Original illustration by PestControlBasics.com. Use anatomical labels above to confirm your identification.
🔍 Identification
Adults: 8-10mm; dark grey; soft golden hairs on thorax (visible close-up); slightly larger and darker than house flies; sluggish movement. Emerge on warm winter days from wall voids, often in large numbers. Found inside windows, struggling to get out. Distinctive: when resting, wings overlap completely (scissor-like) unlike house flies which hold wings at an angle.
🧬 Biology & Behavior
Cluster fly larvae are parasites of earthworms — the adult fly finds earthworms in soil, lays eggs near them, and larvae tunnel into earthworms to develop. Adults emerge in late summer, feed on flower nectar, then seek overwintering sites in September-October. They prefer older buildings with natural siding. Aggregation pheromones bring them back to the same structures year after year.
⚠️ Damage & Health Risk
Large numbers emerging indoors on warm winter or early spring days; clusters of dead or sluggish flies at windows; faint sweet smell from aggregations in wall voids; staining from crushed flies.
🔧 DIY Treatment
September exterior spray (bifenthrin or lambda-cyhalothrin) on south and west-facing walls before aggregation begins. Seal attic vents and gaps where flies enter. Use a light trap or vacuum for indoor individuals. Don't spray walls inside — cluster flies in walls die naturally and emerge dead in spring.
👷 When to Call a Pro
For buildings with persistent severe invasions, professional late-summer perimeter spray is highly effective.