In October, or on a sunny February day, flies show up at the upstairs bedroom windows. They are bigger than a house fly, fly slowly, bump into the glass and are easy to catch. The instinct is to look for forgotten garbage or a dead animal. Most of the time there is none: these are cluster flies (genus Pollenia). They didn’t breed in your house; they came in to spend the winter.
How to recognize them
- Size: 8 to 10 mm, a little bigger than a house fly.
- Colour: dark grey, with a silvery grey checkered abdomen.
- The giveaway: fine golden hairs on the thorax, visible up close or with a magnifier. They wear off with age, but some nearly always remain.
- Wings: at rest they overlap over the back, like closed scissors.
- Behaviour: slow, clumsy, gathered at sunny windows, often upstairs and on the south or west side.
A house fly has four sharp black stripes on its thorax and no golden hairs. The tiny red-eyed flies around your fruit bowl are fruit flies, a different problem altogether.
Where they come from: your lawn
The cluster fly’s life cycle explains everything. Its larvae are parasites of earthworms: the female lays eggs in the soil, and the larva burrows into a worm and feeds on it. Adults develop this way all summer in lawns, fields and grassy lots.
In late summer and fall, the adults look for a place to overwinter. Like Asian lady beetles, they land on warm, sunny walls, then slip in through the smallest gaps: soffit edges, siding, window frames, vents. They cluster in the attic, wall voids and the spaces between floors, where they stay dormant.
That is why homes surrounded by grass, in the suburbs or the countryside, are hit hardest, and why the same houses get them year after year. It is not a sign of poor hygiene.
Why they show up in mid-winter
A sunny day warms the wall or attic, and the heating does the rest. The flies wake up, crawl toward the light and end up in your rooms instead of outside. They die within days, hence the little piles of dead flies on the windowsills. In spring, the survivors try to get out, and you see them at the windows again.
If they are not cluster flies
Flies indoors can also come from a source that does need quick action:
- maggots in the kitchen, the garbage or on the floor: flies are breeding in your home (see our article on maggots in the house);
- large numbers of metallic green or blue flies in one room, with an odour: often a dead rodent or bird in a wall or ceiling.
In both cases, the source has to be found and removed; insecticide won’t change anything.
Removing them
- Vacuum them right off the windows, then throw out the bag. The simplest, cleanest method.
- A light trap in the attic or a little-used room catches the ones moving around, with no product.
- Fly ribbons near windows: useful to gauge numbers, weak on their own (see our article on fly stickers).
- Don’t spray inside the walls. Hundreds of dead flies in a wall or attic attract carpet beetles, whose larvae then go after wool and dry food.
Keeping them out
As with every insect that overwinters in houses, the lasting fix is exclusion, done at the end of summer, before they arrive:
- put fine mesh over attic, bathroom and dryer vents;
- seal the joints between siding, fascia and soffits;
- caulk around windows and doors, on the outside;
- repair screens;
- seal cable, pipe and wall air conditioner openings.
The gaps to seal are the same as for stink bugs coming in for the fall. An exterior treatment of the sunny walls in late summer, with a product whose label covers flies, can reduce the number that land there. It complements sealing; it doesn’t replace it.
When to call
- You are vacuuming up dozens every day.
- You can’t find where they get in, or the attic is hard to reach.
- You see maggots or notice an odour: the source has to be found.
Our home protection plan includes an audit of the house and perimeter treatments. For an assessment, contact us or call 514-638-8705. The estimate is free.
Not sure what you found? Our guide to bugs in the house in Quebec helps you identify it by where and when you see it.
Frequently asked questions
Are cluster flies dangerous?
They don’t bite and don’t breed indoors. They are mostly a nuisance. Still, avoid letting large numbers die inside the walls.
Why always the upstairs bedrooms?
They overwinter in the attic and upper walls, which get the sun. That is where they come down from, into the nearest rooms.
Should I treat my lawn?
No. Their larvae live in earthworms, which are good for the soil. Treating your lawn wouldn’t stop flies from neighbouring lots anyway.