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Occasional Invaders

Cluster Flies

Pollenia species

Danger Level: 1/5Updated September 2026 · Boise, ID

IdentificationHow to Identify Cluster Flies

Quick Answer

Cluster flies are slightly larger than house flies, with dark gray bodies covered in golden-yellow hairs. They move sluggishly compared to house flies and don't buzz as loudly.

Full identification notes

They overlap their wings when at rest (house flies hold wings in a V). They cluster in large groups, hence the name.

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Cluster Flies Behavior & Habits

Understanding how cluster flies behave helps prevent infestations

Cluster fly larvae are parasites of earthworms.

More about their habits

Adults aren't associated with garbage or filth like house flies are. In fall, adults seek shelter in buildings, clustering in attics, wall voids, and around windows. They emerge on warm days throughout winter and become active again in spring. They're attracted to light, which is why they end up at sunny south and west-facing windows.

Cluster Flies Risks & Dangers

What cluster flies can do to your health and property

Health Risks

Cluster flies don't bite, spread disease, or breed indoors.

Property Damage

No structural damage.

Full health and property details

Health Risks

They're purely nuisance pests.

Property Damage

Large numbers can be extremely annoying, especially when they appear at windows or in living spaces on warm winter days.

Signs of Cluster Flies Infestation

Look for these indicators in your home

Signs You Have Cluster Flies

What homeowners spot first

  1. Sluggish flies at windows on warm winter days
  2. Flies clustering in attic spaces
  3. Dead flies accumulating in window sills
More signs to look for
  1. Flies with golden hairs on their bodies
  2. Large numbers of slow-moving flies

Cluster Flies in Boise & the Treasure Valley

Cluster flies hit homes across the Treasure Valley with mature lawns and irrigated landscaping, because earthworm density drives larval populations.

More on what we see locally

We see the heaviest fall invasions in older Boise neighborhoods like the North End, East End, and the Bench, plus Eagle and Meridian properties that back up to greenbelts, canals, or open pasture. October is the peak entry window, when the first cold snaps push adults toward south and west-facing walls on the next warm afternoon. Once they're in, homeowners notice them again in January and February whenever a sunny day warms the attic.

Our Solution

How We Handle Cluster Flies

What our approach looks like

Cluster fly control works best as an early-fall preventive treatment, before adults find their way into your soffits and attic.

Our full approach

We hit exterior walls, eaves, and window frames with an organic-based barrier spray, then point out the soffit gaps and utility penetrations worth sealing before next year. We don't treat attics, so for flies already inside, vacuum them up as they show up at windows. Just $49 to get started on a subscription plan.

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How to Prevent Cluster Flies

Steps you can take to reduce the risk of infestation

  1. Seal gaps around window frames and soffits
  2. Screen attic vents
  3. Caulk around utility penetrations
More prevention steps
  1. Professional exterior treatment in early fall
  2. Light traps in attics can reduce populations

Cluster Flies Questions Answered

Common questions about identification, prevention, and treatment

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Where are these flies coming from in winter?

Quick answerCluster flies entered your home in fall to overwinter.

They hide in wall voids and attics, emerging on warm, sunny days. They're trying to get outside, not breeding indoors.

Are cluster flies dangerous?

Quick answerNo.

Cluster flies don't bite, don't spread disease, and don't breed indoors. They're a pure nuisance pest. The downside is volume: when hundreds wake up on a warm winter afternoon, they're hard to ignore.

How do I stop cluster flies from coming back next fall?

Quick answerYou need to do two things together.

Seal the entry points (soffits, attic vents, gaps around windows and utility lines), and treat the exterior in early fall before flies start landing on the siding. We handle the organic-based barrier spray and point out the gaps worth sealing, so you're not fighting the same swarm next year.

Think You Have Cluster Flies?

Call and we'll talk through your options. We usually get out within 24 to 48 hours.

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$119/quarter

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Or call now: (208) 297-7947

(208) 297-7947

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