It isn’t hard to see that the land is awakening as spring unfolds before our eyes. One revelation I always marvel at in the spring is the rapid emergence of insects, when it is still so cold outside. Even though the outside temperature was only +4C at my house last week, I saw honey bees, a hornet, leaf-footed bugs, several fruit flies, some cluster flies and a few ladybugs. Where the did they come from? Well, for the most part they emerged when the soil and leaf litter warmed and conditions were suitable, for them to fly and try to find food. This is a risky proposition since, if it turns cold again, they may be doomed. But somehow they survive as they have for eons.
Cluster flies follow a slightly different cycle than many of our native species. They are opportunistically closely linked to us and our homes. While many overwinter in barns and other areas where they can glean some heat and protection, many more choose our homes for refuge in the fall. So let’s explore their lives a little bit.
Cluster flies are those small, black, slow-moving flies, with small stripes on their heads and golden hairs and a checkered pattern on their abdomens we see on our windows and siding at this time of year. They favour south or west facing surfaces in particular, because these locations are the first to absorb the weak spring sun’s heat.
Most people confuse them with house flies that thrive on rotting meat, but they are a different type of fly. Sometimes called attic flies, they are frequently found in our attics, hence their name, but they also may winter in walls or other dead spaces in our homes. In the fall, they enter our houses through tiny cracks and crevices or unscreened vents then slowly move to their wintering sites inside the home, where they will wait out the winter. They will share our home and will stay until the temperature rises to about 10 or 12 degrees Celsius. They emerge and try to find their way back outside. Unfortunately, they have poor memories so the tiny crack that allowed them to enter our homes is forgotten and they ‘cluster’ on the windows trying to get outside.

They pose no risk to humans as their only food is worms, which they parasitize. The female lays her eggs near the little burrows that earthworms create. The eggs develop in the soil for about 3 or 4 days and then when the larvae hatch out; they burrow into the nearest worm and feed on it! Then after 2 to 3 weeks, the maggots emerge from the worm and morph into pupae for another 2 weeks and after that into adults – and so the cycle repeats – often 3 or 4 times per year. Adult cluster flies feed on flower nectar, plant juices, fruit and other types of organic matter.
Using pesticides to control them is an over-reaction and an unnecessary practice. They do not carry disease, are not associated with decaying carcasses or rotten meat, nor do they damage homes or food. They are essentially harmless. So what can you do? The best remedy is to stop them from entering your home. Caulk any cracks or crevices in the siding and window or door frames in late summer. Repair damaged screens and put mesh over any vents, including soffits, that lead into the house. This will help keep them out of your home. In the spring, if I find some on my windowsills, I simply open the window or screen door and shoo them out. Works perfectly and doesn’t interfere with their life cycle!
We’re not alone in having them invade our homes, as worldwide there are over 70 species of them. on all continents except Antarctica. And in case you’re asked–cluster flies emit an odour like buckwheat honey when disturbed. No idea why!?
Geoff Carpentier is a published author, expedition guide, and environmental consultant. Visit Geoff on-line at www.avocetnatureservices.com and on LinkedIn and Facebook.