Cluster Fly

What it looks like

• Slightly larger than a house fly
• Dark gray with distinctive golden hairs on the thorax (visible up close)
• Slow flier, clumsy on windows
• Wings overlap when at rest (house flies have spread wings)

Where you'll find it

• Attics and upper-story walls in fall, winter, and early spring
• South-facing windows on warm winter days
• Wall voids, especially around the eaves
• Outside: lawns and gardens (larvae are parasites of earthworms)

Behavior & biology

Cluster fly larvae are parasites of earthworms — adults emerge in fall and seek shelter in attics, wall voids, and behind siding to overwinter. They cluster in large numbers on south-facing windows and become active on warm winter days. Unlike house flies, they don't breed indoors and aren't a sanitation pest. They emerge again in spring before flying back outside.

Iowa activity calendar

Signs of an infestation

• Slow flies clustering on attic and second-floor windows in fall and spring
• Dead flies in window sills, attic floors, and light fixtures
• Buzzing in walls on warm winter days

Health & property risk

No health risk. They don't breed indoors, don't bite, and don't contaminate food. Pure nuisance pest.

How we treat it

1. 1

   Attic + soffit treatment

   Targeted residual insecticide applied to attic spaces and around soffits/eaves where flies cluster to enter.

2. 2

   Late summer perimeter spray

   Exterior treatment in late August-September on the south side of the house intercepts the fall migration.

3. 3

   Vacuum indoor flies

   Vacuum dead flies from window sills and attic floors. They don't reproduce indoors so vacuuming makes meaningful progress.

Why DIY usually fails

Vacuuming is fine for the visible flies. Sealing soffits and treating the attic perimeter prevents the next batch.

FAQ