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.
In the Home:
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.
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.
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.
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 taps or other moist areas, as well as the benches or pots.
Miterid....Scaridfly in conjunction with Penetrator usually solves the problem within 24 hours.