University of Vermont Extension
Department of Plant and Soil Science


Fungus Gnats                                      EL 50

by G.R. Nielsen, Former Extension Entomologist, Plant and Soil Science Department

Fungus gnats* are tiny black flies frequently numerous in greenhouses and around certain house plants. Often called
manure flies or mushroom flies, none of the adults feed on or damage plants. The larvae primarily feed on fungi and
organic matter in the soil. Sometimes these maggots also feed on the roots of flower crops, notably poinsettias,
geraniums, lilies, and chrysanthemums. Adults of some "shore flies" (Diptera: Ephydridae), which also breed in moist
places, including the greenhouse, are similar to fungus gnats but do not harm plants and so not require control.

Plant Damage: The maggots may feed on roots of many plants by chewing or stripping the roots. Severely injured
plants make poor growth, go off color, and may drop their foliage. Potted plants may lose vigor and the leaves turn
yellow, without any visible injury showing on the above-ground parts. Root hairs are eaten off, as are the small feeding
roots. The chewed roots have small brown scars on the surface. In cases of severe feeding, all that is left is the center
part of the root. Maggots can also cause injury to underground stems. Severe infestations have occurred in carnation
benches.

Hosts: Fungus gnats can cause serious damage to such flower crops as African violets, carnations, cyclamen,
geraniums, foliage plants, and poinsettias. Any crop that is grown in a high organic soil like peat moss, may harbor the
pests.

Life Cycle: A generalized life cycle is as follows: the adults (about 1/8-inch long) are slender, black or brown flies
with long wings, long, thin legs, and slender antennae. The eggs (barely visible to the unaided eye) are oval, smooth,
shiny-white, and semi-transparent. They are laid singly, or scattered in groups or strings of 3 to 10. Eggs hatch in 4 to
6 days.

The whitish larvae are so transparent that the food channels can be seen through the body walls. These legless maggots
have dark heads and are visible to the naked eye. The larvae is about 1/4-inch long when full grown, in about 10 to 14
days. All feeding on the roots is done by the larvae.

After full growth, the larva stops feeding, spins a very thin silken cocoon in the soil, sheds its skin, and transforms into
a pupa. The pupal stage lasts 4 to 7 days and no feeding occurs. At the end of this stage, the adult pushes itself out of
the cocoon and is ready to fly in a few hours. The female can mate soon after emergence and quickly starts to lay
eggs. As many as 272 eggs have been counted in a single female fly. One female can lay up to 1,000 eggs during its
lifetime.

In some moist, warm fall periods, fungus gnats abound and swarm to lights at night--becoming a widespread nuisance.

Fungus gnat broods will overlap, because every 1 1/2 to 4 weeks there is a new brood of flies. The life cycle (egg
laying to fly emergence) is 18 to 26 days at 65-75 degrees F. The adults live about one week.

Control:

In the Home:

1.Adults: Fungus gnats may be controlled with one of the many house and garden pressurized space or foliage
sprays. Treat daily or as needed. (Caution: the repeated use of any pressurized sprays in the home without
proper ventilation is not recommended.) Dusting the soil surface lightly with an insecticide dust will also control
adults satisfactorily.

2.Larvae: Best overall control can be attained by treating the soil. Use an insecticide soil drench. Wet the soil
surface thoroughly. Repeat weekly or as needed. Treat plants outdoors and let air and dry during the day
before bringing back inside.

Commercial Greenhouses: Adult gnats or flies are probably readily controlled with most of the common insecticides
used in the greenhouse. But maggot control is important if you hope to reduce or stop the root injury or feeding.

1.Adults: Although the adults do no damage, they may be a nuisance and are often a convenient stage in the life
cycle to attack. Insecticide smokes should reduce adult populations but may have to be repeated often. Dusting
the soil surface with any insecticide used for larval control should control the adults.

2.Larvae: For best results, treat the soil! Larvae may be controlled with soil drenches. Follow label directions for
soil treatment. Weekly applications may be necessary with less persistent insecticides. Treat walk areas and the
soil under the benches, especially around faucets or other moist areas, as well as the benches or pots.

Contact your local lawn and garden dealer or local Extension office for current chemical recommendations.


*Sciara spp.; Diptera: Sciaridae.


Before using any pesticide, read the label and follow all precautions!

Edited in January 1997, based on material developed in 1985.


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