What Attracts Cluster Flies to Your House?

Cluster flies belong to a genus whose larvae are obligate parasites of earthworms, a life cycle documented by the University of Connecticut Home & Garden Education Center: females lay eggs in cracks in garden or lawn soil, larvae hatch and parasitize an earthworm for 11 to 14 days, and the whole cycle from egg to adult takes 27 to 39 days, with several generations completing over a summer. None of that happens anywhere near a house, and none of it involves garbage, human food, or decaying matter of any kind — this species simply isn't a filth fly the way a house fly is.

Our old page's original phrasing described the flies as seeking warmth and a suitable winter refuge, using the word reserved for mammal dormancy rather than the correct insect term — flies do not hibernate; the accurate word is dormancy. That phrasing didn't claim a garbage connection outright, but it also didn't correct the common assumption that flies clustering on a house must be drawn by food or trash nearby. They aren't.

Once the last generation of the summer matures, adult cluster flies stop breeding and start searching for a dry, protected place to spend the winter, often resting on sun-warmed exterior walls before working their way into cracks, attics, and wall voids. That space is purely a winter roost — flies don't lay eggs there, don't develop there, and have no food source there. It's also worth naming a word error directly: cluster flies do not hibernate — hibernation is a controlled, physiological process specific to mammals, while insects instead enter dormancy, and this species can be roused from that dormancy by household heat on a warm winter day, which is why clusters sometimes appear active indoors in the middle of winter even though nothing new has arrived or bred.