Shore flies are small, stocky flies that breed in algae and standing moisture, and they often show up indoors when houseplants are overwatered or drainage trays hold stagnant water. They don’t bite, and they won’t damage your plants directly, but they multiply fast and become a genuine nuisance. Getting rid of them comes down to eliminating the algae and excess moisture they depend on.
What Shore Flies Look Like
Shore flies are easy to confuse with fungus gnats, since both hover around potted plants. But they’re distinctly different insects. Shore flies are darker, stockier, and more compact, with short antennae and a body shape closer to a common housefly than a mosquito. Their wings often have light-colored spots. Fungus gnats, by contrast, are more delicate and leggy, with long antennae and a tendency to fly in lazy, erratic patterns near the soil surface.
The behavioral difference matters for identification too. Shore flies are stronger, more direct fliers. You’ll notice them resting on surfaces near moisture, on windowsills, or buzzing toward lights. If the tiny flies around your plants look robust rather than fragile, and you see a greenish film of algae on your soil surface, shore flies are the likely culprit.
Why They’re in Your House
Shore flies feed on algae, not plant roots or decaying organic matter. That’s the key distinction from fungus gnats. Anywhere algae grows in your home, shore flies can breed: the surface of overwatered potting soil, saucers with standing water, trays beneath plant pots, or even persistently damp windowsills and bathroom surfaces.
Overfertilizing houseplants makes the problem worse, because excess nutrients in moist soil fuel algae growth. That green or brownish crust you sometimes see on the surface of potting mix is exactly what shore flies are looking for. They lay up to 300 eggs in algae or very wet areas, and those eggs hatch in just two to three days. Larvae feed on algae beneath the soil surface for about a week before pupating. The entire cycle from egg to adult takes as little as 8 days at 78°F, or about 14 days at 68°F. Adults live two to three weeks and can reach nuisance levels remarkably fast in warm, moist conditions.
Are They Harmful?
Shore flies don’t bite people, and neither the adults nor larvae feed on plants. Your houseplants aren’t in direct danger. The main concern is that adults can carry spores of plant-pathogenic fungi on their bodies and spread them between pots as they move around. In a home with just a few plants this is a minor risk, but if you keep a large collection, it’s worth addressing an infestation promptly.
Mostly, though, they’re just annoying. A single female can produce hundreds of offspring, and in a warm house with consistent moisture sources, populations build quickly. You’ll find them on windows, around kitchen sinks, and clustered near any light source.
How to Get Rid of Shore Flies
Cut Off Their Moisture and Food
Since shore flies depend entirely on algae, removing the algae removes the population. Let the top layer of soil in your plant pots dry out completely between waterings. Empty drainage saucers after watering rather than letting water sit. Fix any plumbing leaks or dripping faucets near plants. If you see a greenish or crusty mat on the soil surface, scrape it off with a fork or remove and discard that top layer of potting mix entirely.
Improving airflow between pots helps the soil surface dry faster and discourages algae regrowth. Spacing plants apart, placing them near a fan, or moving them to a better-ventilated spot all help. Reducing your fertilizer concentration also slows algae development, since nutrient-rich runoff is one of the main things feeding it.
Trap the Adults
Yellow sticky traps are effective for catching adult shore flies and reducing the breeding population while you address the moisture problem. Cut them into small squares and attach them to wooden skewers stuck into the soil, positioning them just above the pot rim. Replace them as they fill up. Sticky traps won’t solve the problem on their own, but they’ll knock down adult numbers and give you a visual indicator of how bad the infestation is and whether it’s improving.
Improve Your Soil Mix
If your potting mix stays wet for days after watering, it’s probably too dense. Adding perlite or coarse sand to the mix improves drainage and helps the surface dry out faster. A layer of coarse sand or fine gravel on top of the soil can also discourage egg-laying by keeping the surface dry and algae-free, even when the soil beneath stays moist enough for roots.
Biological Controls
For persistent infestations, beneficial nematodes can target shore fly larvae in the soil. The species that works against shore flies is Steinernema carpocapsae. This is important to get right, because the more commonly sold Steinernema feltiae (marketed for fungus gnats) does not control shore flies. You can order S. carpocapsae online and apply it as a soil drench following the product instructions. The microscopic nematodes seek out and kill larvae in the potting mix.
Preventing Them From Coming Back
Long-term prevention is straightforward: don’t let algae establish itself. Water only when the soil surface is dry to the touch. Use pots with drainage holes and never let plants sit in water. Keep humidity around your plants reasonable, especially in winter when indoor air tends to be still. If you grow plants in trays or on pebble trays, clean them regularly so algae doesn’t accumulate.
When you bring new plants home, check the soil surface for signs of algae or excessive moisture. Greenhouses, where shore flies are most common, often overwater plants for display purposes. Repotting new plants into fresh, well-draining mix and letting the soil dry before the first watering gives you a clean start. Once you break the algae cycle, shore flies have nothing to breed in, and the adults that remain will die off within two to three weeks.