Key takeaways
- Cluster flies develop inside earthworms, not garbage, and they do not breed inside your home.
- Extension entomologists advise against spraying insecticides indoors; a vacuum and sealing entry points work better.
- Cluster flies are a pest of places with cold winters, so flies swarming inside a South Florida home usually come from a breeding source nearby.
- A sudden burst of shiny blue or green flies indoors often means a dead animal in a wall, attic or crawl space.
In this article
- What Are Cluster Flies?
- Why Cluster Flies End Up Inside Your Walls
- Are Cluster Flies a Problem in South Florida?
- Cluster Fly or Something Else? How to Tell
- How to Get Rid of Cluster Flies Already Inside
- How to Keep Cluster Flies Out
- When Flies Keep Coming Back: Finding the Real Source
- When to Call a Professional
- Frequently Asked Questions
To get rid of cluster flies, vacuum the ones already inside, seal the cracks and gaps they use to get into walls and attics, and skip indoor insecticide sprays, which extension entomologists say do little against flies hidden in wall voids. Because cluster flies do not breed indoors, removing the adults and closing their entry points solves the problem.
Before you start, make sure you really have cluster flies. They are a classic pest of regions with cold winters, where they crawl into buildings in the fall to sleep through the cold. In a warm climate like South Florida, a crowd of flies at your windows more often points to house flies, blow flies or drain flies breeding nearby, and those need a different fix. This guide covers both: how to deal with true cluster flies and how to track down the real source when the flies are something else.
What Are Cluster Flies?
Cluster flies (Pollenia species) are a little larger than the common house fly, dull gray with darker markings, and covered in short, golden-yellow hairs on the thorax, the part of the body behind the head. Colorado State University Extension puts them at about 3/8 to 1/2 inch long. They fly slowly and clumsily, often buzzing against windows or crawling across sills.
Their name comes from their habit of gathering in large groups in attics, wall voids and upper rooms. According to Penn State Extension, the species is thought to be native to Europe and may have reached North America in ship ballast.
Why Cluster Flies End Up Inside Your Walls
Cluster flies have an unusual life cycle. They are not filth flies: their larvae do not develop in garbage, manure or rotting food. Instead:
- In spring, adults leave their winter shelter and lay eggs in soil cracks.
- The larvae enter earthworms and feed inside them, killing the worms.
- Larvae are full-grown in about 2 to 3 weeks, pupate in the soil and emerge as adults. As many as four generations can develop in one growing season.
- As days shorten in late summer, adults stop reproducing and look for dry, sheltered places to spend the winter.
That last step is what brings them indoors. In regions with real winters, the flies gather on sun-warmed south and west walls from about mid-August into early October, then crawl through gaps around windows, unscreened vents and siding. Inside, they settle into wall voids and attics and become dormant. On warm, sunny winter days or when the heat comes on, some stir and wander into living areas, and in early spring most of them try to leave. That is when homeowners notice them at windows.
UMaine Extension notes that their presence simply means there is a good supply of earthworms nearby, typically in grassy areas with good, moist soil.
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Are Cluster Flies a Problem in South Florida?
Not usually. The cluster fly’s habits are built around winter dormancy, and most of the extension guidance on cluster fly invasions comes from northern states and higher-elevation areas. South Florida’s mild winters do not create the same pattern of flies packing into attics in fall and emerging in spring.
What South Florida does have is year-round warmth for flies that breed close to people. UF/IFAS reports that the house fly’s life cycle can take as little as 7 to 10 days in warm weather, with more than 20 generations a year in subtropical and tropical regions, and a single female can lay up to 500 eggs. If flies are gathering at your windows in Miami-Dade, Broward or Palm Beach, find out where they are coming from before you treat. Our fly identification guide covers the species most common here.
Cluster Fly or Something Else? How to Tell
| Fly | Looks like | Where it breeds | What it usually means indoors |
|---|---|---|---|
| Cluster fly | Gray, slightly larger than a house fly, golden hairs on the thorax, sluggish | Inside earthworms in soil | Adults sheltering for winter in walls or attic (cold climates) |
| House fly | Gray with four dark stripes on the thorax, about 1/4 inch | Garbage, pet waste, rotting food, manure | A breeding source in or near the home |
| Blow fly (bottle fly) | Shiny metallic green, blue or gray body | Dead animals, garbage, pet waste | Often a dead animal in a wall void, attic or crawl space |
| Drain fly | Tiny, fuzzy, moth-like, rests on bathroom walls | Slimy buildup inside drains | Organic buildup in sink, shower or floor drains |
| Fruit fly | Tiny, tan, often with red eyes | Ripe fruit, trash, recycling, drains | Fermenting food or a dirty drain nearby |
If your flies are tiny and hovering around fruit or the kitchen sink, see how to remove fruit flies from your home.
