It is late February, there has been a mild few days, and there are two hundred slow, clumsy flies on the inside of an upstairs window. There were none yesterday.
The most useful thing to know is that they did not come in yesterday. They came in five months ago.
What is actually happening
Cluster flies overwinter in buildings. In late September and October the adults look for shelter, gather on warm sun-facing walls in the afternoon — south and west elevations — and work their way into wall voids, attics and soffits through very small gaps.
There they sit dormant all winter. A mild spell warms the wall cavity enough to wake some of them, and a waking cluster fly moves toward light. In a wall cavity, the light is your window.
So the number on the glass is not the number in the house. It is the fraction that happened to wake up.
They are not a hygiene problem
Worth saying because people find this genuinely distressing and assume it reflects on the house.
Cluster fly larvae parasitise earthworms in soil. They do not breed indoors, they do not lay eggs in food, and they have nothing to do with waste or cleanliness. Their numbers track undisturbed soil — pasture, old sod, orchard floor.
That is exactly why this is a South Georgian Bay problem. The orchard belt running from Thornbury and Clarksburg west to Meaford, and the farmland behind Stayner and through the Clearview concessions, produce them in quantity. A house in the middle of a city does not have that reservoir behind it.
The cleanest, best-maintained farmhouse in Creemore and rural Clearview will get them. Its neighbour will too.
Why nobody can fix this today
This is the part worth being direct about, because it is a month when the wrong thing gets sold.
The flies are inside the wall assembly. An interior spray reaches the ones that have already emerged onto the windowsill — which you could have achieved with a vacuum — and does not reach the several thousand still dormant in the cavity. There is no interior treatment that meaningfully addresses an overwintering population in a wall.
If somebody offers you a winter interior treatment for cluster flies, they are selling you something that does not work.
What actually works, and when
An exterior residual applied to the sun-facing walls, the soffits and around openings in early to mid September — before the aggregation, not after it. That is the cluster fly and fall invader treatment, and it is booked seasonally.
Timing is the entire job. A week or two late and a large share of the population is already inside, where the treatment cannot reach it. That is why we book this seasonally and hold the dates, rather than fitting it in when someone calls.
What to do between now and then
Vacuum, do not spray. A vacuum with a hose is the correct tool for what is on the glass. Empty it outside — the bodies attract carpet beetles otherwise, which is a genuine second problem.
Note which windows. Keep a rough record of which rooms and which elevations produce them. That tells us which walls to treat in September, and it is information only you can collect.
Seal what you reasonably can, in warm weather. Gaps around window and door frames on the sun-facing elevations, soffit and fascia junctions, service penetrations, and failed attic vent screening. On an older house in Thornbury or a farmhouse out in Clearview that is a real afternoon of work. It will not replace the treatment, but it reduces what gets in every year from now on.
For what the timed treatment costs alongside everything else, see pest control prices in Collingwood.
Put a note in your calendar for the third week of August. That is when to call about this, and it is the only call that changes next February.