Fruit flies vs fungus gnats: the fruit bowl or the plant pot
Fruit flies breed in fermenting food. Fungus gnats breed in damp potting compost. Which one you have decides the fix, because the vinegar trap that clears up fruit flies does nothing to fungus gnats, and drying out the compost does nothing to fruit flies.
By The Pest GuyLast updated
The short answer
Watch where they go for a minute. It tells you more than looking at one.
If they're hovering round the fruit bowl, the bin or the recycling, quick and erratic, and the ones you get a good look at have red eyes, they're fruit flies. They breed in fermenting food, and the job is to find that food and bin it. Getting rid of fruit flies is that, then trapping the ones already flying.
If they're dark, slender, long-legged and hanging round your houseplants, or sitting on the window nearest them, they're almost certainly fungus gnats. They look like tiny mosquitoes. They breed in damp potting compost, and the fix is in how you water.
Getting it wrong costs you weeks. The University of Wisconsin says fungus gnats are often mistaken for fruit flies, and that the vinegar traps used for fruit flies don't work on them. So a vinegar trap sat next to a plant pot catches nothing, and the gnats carry on breeding underneath it.
And if they're neither at the fruit nor the plants, but on the bathroom wall or round the sink, look at the plumbing. That's the third common small fly, and the flies in the house page covers drain flies and the rest.
Fruit flies and fungus gnats side by side
Nine tells, each with the source that states it. The bottom two rows are the ones that decide what you do.
| The tell | Fruit fly | Fungus gnat | Source |
|---|---|---|---|
| Size and build | About 3 mm long | 3-4 mm, slender, long legs, looks like a tiny mosquito | University of Kentucky Entfact-657; RHS |
| Eyes and colour | Usually red eyes, tan front and darker rear | Dark greyish brown to black, long antennae | University of Kentucky Entfact-621 and 657; RHS |
| How it flies | Hovers, quick and erratic, around the fruit or the bin | Weak flier, stays near the compost, drifts to windows and screens | University of Kentucky Entfact-657; UC IPM |
| Where it breeds | Fermenting food and damp organic matter: fruit, bins, drains, bottles | Damp potting compost rich in organic matter | University of Kentucky Entfact-621 and 657; RHS |
| The larvae | In the fermenting food itself | Whitish, see-through, black head, up to 6 mm, in the compost | University of Kentucky Entfact-657; RHS |
| Egg to adult | 8 to 10 days at 25 C | About 17 days at 24 C, three to four weeks at room temperature | University of Kentucky Entfact-657; UC IPM; Colorado State |
| Eggs per female, adult life | Over 500 eggs; adults live about a month | 100 to 200 eggs; adults live 7 to 10 days | University of Kentucky Entfact-657; Colorado State |
| Which trap catches it | A vinegar bait trap | Yellow sticky cards. Vinegar traps do not work on them | University of Kentucky Entfact-657; Colorado State; University of Wisconsin |
| What actually ends it | Bin the source, then trap the adults left | Let the compost dry between waterings, or repot | University of Kentucky Entfact-657; Colorado State; RHS |
Sources: University of Kentucky Entomology Entfact-621 and Entfact-657; Royal Horticultural Society; UC IPM Pest Notes 7448; Colorado State University Extension; University of Wisconsin-Madison Extension. The US fact sheets give life cycles in Fahrenheit: 77 F is 25 C and 75 F is about 24 C. Every life-cycle figure depends on temperature, so a cool room runs slower than these.
Three checks that settle it
No magnifying glass needed for the first two, and the third one you may already be running.
Watch them fly. Fruit flies can hover, and they fly fast and erratically. Fungus gnats are weak fliers. They stay close to the compost they came from, and they head for light, which is why you find them on the window nearest the plants or flying round your face and the laptop screen.
The potato slice. Push a slice or wedge of raw potato, about 6 mm thick, into the top of the compost. Fungus gnat larvae come up to feed on it. Lift it after three or four days and look underneath. The larvae are slender, whitish and see-through, with a shiny black head, and up to 6 mm long. Find them and you have your answer, and you've found which pot is the source.
The trap that stays empty. If you already have a vinegar trap out and the flies keep coming without it catching any, that fits fungus gnats, because vinegar traps don't work on them. A yellow sticky card in the same spot settles it: Colorado State says the adults are attracted to yellow. A fruit fly problem does the opposite and fills the vinegar.
If you do have a magnifying glass, the wings give it away. The common fungus gnats have a rounded Y-shaped vein near the wing tip, which is the tell UC IPM and Wisconsin both use. For the fruit fly side of the comparison, the red eyes and two-tone body are covered on what fruit flies look like.
One case muddies it. Flies round a plant can be fruit flies if there's rotting fruit or food waste nearby, and the fruit flies in plants page goes through how to tell when that's what's going on.
Two different clocks
Fruit flies are fast and long-lived. Fungus gnats are slow and short-lived. That changes how long each fix takes to show.
A fruit fly goes from egg to adult in about 8 to 10 days at 25 C, a female can lay over 500 eggs, and the adults live for about a month. That's why the numbers climb so fast, and why adults can still be about for weeks after you've binned the source. The fruit fly life cycle explains why a week of trapping isn't always the end of it.
A fungus gnat takes longer. UC IPM puts a generation at about 17 days at 24 C, and Colorado State and Wisconsin say three to four weeks at room temperature. The RHS says about a month in summer. Females lay 100 to 200 eggs, but the adults only live 7 to 10 days.
Put those together and you get the practical bit. Adult gnats only live 7 to 10 days, but anything already developing in the compost takes up to three or four weeks to come through, so expect to see a few for a while after you change how you water. With fruit flies the adults are the long-lived part, which is exactly what a trap is for.
