Cluster Flies in Your Attic: Why They Appear and How to Stop Them

As fall arrives across Washington, Idaho, and Oregon, homeowners often discover hundreds-sometimes thousands-of sluggish flies clustering in attics, wall voids, and upper floors. These are cluster flies, and they're seeking warm shelter for winter hibernation. While harmless, their sheer numbers and tendency to emerge on warm winter days make them one of the most frustrating fall pests. Understanding their behavior and implementing prevention strategies can keep your attic fly-free.

Identifying Cluster Flies

Physical Characteristics

Distinguishing Features

• Golden hairs: Short, golden-yellow hairs on thorax (visible up close)
• Overlapping wings: Wings overlap when at rest
• Clustering behavior: Gather in large groups, often in corners or on sunny walls
• Sluggish movement: Especially noticeable in cool temperatures
• Buzzing sound: Loud buzzing when disturbed

Life Cycle and Behavior

Annual Cycle

Spring (April-May):

• Adults emerge from overwintering sites
• Leave buildings to mate and lay eggs outdoors
• Eggs laid in soil near earthworm burrows

Summer (June-August):

• Larvae parasitize earthworms (unique behavior)
• Multiple generations develop outdoors
• Population builds throughout summer
• Adults feed on flowers and nectar

Fall (September-October):

• Adults seek overwintering sites as temperatures drop
• Attracted to sunny, south and west-facing walls
• Enter buildings through cracks and gaps
• Congregate in attics, wall voids, and upper floors
• Peak invasion: late September through October

Winter (November-March):

• Remain dormant in overwintering sites
• Emerge on warm, sunny days (false spring)
• Attracted to light and warmth inside living spaces
• Do not breed or feed indoors

Why They Invade Attics

Attics provide ideal overwintering conditions:

• Protection from elements: Shelter from rain, snow, and wind
• Moderate temperatures: Warmer than outdoors but cool enough for dormancy
• Darkness: Undisturbed, dark spaces
• Accessibility: Numerous entry points (vents, soffits, gaps)
• Proximity to warmth: Heat from living spaces below

Problems Caused by Cluster Flies

Nuisance Issues

• Large numbers: Hundreds to thousands can invade a single attic
• Recurring problem: Return to same locations year after year
• Winter emergence: Appear in living spaces on warm days
• Difficult to eliminate: Once inside walls/attics, hard to remove completely
• Odor: Dead flies create musty smell
• Staining: Excrement can stain surfaces

When Problems Occur

Fall invasion (September-October):

• Flies cluster on sunny exterior walls
• Enter through any available opening
• Accumulate in attics and wall voids

Winter emergence (November-March):

• Warm days trigger activity
• Flies emerge from walls into living spaces
• Attracted to windows and light fixtures
• Homeowners discover the infestation

Prevention Strategies

Timing Is Critical

Best prevention window: Late August through early September

Prevention must occur BEFORE flies begin seeking shelter. Once they've entered attics and wall voids, removal becomes much more difficult.

1. Seal Entry Points

Exterior Sealing:

• Attic vents: Cover with fine mesh screening (1/8 inch or smaller)
• Soffits and fascia: Repair gaps and damage
• Roof penetrations: Seal around chimneys, vents, pipes
• Siding gaps: Caulk cracks and gaps in siding
• Windows and doors: Install or replace weatherstripping
• Utility penetrations: Seal gaps around wires, pipes, cables

Materials to Use:

• Silicone or acrylic latex caulk for small cracks
• Expanding foam for larger gaps
• Fine mesh screening for vents
• Weatherstripping for doors and windows

2. Exterior Barrier Treatments

Professional pest control treatments applied to exterior walls in late summer create a barrier that kills or repels cluster flies before they enter.

Treatment Areas:

• South and west-facing walls (primary congregation areas)
• Around attic vents and soffits
• Eaves and roof lines
• Around windows and doors

Timing: Late August to early September, before flies begin seeking shelter

3. Light Management

• Keep attic lights off during fall invasion period
• Use yellow or sodium vapor bulbs for exterior lighting (less attractive)
• Close curtains/blinds on sunny south and west windows in fall
• Minimize indoor lighting visible from outside during peak invasion

Control Methods for Active Infestations

Exterior Control

For flies congregating on walls:

• Vacuum: Use shop vacuum to remove large clusters
• Spray treatment: Apply insecticide labeled for cluster flies
• Timing: Treat in early morning or evening when flies are less active

Attic Treatment

For flies already in attic:

• Vacuum: Remove visible flies with shop vacuum
• Insecticidal dust: Apply to attic spaces where flies cluster
• Residual spray: Treat attic surfaces where flies rest
• Light traps: UV light traps can capture emerging flies

Interior Control

For flies emerging into living spaces:

• Vacuum: Remove flies from windows and light fixtures
• Fly swatter: Slow-moving flies are easy to swat
• Sticky traps: Place near windows where flies congregate
• Seal interior gaps: Prevent flies from emerging from walls

What NOT to Do

• Don't use bug bombs: Won't reach flies in wall voids; creates mess
• Don't spray inside wall voids: Ineffective and wasteful
• Don't wait until spring: Prevention must occur in late summer/early fall
• Don't leave dead flies: Remove promptly to prevent odor and staining

Professional vs. DIY Control

When DIY May Work

• Small numbers of flies (dozens, not hundreds)
• Flies only on exterior, not entering building
• You can identify and seal all entry points
• You have time to apply treatments before fly migration

When to Call a Professional

• Large infestations: Hundreds or thousands of flies
• Flies inside walls/attic: Emerging into living spaces
• Recurring problem: Returns year after year
• Difficult access: High attics, steep roofs, hard-to-reach areas
• Extensive sealing needed: Multiple entry points requiring professional materials
• Prevention program: Want ongoing protection

Regional Considerations

Washington & Idaho

• Peak invasion: Late September through October
• Common areas: Rural and suburban homes near fields or pastures
• Prevention timing: Late August to early September
• Climate factor: Cool fall temperatures trigger early invasion

Oregon

• Peak invasion: Late September through early November (slightly later than WA/ID)
• Common areas: Willamette Valley, rural eastern Oregon
• Prevention timing: Early to mid-September

Arizona

• Occurrence: Less common due to warmer winters and fewer earthworms
• When present: Higher elevations with cooler climates
• Prevention timing: September (if applicable)

Long-Term Management

Why Cluster Flies Return

• Site fidelity: Flies return to same overwintering sites year after year
• Pheromones: Previous generations leave chemical signals attracting new flies
• Favorable conditions: If conditions were good once, they'll be good again

Breaking the Cycle

• ✅ Annual preventive treatments in late summer
• ✅ Maintain exclusion work (seals deteriorate over time)
• ✅ Remove dead flies promptly (reduces pheromone signals)
• ✅ Monitor for early signs of invasion
• ✅ Professional seasonal program for ongoing protection

Conclusion

Cluster flies are a predictable fall nuisance across Washington, Idaho, and Oregon. While harmless, their tendency to invade attics by the hundreds makes prevention essential. The key to successful control is timing-sealing entry points and applying barrier treatments in late summer, before flies begin seeking shelter.

Once cluster flies have entered attics and wall voids, removal becomes significantly more difficult. Proactive prevention is far more effective and less expensive than dealing with an established infestation.

At Patriot Pest Control, we help homeowners across Washington, Idaho, and Oregon prevent cluster fly invasions with timely treatments and comprehensive exclusion services. Our seasonal programs ensure your attic remains fly-free year after year.