Flies showing up inside your home during winter almost always trace back to one of a few causes: they entered in autumn and have been hiding in your walls, they’re breeding in your drains or houseplant soil, or something died in a wall void. The species you’re dealing with determines both why they’re there and how to get rid of them.
Cluster Flies: The Most Common Winter Intruder
If you’re seeing large, sluggish flies gathering at your windows on sunny winter days, you’re almost certainly dealing with cluster flies. They’re slightly larger than common house flies, dull gray with black markings, and have golden-yellow hairs on the upper body that give them a subtle sheen. Their flight is lazy and erratic, and they’re easy to swat or catch.
Cluster flies don’t breed indoors. They entered your home months ago, starting as early as mid-August and continuing into early October, when they gathered on sun-warmed south and west-facing walls looking for a place to ride out winter. Any crack around a window, soffit, ventilation opening, or gap in siding gave them a way in. Once inside, they tucked into wall voids and attics in loose clusters and went semi-dormant.
When a warm winter day heats up the walls, they wake up confused and move toward light, which is why they end up buzzing against your windows. They aren’t trying to infest your home. They’re trying to get back outside but can’t figure out how. This cycle repeats on every warm day from autumn through spring, and in older homes with lots of entry points, you can see dozens or even hundreds at a time.
Drain Flies: Tiny Moths Near Sinks and Showers
Drain flies are small, fuzzy, moth-shaped flies that tend to hover near bathroom sinks, shower drains, and floor drains. Unlike cluster flies, drain flies are actively breeding inside your home, and they can do this year-round because their habitat is your plumbing.
Their larvae develop in the slimy organic buildup that coats the inside of drain pipes: soap scum, hair, algae, bacteria. The drains most likely to harbor them are ones used infrequently, like a guest bathroom shower where water sits stagnant in the trap for weeks. Sink drains, tub drains, toilet overflows, floor drains, and the area beneath leaky pipes are all potential breeding sites. If you clean every visible drain and the flies keep coming back, that can point to a broken pipe or leak under the slab or in a crawlspace.
Fungus Gnats: The Houseplant Problem
Tiny, dark flies hovering around your potted plants are almost certainly fungus gnats. They’re especially common in fall and winter because houseplants that spent the summer outdoors often come back inside already infested. Indoors, they breed continuously in moist potting soil, with overlapping generations where eggs, larvae, pupae, and adults are all present at the same time.
Females lay up to 200 eggs in clusters on the surface of moist soil. The eggs hatch in three to six days, larvae feed in the soil for one to two weeks, then pupate near the surface. Adults emerge about four to five days later but only live about a week. The entire cycle from egg to adult takes three to four weeks under warm indoor conditions, which means a small population can explode quickly if the soil stays wet. At high populations, the larvae can cause real root damage and even kill houseplants.
Blow Flies: A Sign Something Died
If you suddenly notice large, shiny green, blue, or black flies inside during winter, a rodent or bird has likely died in your walls or attic. Blow flies and flesh flies lay eggs on dead animals. The larvae feed on the carcass for 5 to 10 days, then wander off to pupate, and adult flies emerge 5 to 7 days later. These adults then find their way into your living space through gaps around light fixtures, outlets, or baseboards.
The good news is that these flies won’t reinfest your home. By the time the adults emerge, the original carcass is too dry to support another generation. The problem is self-limiting but unpleasant. If you can locate and remove the carcass, the flies stop. If you can’t reach it (common with wall voids), the situation resolves on its own as the food source dries out, typically within two to three weeks.
How to Get Rid of Winter Flies
Cluster Flies
A vacuum cleaner is your best tool. The flies are slow and sluggish, making them easy to suck up from windows and walls. There’s no nest to destroy and no breeding site to clean because they aren’t reproducing inside your home. You’re simply removing the ones that woke up and wandered into living spaces. Sealing cracks around electrical outlet boxes, switches, light fixtures, and window and baseboard molding on interior walls helps keep flies trapped in the wall voids they’re already sheltering in, preventing them from entering rooms. In older homes with double-hung windows that use pulleys, flies commonly enter through the pulley opening. A strip of masking tape over it blocks that route.
Drain Flies
The fix is removing the organic sludge where larvae develop. Physically scrubbing the inside of drain pipes with a stiff brush is the most direct approach. Enzyme-based drain gels break down the organic film that larvae feed on and work well for ongoing maintenance. Chemical drain cleaners are less effective because they don’t fully remove the biofilm coating the pipe walls. If a specific drain seems to be the source, you can test it by placing a strip of clear tape over the drain opening overnight. Flies emerging from that drain will stick to the tape.
Fungus Gnats
Letting the soil dry out between waterings is the single most effective control. Fungus gnat larvae need moist soil to survive, and most houseplants tolerate drier conditions than people think, especially in winter when growth slows. Because the most effective control targets larvae and pupae in the soil rather than the short-lived adults flying around, soil management matters more than traps. Yellow sticky traps near plants will catch adults and help you monitor whether the population is dropping, but they won’t solve the problem alone. Repotting heavily infested plants in fresh, well-draining soil removes larvae directly.
Sealing Your Home for Next Year
The permanent fix for cluster flies and other overwintering insects is exclusion: sealing entry points before they arrive in late summer and early fall. This is a one-time investment that pays off every winter.
Start with the gaps that matter most. Install door sweeps on all exterior doors, paying special attention to the bottom corners where insects enter most easily. If you can see light under a door while lying on the floor, insects can get through. Garage doors need a rubber bottom seal (vinyl stiffens and fails in cold weather). Seal utility openings where pipes and wires enter the foundation and siding, around outdoor faucets, gas meters, dryer vents, and cable wires, using caulk, expandable foam, steel wool, or copper mesh. Caulk cracks around windows, doors, and fascia boards with silicone or acrylic latex caulk. Repair any torn window and door screens.
Gaps under sliding glass doors can be sealed by lining the bottom track with half-inch to three-quarter-inch foam weatherstripping. For soffits and ventilation openings, check that screening is intact and that no gaps have opened around the edges. The goal is to have the work done before mid-August, when cluster flies start scouting for overwintering sites. Sealing the house after they’ve already entered traps them inside and can make the winter problem worse.