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 FLORIDA FLIES
A source for information on flies

Fungus gnat  Family Sciaridae

Gnat is the common name for many small, winged insects in the fly family. Regionally, they are also sometimes called flies, midges and no-see-ums. Contrary to popular belief, these tiny flying insects are not “babies”, they are adults. The tiny flying insects that many people call “gnats” could really be fruit flies or fungas gnats.Depending on species, gnats can be biting or non-biting and will feed on plants, other insects or blood. Males assemble in large mating swarms known as ghosts. These swarms occur most commonly at dusk in large fields and above streets.Depending on the species, gnat eggs are laid on land or water. Larvae may be immobile or capable of movement by way of rocks and water plants. Adults range in size from one to three millimeters. The larval and adult stages of the gnat are considered both beneficial and detrimental. Some species are excellent plant pollinators and feed on crop pests such as aphids and scales. Other gnats, such as the Hessian fly, are crop pests, themselves.

Females of some species, such as the black gnat or black fly, feed on blood. These gnats have been known to carry parasites and spread diseases to humans and livestock. Due to the spread of river blindness and other health concerns, numerous programs have been established throughout the world to control gnat populations. While the small size of the gnat can make it appear less serious as a pest, it is important to remember that gnats reproduce quickly and populate infested areas in swarms. This is how gnats become larger problems to homeowners or apartment residents. Many gnat species are capable of being a pest and causing damage to the material where it lays its eggs. Most damage caused by these “gnats “occurs during the larval stage, when they are feeding. They lay their eggs in damp soil of plants that have been over-watered. When the eggs hatch, the immature gnats feed on decomposing, organic material in the soil. These gnats are attracted to fungus that grows in the soil of houseplants that have been over-watered. Leaves, stems and roots are commonly affected in areas infested by these gnats. Gnats feed on the roots and root hairs of plants, as well. They are most harmful to small plants, but can also destroy large plants if the plant is already unhealthy or if the gnat population is extremely large. Growers go to great lengths to prevent and eliminate gnat populations, as the damage caused by a gnat infestation can damage both ornamental and edible plants.

Appearance:
Adult fungus gnats are grayish-black in color, slender, mosquito-like and delicate with long legs, antennae and one pair of wings.

Behavior:
Fungus gnats reproduce in moist, shady areas in decaying organic matter such as leaf litter or in homes where warm temperatures are maintained. Adults live about 7-10 days and deposit eggs on the moist soil surface or in soil cracks. Females can lay up to 100-300 eggs in batches of 2-3. Eggs hatch in 4-6 days; larvae feed for 12-14 days before emerging as adults. There are many overlapping generations throughout the year. Sometimes occurring in large numbers, they gravitate toward windows, attracted by sunlight, but may also fly about desks or be attracted to persons by cologne or perfumes. The tiny flies flitting about one’s personal workspace can be disconcerting, prompting calls to have the insects controlled.

Habitat:
Fungus gnats occasionally become nuisance indoors when adults emerge in large numbers from potted plants or flower boxes containing damp soil rich in humus. Damage often occurs in greenhouses and plant beds. The larvae can injure the roots of bedding plants, African violets, carnations, geraniums, poinsettias and foliage plants. Some are serious pests in mushroom houses; larvae feed on fungi and decaying organic matter as well as living plant tissue, particularly root hairs and small feeder roots.

Fungus gnat infestation can cause brown scars to appear on the chewed roots. The underground parts of the stem may be injured and root hairs eaten off. Plant symptoms may appear as sudden wilting, loss of vigor, poor growth, yellowing and foliage loss. Fungus gnats may breed anywhere that suitable molds might grow. In nature, fungus gnats breed in the soil, in fungi, and any place where suitable molds might grow, e.g., a rotting log. In buildings, they are most often associated with the soil in potted plants and in atriums. When plants are overwatered, molds in the soil are capable of reproducing thus providing a breeding medium for fungus gnats. On occasion, these flies have been found breeding in ceilings and walls where water leaks are present or have occurred, but have not thoroughly dried. Molds thrive in such damp areas and provide a place for fungus gnats (and certain fungus-feeding beetles) to breed.

A Fungus gnat infestation indoors may also mean a moisture problem is present. Although fungus grows in areas such as moist wood (the result of leaks or poor insulation) and poorly ventilated crawlspaces and attics, most infestations are associated with moist potting soil.

Interesting Fact:
Fungus gnats are attracted to light, so ultraviolet-light fly traps or “bug zappers” are capable of attracting and killing many adult fruit flies at night, as well as other winged pests.

Control:
Inspect plants carefully before purchase for signs of insect infestation. Always use sterile potting soil to prevent introduction of fungus gnats. Over-watering, water leaks and poor drainage can lead to concentrations of fungus gnats. Allowing the soil to dry as much as possible, without injury to the plants, is effective in killing many larvae. Remover all old plant debris in and around the home.

The control of fungus gnats involves discovering the breeding sites then drying them out sufficiently to stop the growth of molds and thus eliminating the breeding media used by the flies.

  • The top inch or so of the soil in potted plants can be turned over several times to dry out the soil.
  • Plants should then be watered less frequently and only when watering is necessary. Soil in potted plants should not be allowed to stay constantly wet.
  • Areas where water leaks have occurred should be dried thoroughly using fans, or wet wood or other building materials need to be replaced.
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