Late summer nighttime temperatures — dipping into the low- and mid-40s — create the conditions that force cluster flies to seek winter harborage. They follow the sun during the course of the day, starting on the east side of a structure, ending on the west. As it cools down at night, these flies then move inside, typically through the grooves on soffit panels. They then “cluster” together in attics and crawlspaces.
While we hold on dearly to the last vestiges of summer, warm temperatures give way to cooler, fall air. That’s the green flag for many fall invaders to barge in, definitely uninvited.
Anyone living with cluster flies can easily relate to the fright flick “Amityville Horror,” specifically the scene when the home is inexplicably filled with thousands of flies. While cluster flies invade by the hundreds, and there is an definite explanation at to why there is an infestation, it does not make the situation any better, especially when your spotless abode is filled with flies. And especially on a winter day.
I have seen homeowners spend thousands of dollars on repairs trying to solve this dreadful problem. There have even been lawsuits over alleged inferior building practices. Because the flies are naturally attracted to ultraviolet light (daylight), faulty windows are first to be blamed. New replacement windows can be installed, and the problem continues. The flies are almost always on the second floor, so the roof must be to blame. New roofing can be installed, and still there are flies. It’s common to find the flies resting on the exterior siding, soffit and fascia, so it’s time for a complete exterior makeover. No resolution. The only thing that goes away is the money from your wallet. With a good understanding of your enemy, control can be achieved with my favorite expression: “Better living through chemistry.”
Cluster flies (Pollenia rudis) are somewhat larger than house flies, dark gray, with clumps of golden hairs under each wing (resembling skin patches). They also have a distinctive wing pattern, folded parallel with their body. Females lay their eggs in soil cracks. The developing larvae then search out earthworms to feed upon. Once completed, to add insult to injury, these larvae then burrow into these worms to pupate. This entire cycle takes 4-6 weeks, with four generations of adults each season. It is the adults, in late summer, that star in your nightmare.
By living in the walls and attics, it only takes a sunny day, even in the winter, to warm up their surroundings, fooling them into believing its springtime. Now they make their guest appearance out of nowhere, falling onto floors, acting lethargic and coma-like. Sorry, no awards this time. This winter arrival is what stymies most people. They do no damage nor carry disease; just their presence makes them pests.
Once spring arrives (read: warmer temperatures), these flies exit from their harborage, being more plentiful and annoying. Because they are not breeding or multiplying within your structure, but rather using it as an overwintering site, they will all exit and leave you alone during summer months. It is common, on the first warm day of spring, for a lawn or siding to “come alive” with hundreds of flies.
The key to successful cluster fly control is a properly timed treatment. Because these flies are temperature driven, a treatment application should be made before the flies arrive. This translates to an application in the late summer or early fall. Unfortunately, it needs to be repeated every year. I like to set up multiple “lines of defense.” These include a liquid application on the exterior, to stop them as they are resting. If accessible, I then like to create a second defensive line in the attic/crawl space with a dry dust formulation.
This one-two punch has proved effective for me against these pain-in-the-you-know-where pests. Properly-labeled treatment products are readily available through most outlets.