Bradysia spp. Winnertz, 1867

Fungus gnat

Tiny dark flies hovering over houseplant soil, their larvae feed on fungi in damp potting mix.

Order: Diptera
Family: Sciaridae
Group: Flies
Ontario status: Established, common (greenhouses, houseplants)

Why it matters

Mostly a houseplant nuisance. In greenhouses and nurseries, heavy larval feeding damages seedlings and spreads plant pathogens, an economic pest of horticulture, not of homes.

Origin

A worldwide family of soil-dwelling flies; the houseplant species arrived with the potted-plant trade.

Behavior

Adults hover weakly over soil and run across the surface. Harmless to people, they don't bite, they just annoy.

Living conditions

Larvae live in damp potting soil, feeding on fungi, algae and fine root hairs. Overwatered plants are the entire problem.

Signs of presence

Tiny dark flies rising when you water or disturb a plant
Flies stuck to nearby windows
Seedlings wilting for no clear reason (heavy larval root feeding)

Weaknesses

Larvae need constantly moist soil, letting the top centimetres dry between waterings kills them
Bottom-watering and good drainage deny them habitat
Yellow sticky traps catch the flying adults and show whether numbers are falling
Sterile, fresh potting mix starts clean

Myths and facts

Myth They bite like mosquitoes.
Reality Adult fungus gnats have no biting mouthparts at all. Anything biting near your plants is something else.

Research notes

Morphology

Adult 2-4 mm; slender, dark grey-black, mosquito-like with long legs and long beaded antennae. Wing venation distinctive: the media and cubitus form a Y-shaped fork, the family field character. Larvae: translucent white worms ~5 mm with a shiny black head capsule, in soil.

Diagnostics

Confirm by: 2-4 mm dark delicate flies over moist soil, Y-forked wing veins, black-headed translucent larvae in the top layer of potting mix.

Easily confused with

Drain fly, Psychoda spp. Drain flies are moth-like and furry with broad wings; fungus gnats are slender and mosquito-like with narrow wings.
Fruit fly, Drosophila melanogaster Fruit flies are tan with red eyes and hover over fermenting matter; fungus gnats are dark and stay over soil.

Life cycle

Egg: Laid in moist organic soil; hatch in days
Nymph or larva: Translucent larvae with black head capsules graze fungi, algae and root hairs in the top 2-3 cm of soil; 4 instars
Adult: Lives about a week; does not feed significantly
Egg to adult: Approximately 3-4 weeks at 20-25 °C
Generations per year: Continuous in greenhouses and homes; overlapping generations

Development

Complete metamorphosis: egg → larva (4 instars, in soil) → pupa (in soil) → adult.

Reproduction

Females lay 100-200 eggs in moist soil; populations track watering frequency directly.

Distribution

Native range: Cosmopolitan family
Ontario: Common in greenhouses, nurseries and homes year-round
Global: Worldwide

Ecology

Breeding media: moist, organically rich soil and soilless media. Larvae are primarily fungivores, the fungi, not the plant, are the attractant; root damage is secondary. Greenhouse and interior-plantscape pest; vectors of plant pathogens (Pythium, Fusarium) on their bodies.

Resistance

Unknown, managed culturally and biologically; no significant resistance documented.

IPM implications

Cultural control is decisive because the larvae are moisture-obligate: allow the soil surface to dry between waterings, improve drainage, remove algae from surfaces. Yellow sticky traps monitor adult trends. Biological controls (the predatory mite Stratiolaelaps scimitus, the nematode Steinernema feltiae) target larvae in the soil, the biology says treat the medium, not the air.

Chemical ecology

Adults attracted to moist organic media volatiles; larvae follow fungal growth in the soil profile.

Genetics and DNA

No reference genome published as of 2026.

Taxonomy

Diptera: Sciaridae (dark-winged fungus gnats). Genus Bradysia Winnertz, 1867; B. impatiens and B. difformis are the common greenhouse species.

Open questions

Species boundaries among the common greenhouse Bradysia (morphologically cryptic); quantitative thresholds for economic damage in ornamentals.

References

Cloyd, Kansas State Extension, fungus gnat biology and management
Mead & Fasulo, UF/IFAS Extension, dark-winged fungus gnats
Hungerford et al., Sciaridae larval ecology, primary literature