How to Get Rid of Cluster Flies Already Inside
Since cluster flies indoors are dormant and cannot reproduce, the goal is to remove them and keep more from getting in.
- Vacuum them. UMaine Extension calls a vacuum cleaner a quick, effective way to reduce indoor numbers. Empty the bag or canister outside right away.
- Use a swatter for strays. University of Kentucky entomologists suggest swatters or vacuum sweepers to remove flies as they appear.
- Try light traps in attics. Cluster flies ignore most fly traps, but ultraviolet traps with sticky surfaces can help in attics and wall areas, according to Colorado State.
- Clean up dead flies. Penn State notes that carpet beetles may feed on dead cluster flies and then attack woolens and stored dry goods. Colorado State adds that dead flies behind walls may also attract rodents and other scavengers.
Why Indoor Sprays Are Not the Answer
Penn State Extension states that it is not advisable to use an insecticide inside structures for cluster flies. Foggers may kill flies in an attic but will not stop more from emerging from wall voids, and spraying cracks indoors is not a recommended treatment. Insecticide dusts injected into wall voids can kill hidden flies, but Colorado State notes these usually require a professional applicator.
How to Keep Cluster Flies Out
Penn State calls mechanical exclusion the best way to keep cluster flies out of a building. Timing matters: Colorado State recommends sealing entry points by mid-August, and no later than Labor Day, before the flies start looking for winter shelter.
- Caulk cracks around window frames, door frames, fascia boards and where siding meets the foundation, focusing on south and west walls and upper floors.
- Screen attic soffit vents, gable vents and other openings.
- Repair loose or gapped siding; UMaine notes that tight siding gives the flies fewer ways in.
- Replace torn window screens and fit door sweeps.
- Ask a licensed pest control company about an exterior treatment of cracks and resting areas in late summer, which supplements sealing but does not replace it.
Treating the lawn will not help. Colorado State notes that adults can fly long distances, so flies in your home may have come from several hundred yards away.
When Flies Keep Coming Back: Finding the Real Source
If you are in South Florida, or the flies you see do not match the cluster fly description, think breeding source, not winter shelter.
- Shiny metallic flies appearing suddenly indoors. NC State Extension says large emergences of blow flies are often associated with dead animals in wall voids, crawl spaces or attics, and a foul odor is often the first sign. Maggots feed for roughly a week, then pupate, and adults emerge about 8 days later, so the flies will keep coming until the carcass is removed.
- Gray house flies around the kitchen and trash. UF/IFAS stresses that killing adults may reduce an infestation, but eliminating the breeding areas is necessary. Keep tight lids on trash cans, bag food waste and pick up pet waste promptly.
- Small fuzzy flies in bathrooms. These are likely drain flies breeding in the film inside your drains.
When to Call a Professional
Call for help when flies keep returning after you have cleaned up the obvious sources, when you smell something dead in a wall or attic, or when small flies are breeding in drains you cannot reach. Toro’s drain cleaning service treats drain and fruit fly breeding sites with Bio-Foam, which digests the buildup where the flies breed. If rats or mice are getting into the attic or walls, where they can die and feed blow flies, Toro’s rodent control service finds and seals their entry points, and if raccoons or opossums are living in the attic or under the house, Toro’s wildlife removal service traps and removes them in Miami-Dade.
For flies and other pests that keep finding their way inside, the residential pest control plan covers the inside and outside of your home every other month. Every service starts with a free inspection, and if the bugs come back, so will Toro, at no additional cost to you.
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305-594-4767Frequently Asked Questions
Why do I suddenly have a lot of flies in my house?
Do cluster flies lay eggs in your house?
Are cluster flies harmful?
What smell keeps cluster flies away?
Will a fly trap catch cluster flies?
Sources
- Cluster Flies, Penn State Extension
- Cluster Flies and other "Winter Flies", Colorado State University Extension
- Cluster Flies, University of Maine Cooperative Extension
- Blow Fly Biology & Management, NC State Extension
- Cluster Flies, Face Flies, and Blow Flies in Homes, University of Kentucky Entomology
- House fly, Musca domestica Linnaeus (Insecta: Diptera: Muscidae), UF/IFAS Extension
Educational content. This article is for general information about pests in South Florida and is not medical, legal or professional advice. If you have a bite, sting or allergic reaction, contact your doctor; in an emergency, call 911. Pest problems vary from home to home, so a licensed professional should inspect before treatment. When using any pesticide, always read and follow the product label.
