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Cluster Flies in Iowa Attics and Homes

Updated · By Kasey, Waukee, Iowa

If large, slow black flies keep turning up on the windowsills of your Ankeny home in the middle of winter, you are probably dealing with cluster flies. Homeowners in Johnston, Des Moines, Altoona and across Iowa see the same thing: a few flies buzzing at a bedroom window in December, and sometimes dozens on a sunny day in early spring.

Iowa State University (ISU) Extension calls cluster flies a common household pest, and the good news is that they are a nuisance rather than a sign that something is wrong inside your house. This guide explains what they are, why they pick some homes, what you can do yourself and when it makes sense to call a licensed pest professional.

What it is and how to identify it

ISU describes cluster flies as the large, black, pesky flies that show up in bedrooms and on window sills from late fall through early spring. The name comes from their habit of clustering in large numbers inside attics. ISU's older "attic flies" article adds that they also cluster on outside walls in September and October.

A few things set them apart from the flies you see in summer:

Not sure what you have? The ISU Plant and Insect Diagnostic Clinic identifies insects for Iowans. Check its website for current instructions, forms and fees before sending a sample. ISU's winter insect article also encourages people to learn what they found, since we are often less worried about the things we understand.

Is it harmful?

ISU groups cluster flies with the other fall pests that wander indoors to spend the winter. Its accidental invader articles say these insects are generally harmless to people and property: they do not feed on people, pets, houseplants, stored food or furnishings, they cannot sting, and they cannot reproduce indoors. They are a nuisance mainly because of how many can show up at once.

Why it shows up (season and local angle)

According to ISU, there are three generations of cluster flies each summer. The last generation moves to houses and other buildings in mid to late September.

Here is how they get in, based on ISU's description:

  1. During the day in late summer, the flies cluster on the warm sides of buildings.
  2. When the sun goes down and it cools off, they crawl in through cracks under the eaves, around windows and through gaps in the siding.
  3. Once they find a protected spot in the attic or walls, they hibernate until heat from the furnace or the sun warms them up.
  4. As they warm through the winter, and especially in early spring, they start crawling around and find their way into living space through electrical outlets, window pulley holes and small gaps around windows, moldings and baseboards.

That is why a fly you see in February actually got into the house back in September.

Why some homes get more. ISU notes that, based on reports from homeowners, houses on an exposed hilltop or high ground seem to be the most attractive to migrating flies. ISU also says their numbers vary a lot from year to year and tend to be worse after a wet summer. So if your Ankeny neighborhood sits on higher ground, or you had a rainy summer, a bigger fly season is not unusual.

What you can do yourself

Seal up the outside in summer and early fall

ISU says preventing attic flies is a job for the summer and fall. Seal cracks and openings around the outside of the house, especially under the eaves, the same way you would for energy savings. ISU's accidental invader articles add:

ISU is honest that you will never make a house perfectly insect-proof, but this sealing also helps with boxelder bugs, Asian lady beetles and stink bugs. ISU's winter article points out it can help with your heating bill too. See our guides to Asian lady beetles and boxelder bugs for more on those fall pests.

Flies that are already inside

ISU says there is little that can be done for flies already inside the attic and walls. Once they are hidden under insulation or deep in cracks, they are protected from most treatments.

For the ones that make it into your rooms, ISU recommends simple tools: a fly swatter or a vacuum. ISU's winter insect article also suggests collecting insects in a cup or container and releasing them outdoors.

ISU specifically says some common approaches do not help with cluster flies:

Outdoor sprays: rarely worth it for homeowners

ISU says insecticides can be used on the outside of the house in mid-September if you have a persistent problem. But ISU also calls this outdoor treatment difficult and potentially messy and says it would not routinely advise it for most homeowners. ISU's accidental invader articles add that sprays must go on before insects start entering, protection is short lived, and these products can have non-target risks.

If you do use any pesticide, read and follow the label. The U.S. EPA notes that every pesticide label says it is a violation of federal law to use the product in a way that does not match its labeling. We do not recommend specific brands.

When to call a licensed pro

A professional can make sense when:

How to choose a licensed Iowa applicator

Under Iowa's administrative rules, anyone in the business of applying pesticides to another person's property must be licensed and certified by the state. Household pest work is Category 7A, General and Household Pest Control. Before you hire:

  1. Ask for the company's Iowa license and the technician's certification.
  2. Verify both in the Iowa Department of Agriculture and Land Stewardship lookup.
  3. Ask whether the plan includes sealing entry points, not just spraying, and when they plan to treat.

See our guide to choosing a pest pro for more questions to ask.

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Listings are informational, not endorsements or rankings, and no company paid to be listed. Details come from each company’s own website. Always verify an Iowa pesticide license with IDALS before hiring. Companies can request a correction or removal.

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Frequently asked questions

Are cluster flies breeding in my attic?

No. ISU says cluster flies do not reproduce indoors. They develop outdoors as parasites inside earthworms, and the ones in your home moved in during the fall to spend the winter.

Why am I seeing flies in winter and early spring?

ISU says the flies hibernate in attics and walls until heat from the furnace or the sun warms them. Then they crawl into living space through outlets and small gaps around windows, moldings and baseboards, especially in early spring.

Will fly strips or a bug zapper in the attic help?

ISU says no. Cluster flies do not fly around much in attics, so flypaper, fly strips and bug zappers are of no value there. Attic sprays and fogs also have little if any effect.

Why are there so many more this year?

ISU says cluster fly numbers vary greatly from year to year and tend to be worse after a wet summer.

What is the best way to keep them out?

ISU says prevention is a summer and fall job: seal cracks and openings on the outside of the house, especially under the eaves, before the flies move in during mid to late September.

Nearby towns we cover

Related guides

Sources

  1. Cluster Fly - Iowa State University Extension and Outreach (checked 2026-10-25)
  2. Attic Flies - Iowa State University Extension and Outreach (checked 2026-10-25)
  3. It is Accidental Invader Season Again - Iowa State University Extension and Outreach (checked 2026-10-25)
  4. Fall Nuisance Invaders Are On the Move - Iowa State University Extension and Outreach (checked 2026-10-25)
  5. Keeping Insects Out of Your Home in the Winter - Iowa State University Extension and Outreach (checked 2026-10-25)
  6. Submitting an insect for identification - ISU Plant and Insect Diagnostic Clinic (checked 2026-10-25)
  7. Introduction to Pesticide Labels - U.S. EPA (checked 2026-10-25)
  8. Iowa Admin. Code r. 21-45.22, License and certification standards for pesticide applicators (checked 2026-10-25)
  9. Pesticide applicator and company license lookup - Iowa Department of Agriculture and Land Stewardship (checked 2026-10-25)

We link to the original sources so you can read them yourself. Product names are not endorsements.