GREENER PASTURES

By Peter Warren, Extension Agent
Published: November 20, 2008

With winter weather approaching many of us bring our tender plants indoors for the winter.  With the plants we sometimes get their pests.  When these insects have a little population explosion inside our homes it is not always greeted with the curiosity and wonderment you might expect.  One common suspect in these situations is the fungus gnat.
Fungus gnats occasionally become a nuisance indoors when adults emerge in large numbers from potted plants or flower boxes containing damp soil rich in humus. Adults are attracted to lights and are often first noticed at windows. The immature fungus gnats or maggots feed in soil high in organic matter and can injure the roots of bedding plants, African violets, carnations, cyclamens, geraniums, poinsettias and foliage plants. Plant symptoms may appear as sudden wilting, loss of vigor, poor growth, yellowing, and foliage loss. Some are serious pests in mushroom houses. Fungus gnats inhabit fungi or dead plant materials and are harmless to humans and animals.
Adult fungus gnats are about 1/8 to 1/10 inch (2.5 mm) long, grayish to black, slender, mosquito-like, and delicate with long legs, antennae and one pair of wings. Identification can be made by the vein patterns in the wings if you are the kind of person who enjoys examining insect wing venation under a microscope (I did not think so).  Eggs are hardly visible, oval, smooth, shiny white and semi-transparent. Larvae or maggots are legless, thread-like, white, shiny blackheaded, up to 1/4 inch (5.5 mm) long and transparent so food in the gut can be seen through the body wall. Pupae occur in silk-like cocoons in the soil.
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. Broods overlap, with all life stages present during the breeding season. 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. Damage occurs most often in greenhouses or plant beds.
Adults live about 7 to 10 days and deposit eggs on the moist soil surface or in soil cracks. Females lay up to 100 to 300 eggs in batches of 2 to 30 each in decaying organic matter. Eggs hatch in 4 to 6 days; larvae feed for 12 to 14 days. The pupal stage is about 5 to 6 days. There are many overlapping generations throughout the year.
An ounce of prevention is worth a pound of cure as usual.  Inspect plants carefully before purchase for signs of insect infestation. Always use sterile potting soil to prevent introduction of fungus gnats. Overwatering, water leaks, and poor drainage may result in buildup of fungus gnats. Allowing the soil to dry as much as possible, without injury to the plants, is effective in killing many maggots. Since houseplants taken outside during warm weather may become infested with insects before being brought back indoors is a good idea to inspect plants carefully and you may want to discard some if they are heavily infested. Remove all old plant material and debris in and around the home. Practice good sanitation.
Many counties offer Master Gardener Volunteer training beginning in the New Year.  Contact your local Extension Office for more information on classes near you.
For more information about landscape topics contact your local Virginia Cooperative Extension office.  The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.