It’s early November in Maine. The nights are getting colder, you’ve probably turned the heat on, and suddenly, there are flies crawling around an upstairs window. Maybe you’ve found several in a bedroom you rarely use, or noticed a few buzzing around a ceiling light.
Where are they coming from? And why are there still flies in the house this time of year?
There’s a good chance you’re dealing with cluster flies.
These flies are a familiar fall nuisance for homeowners throughout Central and Coastal Maine. Unlike ordinary house flies, cluster flies aren’t looking for garbage or leftover food. They’re looking for a protected place to spend the winter, and your house may be exactly what they’ve found.
How to Tell If Those Flies Are Cluster Flies
Cluster flies look enough like ordinary house flies that it’s easy to confuse them. But there are some differences worth watching for.
Cluster flies are generally:
A little larger than house flies, with a dull gray or grayish-black appearance.
Slower and more sluggish, especially when crawling or flying around indoors.
Covered with short, golden-yellow hairs on the thorax, the section of the body behind the head.
Likely to gather in groups, particularly around windows and other sunny areas.
Their behavior can be just as helpful for identification.
A house fly may dart around the kitchen or land on food. Cluster flies are more likely to move slowly along an upstairs window, gather in a sunny corner, or appear in a room that hasn’t been used for several days.
The University of Maine Cooperative Extension’s cluster fly fact sheet provides additional identification information and explains why these insects become such a nuisance in Maine homes.
If you’re finding flies around garbage, food waste, or another obvious breeding source, don’t automatically assume they’re cluster flies. Different flies require different approaches to control.
Why Cluster Flies Have Moved Into Maine Homes for Winter
Cluster flies begin searching for protected overwintering locations in late summer and fall. By early November, many have already found their way into buildings, although some may still be gathering on exterior surfaces during warmer afternoons.
On a sunny fall day, you might notice them resting on the south- or west-facing side of your house. From there, they can find their way through small gaps around siding, windows, rooflines, and other exterior openings.
Once inside, they often settle into spaces homeowners rarely see:
Attics and lofts
Wall voids
Spaces behind siding
Gaps around window frames
Protected areas beneath rooflines and eaves
Older Maine homes, seasonal camps, and buildings with gaps around exterior materials can offer plenty of potential entry points. Newer homes aren’t immune, either.
The important thing to understand is that cluster flies may enter your home long before you actually notice them indoors.
That’s why a homeowner might see very little activity in October, only to find flies gathering around a window in November or later in winter.
Why Are Cluster Flies Coming Out When It’s Cold Outside?
This is one of the more confusing parts of a cluster fly problem.
You might go several weeks without seeing a single fly. Then, on a bright November afternoon or a mild day in January, several suddenly appear in an upstairs room.
It doesn’t mean a new group of flies has just entered your home.
Cluster flies become relatively inactive while overwintering, but warmth can cause them to start moving again. Sunlight warming an exterior wall or heat inside the house may bring them out of their hiding places.
Once active, they often move toward light, which explains why homeowners find them:
Crawling along the upstairs windows
Gathering near skylights
Buzzing around ceiling lights
Resting on walls in seldom-used bedrooms
Dead on windowsills or floors
In a Maine home with a finished attic or upstairs guest room, this can be especially frustrating. You may clean up the flies one day, only to find more after the next sunny afternoon.
The flies aren’t necessarily entering from outdoors each time. They may be emerging from protected spaces already inside the structure.
Do Cluster Flies Mean Something Is Wrong With Your House?
Not necessarily.
One concern homeowners often have is whether flies indoors mean something is rotting, a dead animal is hidden in the attic, or there’s a sanitation problem.
With cluster flies, that’s generally not the case.
Their life cycle is very different from that of common house flies. Cluster fly larvae develop as parasites of earthworms outdoors. They aren’t breeding in your garbage, drains, or spoiled food.
And despite their habit of appearing in large numbers, cluster flies:
Do not bite people or pets.
Are not considered a significant disease-transmission concern for homeowners.
Do not reproduce inside homes during winter.
Do not eat wood or damage the structure itself.
They’re primarily a nuisance, particularly when dozens of flies gather around windows or accumulate in seldom-used rooms.
That doesn’t mean every fly found indoors is harmless or that identification doesn’t matter. If you’re seeing flies with a different appearance or behavior, especially around an unexplained odor, the source may need further investigation.
What Can Maine Homeowners Do About Cluster Flies in November?
By November, the approach to cluster fly control may differ from what it was earlier in the fall.
If you’ve already found cluster flies indoors, removing the ones you can see is a reasonable first step. A vacuum can help collect sluggish flies around windows, walls, and floors without leaving crushed insects behind.
But there’s an important limitation: removing the visible flies doesn’t address the others that may already be sheltering inside walls or attic spaces.
For a more lasting approach, attention needs to turn to how the flies are getting into the building.
Check for gaps around window trim, siding, rooflines, soffits, and other exterior openings. Repairing damaged materials and making sure vents are properly screened can help reduce access without interfering with necessary ventilation.
For Maine camp owners, this is also worth considering when preparing a property for winter. A building that’s closed up for several months may provide undisturbed shelter for cluster flies, and the first sign of a problem could be a collection of dead flies when you reopen in spring.
Is It Too Late to Treat for Cluster Flies?
Not necessarily, but the timing and location of the flies matter.
Professional exterior treatments are generally most effective before cluster flies enter buildings in late summer and early fall. By November, many may already be sheltered inside, so treatment options and expectations depend on where the flies are active.
An exterior treatment cannot be expected to eliminate flies that are sheltered deep within wall voids or attic spaces. And spraying the flies you can see indoors won’t necessarily stop more from emerging later.
That doesn’t mean homeowners need to wait until next fall to ask for help. Professional identification and an evaluation of the property can help determine whether treatment is appropriate now and what steps could reduce problems in future seasons.
Professional Cluster Fly Control in Central and Coastal Maine
If cluster flies have become a recurring problem at your home or camp, Advanced Pest Solutions can help.
We help homeowners throughout Central and Coastal Maine, including the greater Augusta and Lewiston-Auburn areas, as well as communities along the Midcoast.
Our team can help identify cluster fly activity, assess where flies may be entering, and recommend appropriate treatment options based on your property's conditions and the time of year.
Cluster flies are not included in our standard residential pest control plans, but for an additional charge, treatment can be added. If you’re finding cluster flies around upstairs windows, in attics, or in rooms you rarely use this November, give us a call. We can discuss the current activity, available treatment options, and how to better prepare for future fall seasons.
Contact Advanced Pest Solutions to ask about cluster fly control for your Maine home.