relatively inoffensive insecticides, include Enstar, Precision and Azatin. To kill adults, common diazinon or pyrethrins are effective, or you may use Sunspray horticultural oil. Horticultural oils are the more organic choice, but be careful not to apply it to foliage in hot sun or the plants may suffer foliar burn.

Many gardeners have reported success with simply laying a quarter- to a half-inch of coarse sand on top of the soil. This disrupts their life cycle—it dries out quickly, or it isn’t an attractive place for females to lay eggs, and any larvae that do hatch can’t seem to burrow through it to the soil.

Strange growth, no blooms or are you wondering the best way to transplant? Just ask, and the Horticulture editorial team will take a stab at answering your ailment or query. E-mail edit@hortmag.com

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