Species: Darkwinged fungus gnats (Orfelia and Bradysia species, family Sciaridae); Small fruit flies or vinegar flies (Drosophila species); Drain fly or moth fly (Psychoda phalaenoides)
Side by side
| Fungus gnat | Small fruit fly | Drain fly | |
|---|---|---|---|
| Length | Commonly 1/16 to 1/8 inch, 1.6 to 3.2 mm | 2.0 to 2.5 mm, about 0.08 to 0.10 inch | 2 mm, about 0.08 inch; Maryland gives 1/16 to 1/4 inch |
| Look | Dark and delicate, mosquito-like, long segmented antennae | Light brown, bright red eyes in the common species | Pale brownish gray to black, fuzzy, moth-like |
| Where the larvae live | Moist potting soil, organic debris and decomposing matter | Overripe fruit and fermenting residues such as unwashed beverage containers | The gelatinous slime film inside drains and traps |
| Where you see adults | Near windows and on the soil of potted plants | Around fruit bowls, recycling and kitchen spillage | Resting on walls and ceilings near sinks, tubs and floor drains |
| Why it matters | Adults do not damage plants or bite; larvae in numbers can damage roots | Breeds in fermenting food residue; Colorado State's remedy is removing the source | Larvae feed on bacteria coating wet plumbing |
What to look for
- Shape. UC IPM describes fungus gnats as dark, delicate-looking flies similar in appearance to mosquitoes, with slender legs and segmented antennae longer than the head.
- Size. UC IPM gives fungus gnat adults as commonly 1/16 to 1/8 inch (1.6 to 3.2 mm). Colorado State gives small fruit flies as 2.0 to 2.5 mm (about 0.08 to 0.10 inch) and moth flies as 2 mm (about 0.08 inch).
- Eyes. Colorado State says the most common small fruit fly species have bright red eyes, and gives their body color as light brown.
- Wings. UC IPM says the common Bradysia fungus gnats have light gray to clear wings carrying a Y-shaped vein. Colorado State says moth fly wings make the adult superficially resemble a tiny moth.
- Flight. UC IPM notes fungus gnats are relatively weak fliers compared with a house fly and usually stay near potted plants, running across or resting on the growing media.
- One more that is often in the mix. Colorado State describes humpbacked or phorid flies at 3 mm (about 0.12 inch), resembling a small fruit fly but separated by a large humped thorax seen from the side.
Where and when
UC IPM says fungus gnats thrive in moist conditions, so overwatered houseplants and poor drainage are the usual indoor source, and a generation takes about 17 days at 75 degrees Fahrenheit. Colorado State reports small fruit flies are most numerous in late summer and early fall, when they infest fruit during harvest season and get carried indoors, and that fungus gnats gather at windows through fall and winter.
Does it matter?
UC IPM says adult fungus gnats do not damage plants or bite people and are primarily a nuisance, while larvae in large numbers can damage roots and stunt seedlings. Colorado State says moth fly larvae feed on bacteria coating continuously wetted plumbing, and that humpbacked flies often breed in drain food waste. Both point at the source: let soil dry between waterings, and remove or refrigerate overripe fruit.
Related guides
- Drain fly: A drain fly is a tiny, fuzzy, moth-like gnat that sits still on the wall near a sink or a floor drain and makes short hopping flights when disturbed.
- House centipede: The fast, many-legged thing in a bathroom is almost always a house centipede.
- Weevils and flour beetles: The snout tells you which group you have.