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Cluster flies in your Delaware house
Slow, dark-gray flies that moved into attics and walls in late summer and wake up at sunny windows on warm days.
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Quick answer: Cluster flies are slow, dull-gray flies about 3/8 inch long that moved into attics and wall voids in late summer. Warm days wake them at sunny windows. They don’t breed indoors, bite or come from garbage. Vacuum them, seal the gaps where they emerge, and ask a pro about early-fall prevention next year.
| Peak season | Sep; indoors through winter |
|---|---|
| Size | About 3/8 inch |
| Risk | Nuisance — no bites; dead flies can draw carpet beetles |
How to identify them
Adults are about 3/8 inch long, dull gray to grayish-brown, and slightly larger than a house fly. They have short golden hairs on the sides of the thorax and a checkered grayish abdomen, and at rest their wings overlap over the abdomen like a pair of scissors. They are slow, noisy fliers, and a crushed one smells like buckwheat honey. Blue bottle flies, which also shelter indoors in cool months, have a dull metallic-blue abdomen and lack the golden hairs.
Why cluster flies move in during late summer and fall
We did not find a University of Delaware or DNREC page on cluster flies, so this page relies on University of Maryland, Penn State, Virginia Tech, Colorado State and Connecticut extension sources. Cluster flies are not a garbage fly: the larvae are parasites of earthworms, so they come from lawns and pastures, not from your trash. As days shorten, from mid-August into early October, adults rest on sun-warmed south and west walls and crawl into cracks around windows, soffits, siding and vents. Maryland says they usually enter in late August, and Colorado State says most move in during September. They then cluster in attics and wall voids and go dormant, but warm sunny days (Connecticut says above 54°F) can stir them and bring them into living areas. They don’t breed indoors, they leave in early spring, and the same buildings often serve as overwintering sites year after year.
When they’re active
Swipe to see all 12 months →
What you can do now
- Vacuum flies off windowsills and floors, then empty the canister into a sealed bag or an outdoor trash can so the dead flies don’t draw carpet beetles (Virginia Tech). Swatting works too, but a swatted fly can leave a greasy spot.
- Find where they come out inside, which is typically gaps around ceiling lights and exhaust fans, cracks behind baseboards, around window and door trim, and poorly fitting attic doors, and caulk those openings.
- Seal outside gaps now so next fall goes better: caulk cracks around windows, doors, siding, utility pipes and soffits, repair screens, and put fine screen over vents. Colorado State says sealing should be done by mid-August and certainly before Labor Day, so it is too late to keep this year’s flies out.
- Light traps are a supplement at best, and the sources disagree. Colorado State says a UV light trap with a sticky surface can help intercept flies in attics and ceiling corridors, and that fly paper and food-lure traps don’t attract them. Virginia Tech calls light and sticky traps good choices indoors. Connecticut says flypaper, bug zappers and sticky traps are of little use because the flies aren’t active fliers indoors.
- Don’t treat the lawn. Colorado State says lawn insecticide has never been shown to kill developing cluster flies, and flies seeking shelter can travel several hundred yards.
When to call a pro
Call a pro if flies keep appearing after you’ve sealed the gaps, or if there are heavy numbers; Connecticut advises consulting a licensed pest management professional for heavy infestations. Exterior perimeter treatments for cluster flies are professional-use products that Virginia Tech and Penn State say must go on in early fall, before the flies cluster, so that window has passed for this year. Penn State says insecticide inside the structure isn’t advisable once flies are in wall voids or attics, and Colorado State says drying dusts injected behind walls need special equipment and usually a professional. Ask about next August and September.
Tried the steps above and still seeing them? The Bug Guy and Mosquito & Pest Authority both serve New Castle, Kent and Sussex counties. Call The Bug Guy 302-242-5254 or Mosquito & Pest Authority 302-346-2970, or request a free quote.
Cluster Flies: common questions
Why are there so many flies at my windows in October?
Cluster flies move into attics and wall voids in late summer and early fall, usually from late August through September, and then wake up on warm, sunny days and head for windows and lights. Connecticut’s experiment station says they can become active when outdoor temperatures rise above 54°F.
Are they just house flies?
No. Cluster flies are slightly larger than a house fly, have short golden hairs on the thorax, and overlap their wings over the abdomen like scissors when at rest. They are also slow, sluggish fliers, and a crushed one smells like buckwheat honey, per Connecticut and Virginia Tech.
Do they breed in my house or come from garbage?
No. Their larvae parasitize earthworms in lawns and pastures, and the flies indoors don’t reproduce there. Colorado State says they are not a filth fly, and Virginia Tech says the larvae are parasites of earthworms.
Do they bite or spread disease?
Virginia Tech says cluster flies don’t bite and aren’t associated with the transmission of disease. Penn State says they won’t damage your home, though they can leave small dark spots of excrement on windows and walls.
Do light traps work?
The sources disagree. Colorado State says UV light traps with a sticky surface can help in attics and ceiling corridors as a supplement to sealing, while Connecticut says flypaper, bug zappers and sticky traps are of little use. Sealing the gaps they come out of matters more.
Will they go away on their own?
Mostly yes. Colorado State says the great majority of cluster flies that spend winter in a building move outdoors in early spring.
Will they come back every year?
They often do. Connecticut’s experiment station says the same buildings typically become overwintering sites year after year, which is why sealing gaps and early-fall prevention matter.
Where in Delaware
Where cluster flies rank on each county’s top-pest list:
- New Castle County Not in the county top six
- Kent County Not in the county top six
- Sussex County Not in the county top six
Sources
Still dealing with cluster flies?
Call a local pro, or tell us what you’re seeing and we’ll send your request to one of the two licensed Delaware companies.