Fungus Gnats

Common Name Scientific Name 
Darkwinged Fungus Gnat Sciara spp.
Fungus Gnats Orfelia spp.

Darkwinged Fungus Gnat
Adult, Larva

Fungus gnats occasionally become a nuisance indoors when adults emerge in large numbers as mosquito-like insects from potted plants or flower boxes containing damp soil rich in humus. Adults are attracted to lights and are often first noticed at windows. Larvae or maggots, which feed in soil high in orangic matter, 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.

Identification
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. Darkwinged fungus gnat adults have eyes that meet above the base of the antennae. 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.

Life Cycle and Habits
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.

Control Measures

Prevention
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. Houseplants taken outside during warm weather may become infested with insects before being brought back indoors. Inspect plants carefully and discard if heavily infested and unable to save. Remove all old plant material and debris in and around the home. Practice good sanitation. Electrocutor-light fly traps will attract and kill many adults at night.

Monitoring
Use yellow sticky cards (traps) for adult fungus gnat detection. Place traps just above the plants at a frequency of one per 500 to 1,000 square feet. Replace when covered with insects. Check traps 2 to 3 times each week.