Be prepared for the invasion of the fungus gnats

It’s about this time of year when we start getting desperate phone calls about fruit flies in people’s homes.

“My house has been invaded with fruit flies, but there’s no fruit in my home” is a common lament. The invasion is rarely fruit flies and is almost always fungus gnats. Fungus gnats occasionally become a nuisance indoors when adults emerge in large numbers as mosquito or fruit fly-like insects from potted plants or sink drains containing organic material. Adults are attracted to lights and are often first noticed at windows.

Larvae or maggots, which feed in soil high in organic matter, can injure the roots of bedding plants, African violets, geraniums, poinsettias and foliage plants. Plant symptoms may appear as sudden wilting, loss of vigor, poor growth, yellowing and foliage loss.

Fungus gnats reproduce in moist, shaded areas in decaying organic matter such as leaf litter. The life cycle is about four weeks, with continuous reproduction in homes or greenhouses where warm temperatures are maintained. Larvae not only feed on fungi and decaying organic matter, but on living plant tissue, particularly root hairs and small feeder roots. Brown scars may appear on the chewed roots. The underground parts of the stem may be injured and root hairs eaten off.

Wherever organic material and moisture accumulates there is potential for fungus gnat breeding. This is particularly true of water drains with gooseneck plumbing in kitchens where such debris can accumulate and provide a breeding site for these flies and others, such as drain flies.

A regular (at least once a month) cleaning with a gallon of near-boiling water poured down the drain, followed by a cup of bleach diluted with water in a 1:5 ratio should render this hard-to-reach environment maggot-free for two weeks in most cases. Fungus gnat populations in potted plants can be cured by letting the top inch of soil completely dry before watering the plant.

For more information about horticulture, nutrition, health, home and family or 4-H and youth development programs, contact the Mat-Su/Copper River District office of the University of Alaska Fairbanks Cooperative Extension Service at 745-3360. Find us on the web at uaf.edu/ces.

Stephen Brown is the Mat-Su/Copper River District agriculture agent.