Fungus Gnats in Houseplants

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You might be one of those that propagates houseplants. Some are heirlooms that have been in the family for many decades and you share the new starts to keep family traditions going. But every year is different and something might be bugging you. Literally. Some of your pots might have fungus gnats. Gnats are annoying anyway. As a plant lover, however, the fungus gnat really is a nuisance whose larvae are munching away at the roots of your transplants.

Gnats are a group of species in Order Diptera, the same group that contains craneflies and mosquitoes. While it’s a familiar pest in the greenhouse where humidity levels are usually high, sometimes fungus gnats become a bother indoors in both residential and commercial buildings when adults emerge in large numbers from potted plants or some other chronic source of moisture (and subsequent mold growth). Adults are attracted to lights including televisions or computer monitors at night, which can be particularly aggravating and are often first noticed at windows or light fixtures.

These tiny gnats prefer overly-moist soil full of decaying plant matter. They complete their life  cycle in about four weeks depending on temperature and other environmental conditions. With indoor infestations there will typically be continuous reproduction and overlapping generations with all life stages likely to be foundFungus Gnats throughout the year. Adults fungus gnats live about seven to ten days. The female deposits 100 to 300 eggs in batches of two to 30 each in decaying organic matter. Eggs hatch in four to six days; larvae feed for 12 to 14 days. The pupal stage lasts about five to six days. Larvae primarily survive off fungi found in the soil but also feed on root hairs.

The root-feeding that is done by the larvae can harm many plants. They are general feeders, but are often found around African violets, carnations, cyclamens, geraniums, poinsettias and foliage plants. Damage to plant roots may promote root diseases. Plant damage symptoms may appear as sudden wilting, loss of vigor, poor growth, foliage yellowing and early leaf drop.

But you can get rid of fungus gnats without having to lose your houseplants. The key to solving indoor fungus gnat problems is to find and eliminate the source, i.e., find the area(s) of excess moisture. Drain any excess water from the dish/container below flower pots. If the weather permits, move the plants outdoors or allow the soil to dry (not to the point where plants will wilt). Then, simply add a day (or more) between regular watering and the problem should decline.

Generally, use caution when considering the use of insecticides to control fungus gnats within a home or building. Inside, insecticides are not usually necessary for control, because there are easy cultural changes you can make to solve the problem (such as exclusion, sanitation, and potted plant management are effective approaches). However, indoor plants or interiorscaping that are difficult to remove can be treated with a number of pesticides that are listed in the North Carolina Agricultural Chemicals Manual. Among the less hazardous products available to consumers are those containing Bacillus thuringiensis var. israelensis (Gnatrol, Vectobac). These products can be applied as soil drenches to potted plants indoors or to outdoor areas. Outdoor landscaped areas can also be treated with beneficial nematodes. Regardless of how “safe” you consider any pesticide or insect control product, always read the label and follow directions and safety precautions.

Amanda Wilkins is Horticulture Agent for North Carolina Cooperative Extension in Lee County.