Fruit fly, drain fly or fungus gnat? How to tell which one you have

Three small flies get called "fruit flies" in a kitchen, and only one of them is. The fix for each is different, so a minute of identification saves a week of doing the wrong thing. Here is how to tell them apart by sight, by where they sit, and by a tape test, with what to do about each.

The quick table

The overnight tape test

If flies gather at a sink and you aren't sure which, dry the drain opening at night and press clear tape across it, sticky side down, leaving a gap at one edge for air. Check it in the morning. Flies stuck to the tape came up out of the drain. Fruit flies and drain flies both breed in drains, so the tape tells you where, and the shape tells you which. Extension entomologists at Purdue use this test for both (HN-66).

If it's a fruit fly

Fruit flies breed in any moist, fermenting film, and a female can lay about 500 eggs (University of Kentucky, ENTFACT-621). The job is to find the film and take it away, then make the counter uninviting while the last generation ages out.

• Brush the drain throat, run the hot tap, then add Sink Drops. Three nights in a row, then two or three times a week.
• Pull the disposal splash guard and scrub both sides.
• Rinse the trash can and every bottle and can in the recycling.
• Put ripening fruit in the fridge for a week.
• Set a peppermint and clove disc beside the fruit bowl, by the sink and on the trash lid. Peppermint oil and its compounds repelled the common fruit fly in a lab study (Wang et al., 2021). The study tested the ingredients, not our discs; judge it in your own kitchen with the 30-day guarantee.

Full routine: how to get rid of fruit flies.

If it's a drain fly

Drain flies (moth flies) breed in the slimy layer that builds in drains that rarely run: a guest bathroom, a basement floor drain, a shower that gets used once a week. Sink Drops are formulated for fruit flies in kitchen drains, so we'd rather tell you now than sell you the wrong thing. The method that works is mechanical:

• Run water in the drain every day for a week, or pour a bucket of water into a floor drain to refill the trap.
• Scrub the inside of the pipe with a long stiff brush. The slime is where the larvae live, and water alone doesn't remove it.
• Clean the overflow hole in a bathroom sink with a bottle brush.
• Repeat weekly for three weeks, which outlasts the larval stage (Texas A&M AgriLife, ENTO-050).

If it's a fungus gnat

Fungus gnats come from potting mix that stays wet. Adults are harmless; the larvae feed on roots in soggy soil.

• Let the top two inches of soil dry out between waterings.
• Water from the bottom for a few weeks so the surface stays dry.
• Yellow sticky cards near the pot catch adults and tell you whether numbers are falling.
• Repot anything sitting in a saucer of standing water.

Still not sure?

Catch one on tape and look at it against a white sheet of paper. Red eyes and a rounded tan body is a fruit fly. Fuzzy and moth-like is a drain fly. Thin, black and long-legged is a fungus gnat. If you want a second opinion, email a photo to donny@fruitflydefense.com and a biologist will tell you which one you have.

Sources

• University of Kentucky Entomology, Fruit Flies, ENTFACT-621. entomology.mgcafe.uky.edu/ef621
• Purdue Extension Entomology, Fruit Flies, HN-66. extension.entm.purdue.edu
• Texas A&M AgriLife Extension, Indoor Flies and Their Control, ENTO-050. agrilifeextension.tamu.edu
• Wang Q et al. (2021). Behavioral and physiological responses of Drosophila melanogaster and D. suzukii to volatiles from plant essential oils. Pest Management Science 77:3698-3705. doi:10.1002/ps.6282