The Cluster Fly (Pollenia rudis) is dark gray with golden-yellow thoracic hairs and typically measures ⅜–½ inch. 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 slugg.

The Cluster Fly is generally low-risk, but caution is still advised. No medical risk. No disease transmission. No structural damage. Purely a nuisance pest due to numbers and persistent re-entry.

In Arizona, cluster flys are most often encountered in the following situations: 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.

The Cluster Fly is most active during: Adults enter structures late summer–fall; emerge on warm days through spring. Arizona's warm desert climate can extend activity periods compared to other states, so year-round vigilance is recommended in the Phoenix metro and surrounding areas.

Seal gaps around windows, eaves, siding, and utility penetrations before late summer. Install tight-fitting screens. Caulk exterior cracks before the overwintering season begins.

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.

Most fly species are drawn to food sources, moisture, organic waste, and breeding sites. For Cluster Flys specifically: 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 after. Eliminating attractants is the first step — Pest Control Bros can identify the source and recommend targeted control.