Fungus Gnat

The common name of gnat is applied to certain small flies, and that of fungus comes from their common occurrence on fungi which serve as a major food source for their larvae; dark-winged describes the smoke-colored wings commonly found within the Sciaridae. These flies are nuisance pests in and around structures, but a few species are agricultural pests. For the Sciaridae, about 137 species, and for the Mycetophilidae, about 714 species, are found in the United States and Canada.

Recognition

(Adults about 1/32-7/16” (1-11 mm for Sciaridae; 2.2-13-3 mm for Mycetophilidae) long; slender to moderately robust, long-legged, somewhat mosquito-like. Color usually black, brown, or yellowish, sometimes brightly colored (Mycetophilidae); wings usually smoke-colored (Sciaridae) or sometimes patterned with darker areas (Mycetophilidae). Head with eyes separated (Mycetophilidae) or touching (Sciaridae) just above antennal bases; ocelli (simple eyes) present. Antenna usually with 16 segments (range 11-17). Wings with front margin (C) thickened to near wing tip; Sciaridae with only 2 branched/forked veins, media (M) usually appearing unattached basally and forking beyond middle of wing and cubitus (Cu) forking in basal 1/4th of wing; Mycetophilidae with radial sector (Rs) simple/unbranched or 2-branched (if 2-branched, fork is beyond r-m crossvein or r-m is obliterated by fusion of Rs and M). Legs with coxae elongated, tibiae with 1 or 2 apical spurs, pulvilli (pads beneath tarsal claws) absent or minute (Mycetophilidae) or narrow to broad but much shorter than claws (Sciaridae).

Mature larvae usually slightly longer than respective adults; sciarid larvae with shiny black head and a 12-segmented essentially featureless white translucent body, terminal abdominal segment with a ventral/bottom lobe which acts as a proleg; mycetophilid larva usually cylindrical and slender, with a well-developed head, and 11 or 12 segments, most segments often with ventral creeping welts (roughened areas).

Similar Groups

  1. Humpbacked flies (Phoridae) with humpbacked appearance in side view, wing with strong, heavily pigmented veins along front, remaining 3-4 veins weak, oblique, and unbranched, and hind femur flattened.
  2. Minute black scavenger flies (Scatopsidae) have wing with radial sector (Rs) not forked, veins along front strong, heavily pigmented, remaining veins weak, antenna 7-12 segmented, short.
  3. Gall gnats (Cecidomyiidae) with long 14-16 segmented antenna, wing with costal (C, front edge) vein usually continuous around wing, usually with a break just beyond where R5 meets wing margin, and tibial spurs absent.

Biology

Very little is known about the biology of the species in these two fly families. For the Sciaridae, there are 4 larval instars and apparently one species (Moehina erema Pitchard) is parthenogenetic (eggs develop without fertilization).

For the Mycetophilidae, females lay their eggs singly on larval food material, and eggs hatch within a few days. Under optimal conditions, they pass through 5 instars in 6-8 days. They usually pupate in the ground, and adults emerge in about 3 days. The larvae of some species, such as Ortelia fultoni (Fisher), are luminescent (glow).

Habits

Adults are typically found on or near larval food materials. Larvae feed primarily on fungi growing in the soil and moist decaying organic matter. Sciarid larvae mostly feed on decaying plant material, animal excrement, or fungus, but some feed in rotting wood or under bark of fallen trees. Several sciarid species are economic pests in greenhouses and commercial mushroom houses, and the larvae of the potato scab gnat (Pnyxia scabiei (Hopkins)) bore into potatoes grown on low ground. The larvae of some species migrate in snake-like formation over the ground. Mycetophilid larvae mostly feed on fleshly or woody fungi, on or in dead wood, under bark, or in the nests of birds or squirrels. Larvae of some species spin webs and capture and eat small arthropods.

Inside, fungus gnat infestations are almost always associated with the soil of overwatered potted plants and atriums. Secondarily, consider signs of water leaks or moisture problems, such as water stains, peeling paint, swelling of walls or wall coverings, to indicate where fungi may be growing which can support larval development. Consider flat roofs which are particularly prone to water leaks. Also, the feces in pet bird cages can be the source if the feces are not removed in a timely manner.

Outside, mulched areas and the soil associated with them is usually the source, especially if new mulch is added annually without removing the old first and/or if sprinkler systems are present. Other sources include accumulations of dead leaves, old firewood piles, compost piles, etc.

Adults are usually found in moist areas where larval food is present. During the day, adults of many species congregate in dark moist places. Some species are most active at dusk and less active at dawn. Adults of most species are attracted to light.

Control

Follow the basic 5 steps of identification, inspection, sanitation, mechanical control/exclusion, and insecticide application if required. The key is finding and eliminating all of the breeding sources, so don’t stop until all potential sources have been inspected. In this case look for moist/damp areas which will support fungal growth. Inside, first check the soil of potted plants and atriums and then move on to the other areas mentioned above. If adults are being seen primarily at windows near outside doors, check for the same adults in the immediate area outside; remember adults are attracted to lights. In offices and healthcare facilities, it may be necessary to have all live plants replaced with artificial ones. Occasionally, infestations can develop in wall voids of new structures when damp construction materials are used, trapping enough moisture for fungal growth. Remove the wall plates and put a plastic bag that has been perforated several times with a fine sewing needle over the opening; the flies will follow the airflow and come to the light.

The key to solving the problem once the source(s) have been found is to either remove or dry out the moist material so that it can no longer support fungal growth. Sometimes turning over the top 2-3” (51-76 mm) of soil and/or adding fans to blow over the moist surface will speed the drying process.

The senior author resolved a large dark-winged fungus gnat problem in the lab section of a pharmaceutical plant by tracing the infestation to the flat roof. A new membrane roof had been installed over the previous leaking roof without first drying out or removing the old roofing materials. Adults could gain access to the old moist roofing materials via an improperly sealed front edge and unscreened vent pipes which had been added (going to the old roof through the new roof) in an attempt to let the moisture escape from the old roofing materials. Temporary relief was accomplished by putting portable electrocutor ILTs in the false ceiling to harvest adults until the new roof could be removed, the old roofing materials replaced, and the new roof re-installed. Insect light traps (ILTs) work well in reducing the numbers of adults indoors. Once the source(s) have been eliminated, a non-residual ULV/space treatment can be used to quickly kill the remaining adults if necessary.