Cluster flies (Pollenia rudis) are parasitic flies whose larvae develop inside earthworms. Adults are slightly larger than house flies, dark gray with golden-yellow hairs on the thorax, and move sluggishly compared to other fly species. They are best known for their overwintering behavior — in late summer and fall, adults seek sheltered locations to spend the winter, often entering homes and buildings in enormous numbers through gaps around windows, eaves, and siding. They cluster in attics, wall voids, and upper rooms, becoming torpid in the cold. On warm winter or spring days, they revive and congregate at south-facing windows, creating alarming clusters of slow-moving flies. Cluster flies do not breed indoors, do not bite, and do not transmit disease — they are purely a nuisance pest. However, their sheer numbers and the difficulty of preventing entry make them a persistent problem in some Arizona homes.
👁Warning Signs
Large numbers of slow, sluggish flies at windows on warm days in fall, winter, or spring. Flies emerging from wall voids or attic spaces. Clusters of flies on south-facing exterior walls in late afternoon sun.
⚠Health & Property Risks
No medical risk. No disease transmission. No structural damage. Purely a nuisance pest due to numbers and persistent re-entry.
Prevention Tips
Seal gaps around windows, eaves, siding, and utility penetrations before late summer. Install tight-fitting screens. Caulk exterior cracks before the overwintering season begins.
🛡Professional Treatment
Vacuum up clusters indoors — do not swat, as crushed flies attract other pests. Residual insecticide treatments on exterior surfaces in late summer before entry. Attic treatments for established overwintering populations.
Frequently Asked Questions — Cluster Fly
Dealing with Cluster Fly?
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