Which fix for which
The University of Kentucky says it about both: killing the adults only buys you time unless the breeding stops.
Fruit flies: bin the source, then trap. They lay on fermenting food and other damp organic matter, so the source is overripe fruit, the food caddy, the recycling, a bottle with dregs in it, a drain. Infestations often arrive on fruit that already had eggs on it, which is where fruit flies come from when they seem to appear from nowhere. Once the source is gone, the University of Kentucky's advice is a vinegar trap with a drop of washing-up liquid for the adults that are left. Why cider vinegar works has the detail on the bait, and what makes a fruit fly trap work covers why some traps keep what they catch and others leak.
Fungus gnats: dry the compost. Colorado State calls letting the compost dry between waterings the most important step. Wisconsin says let the top inch dry before you water again, or put a half to one inch layer of coarse sand or fine gravel on top so the surface stays dry. The RHS adds repotting into fresh compost and throwing the old compost away. Yellow sticky cards near the plants cut the adults down, and catching egg-laying females means fewer larvae next time round.
If drying the compost isn't enough, the RHS lists biological controls for fungus gnats: a nematode, Steinernema feltiae, predatory mites, and a rove beetle.
Both at once. A kitchen with a fruit bowl and a windowsill of herbs can have both, and then you run both fixes. The trap goes by the fruit and the bin, the yellow cards go in the pots, and the watering can is the thing that ends the gnats. A fly spray kills whatever it hits on either side and leaves both sources breeding.
For the fruit bowl side, one trap does the job
This is for fruit flies only. It's baited with cider vinegar, and vinegar traps don't work on fungus gnats, so it won't do anything for a plant pot. Bin the fruit first, then stand it by the bowl or the food bin to catch the adults that are still flying.
What I’d use
Reusable cider vinegar trap
Escape-proof fruit fly trap
A two-part cup and lid you fill with about 25 ml of cider vinegar and stand by the fruit bowl.
Eight slots in the lid feed a funnel that drops flies through one 4 mm hole into the dark middle of the cup. The brightest points a trapped fly can see are pinholes too small to get through, set away from the real exit, so the way out is the place they are least likely to look.
- Ways in
- Eight slots in the lid, into one funnel
- Way out
- One 4 mm hole, under the solid centre of the lid
- Holds
- About 25 ml of vinegar, to the moulded fill line
- Size
- 6.7 cm across, 5.4 cm tall
- Refills
- None to buy. It runs on cider vinegar from the cupboard
Watch out for
- Vinegar and washing-up liquid not included.
- Made for fruit flies. Flies from a plant pot or drain need the source found.
- Tip, rinse by hand and refill once it fills up.
That link earns me nothing. It is not an affiliate link, no brand pays for a mention here, and I get the same whether you buy this or ignore it. It is on the page because it is what the job takes.
No brand pays for a mention here, and nothing on this page earns me a commission. The card above links out to a shop, and I get the same whether you buy or ignore it. The link is there because finding the thing is the boring half of acting on advice, which is also why the watch-outs sit on the same card as the reasons to buy.
About this resource
The weight on this page is carried by university extension services and the RHS rather than by pest-control marketing: the University of Kentucky's "Small flies" and "Fruit flies" fact sheets, the RHS profile of fungus gnats (sciarid flies), UC IPM's Pest Notes, and the fungus gnat pages from Colorado State University and the University of Wisconsin-Madison. The RHS counts over 250 species of fungus gnat in Britain and says they're active all year round on houseplants.
The honest limit is identification. This page tells you what separates the two and how to check for yourself; it can't tell you which fly is in your kitchen from a description. No insecticide is named here, because the ones in the US sources weren't checked against what's approved for home use in the UK.
Common questions
How do I tell if I have fungus gnats or fruit flies?
Look at where they are. Fruit flies hover round the fruit bowl, the bin or the recycling, and most have red eyes. Fungus gnats are slender and dark with long legs, look like tiny mosquitoes, and stay close to the compost of your houseplants or drift to the nearest window. If you want proof, push a slice of raw potato into the compost and check it after three or four days. Fungus gnat larvae come up to feed on it.
How can I get rid of fruit flies and fungus gnats?
With two different fixes, because nothing does both. For fruit flies, bin whatever they are breeding in and then trap the adults that are left with cider vinegar and a drop of washing-up liquid. For fungus gnats, let the compost dry out between waterings, or repot into fresh compost, and use yellow sticky cards for the adults. A vinegar trap does not work on fungus gnats, so it will not touch the plant pots.
What's worse, fruit flies or gnats?
Fruit flies breed faster, with over 500 eggs a female and egg to adult in about 8 to 10 days in the warm, and they walk on your food, which is how they can carry bacteria onto it. Fungus gnats are slower but stubborn: once they are settled indoors they are hard to shift, and their larvae can eat the roots of seedlings and cuttings. Neither of them bites.
Why do all my houseplants have gnats?
Because the compost stays damp. Fungus gnats breed in damp potting compost with plenty of organic matter in it, so plants that are watered often and never dry out suit them best. Plants that spent the summer outside often come back indoors already carrying them.
Can I spray vinegar on my plants to get rid of gnats?
Vinegar is fruit fly bait, and it does not work on fungus gnats. The University of Wisconsin says plainly that the vinegar traps used for fruit flies do not work on them. The gnats are breeding in the compost, so the fix is drying the compost, a layer of coarse sand or fine gravel on top, or fresh compost, with yellow sticky cards to catch the adults.
What kills gnats but not plants?
The RHS names biological controls for fungus gnats: a nematode, Steinernema feltiae, two predatory mites, Hypoaspis miles and Macrocheles robustus, and a rove beetle, Atheta coriaria. Drying the compost between waterings costs nothing, and Colorado State calls it the most important step.