How to Kill Fungus Gnats

Fungus gnats are small, "mosquito-like," gray-black, flies of the Sciaridae fly family often noticed around house plants and windows. Found throughout the United States, most species inhabit fungi or dead plant materials, but sometimes young maggots injure plants by feeding on roots. This occurs particularly if the maggots become very numerous and thus limit their food supply.

Host Plants and Damage

Fungus gnats are general feeders. They attack a wide range of ornamental and vegetable plants. Some species can be serious pests in mushroom houses. The maggot or larvae is the damaging state. However, the adult flies are usually noticed before larval damage to the plant is apparent. When maggots become numerous they strip the roots resulting in loss of plant vigor and yellowing and wilting of the leaves.

Description and Life History

The adult flies are slender, approximately 1/8 inch (2.5mm) in length, and have long legs and antennae. They are weak flies but can run quite rapidly across the soil surface. During the female’s lifetime, of about one week, she lays a hundred or more eggs.

The shiny white oval eggs are semitransparent and are barely visible to the naked eye. They are laid either singly or in strings or groups of 10 or more in the soil surface, usually near host plants. They hatch in four to six days.

The mature larvae or maggots are about ¼ inch (5.5mm) long with shiny black head capsules and white transparent bodies. The maggot reaches maturity in about two weeks, when it ceases feeding, spins a silken cocoon and sheds its skin. After about a week, it transforms into a pupae. At the end of the pupal period, the adult fly emerges from the soil and starts the cycle over again. There are many overlapping generations throughout the year.

Control

For homeowners, chemical control of this pest is restricted to controlling the adults. Ideally, larval control would be the best approach for achieving complete cleanup of fungus gnat problem, but no insecticides as soil drenches are registered for home use to control the larvae. Household sprays containing pyrethrins or pyrethroids and labeled for "gnats" or "flying insects" will control adults. Spraying for adults must be done several times. One application will not eliminate the gnat population. Larval control in the home environment can be achieved by "trap cropping." Pots of sprouting grain often attract female gnats who lay their eggs near these plants. After a few days these pots should be submerged in boiling water to destroy the eggs and maggots or discarded outdoors. This practice should be repeated every two weeks until flies are no longer noticed.

Fungus gnats live in moist, shady situations with decaying organic material present, so cultural practices such as the elimination of dead leaves or other decaying organic matter and the avoidance of excessive watering will greatly reduce their numbers.

Monitoring

Successful fungus gnat control depends on a monitoring program for detection of adults. Early detection will result in quicker suppression. Yellow sticky traps should be used for detection. Place trays just above the plants at the frequency of one per 500-1,000 square feet. Replace them each week and count the number of fungus gnats on each trap.

Warning:

Use pesticides with care. Apply them only to plants, animals or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.