Cluster Flies Identification Guide

Cluster Fly Identification Guide

Physical Characteristics

Cluster flies measure 8 to 13 millimeters long. This makes them slightly bigger than common house flies. Their most noticeable feature is the golden-yellow hairs covering their thorax (the middle body section). These hairs are easiest to see on young flies. As flies age, the hairs wear off and they look darker.

The body is dark gray to black with a checkered pattern on the belly. Light and dark patches create this pattern. Unlike house flies, cluster flies do not have dark stripes on their thorax. At rest, they fold their wings flat over their back. This gives them a narrower look than house flies, which hold their wings slightly apart.

Cluster flies move much slower than other flies. They are easy to swat because they lack quick reflexes. This sluggish movement is the easiest way to tell them apart from house flies. When you disturb them, they often bump into windows over and over instead of flying away quickly.

Common Species

The most common cluster fly in North America is Pollenia rudis. People also call it the common cluster fly. Other species that may enter homes include Pollenia vagabunda (vagabond cluster fly) and Pollenia pediculata (tufted cluster fly). All these species look and act alike.

The name “cluster fly” comes from how they gather in large groups to spend the winter. Other names include attic flies, loft flies, and buckwheat flies. The buckwheat name comes from the sweet smell they make when crushed. It smells like buckwheat honey.

Cluster Fly Behavior and Biology

Cluster flies have a life cycle unlike other pest flies. As larvae, they are parasites of earthworms. They do not feed on garbage or rotting food like house flies or blow flies.

In spring, female cluster flies lay eggs in soil cracks near earthworm burrows. The tiny larvae hatch and seek out earthworms. They burrow into the worms and feed on their tissues for several weeks. Then they pupate in the soil. Adult flies come out in late summer.

Adult cluster flies drink plant nectar and other sweet liquids. You often see them on flowers in late summer and early fall. When temperatures drop in September and October, they start looking for warm places to spend the winter. They are drawn to the sunny sides of buildings, especially light-colored ones.

Overwintering Behavior

Cluster flies get their name from how they gather in large groups to survive winter. They enter buildings through small gaps in walls, roofs, and around windows. They crowd into attics, wall voids, and other sheltered spaces. Hundreds or even thousands may gather in one spot.

Once inside, cluster flies go dormant. This state is called diapause. They live off stored body fat all winter. They do not eat, breed, or grow during this time. On warm winter days, the flies may wake up and move toward light. This is when homeowners often notice them inside.

In spring, the flies become active again and try to leave the building. They want to mate and continue their life cycle outdoors. Flies that cannot find a way out may die inside walls or attics.

Treatment Methods for Cluster Flies

Cluster flies breed outdoors in soil, so you cannot remove their breeding sites. Control focuses on keeping them out of your home instead.

Exclusion and Prevention

The best long-term solution is stopping cluster flies from getting inside. Seal entry points before fall arrives:

• Seal gaps around windows and doors: Use weatherstripping and caulk on window frames. Pay extra attention to south and west-facing walls that get afternoon sun.

• Fix damaged screens: Check all windows and vents for holes or tears. Even small openings let flies in.

• Close openings in siding: Seal gaps around pipes and wires. Fill cracks in foundations. Make sure siding fits tightly.

• Screen roof vents: Keep soffit vents and ridge vents covered with fine mesh screens while still allowing airflow.

Timing Matters

Start prevention work in late summer, before flies begin seeking shelter in September and October. Once flies get into wall voids and attics, they are much harder to remove.

Managing Active Infestations

If cluster flies are already inside, these methods can help reduce their numbers:

• Vacuum them up: Use a vacuum with a hose to remove flies from windows, walls, and ceilings. Empty the vacuum outside right away.

• Use light traps: UV light traps in attic spaces can catch flies when they wake up. These work best in spring when flies move toward light.

• Try sticky traps: Place fly traps near windows where flies gather. This catches them without mess.

What to Avoid

Do not spray chemicals inside your home for cluster flies. Since they do not breed indoors, killing adults does not solve the problem. Spraying in attics leaves residue and dead flies that attract other pests.

Do not crush cluster flies. They release a sweet odor and yellow secretion that stains. This secretion is hard to clean and may leave permanent marks on walls, curtains, and carpets.

Professional Help

For bad or repeat infestations, pest control pros can help. They can find all the entry points that need sealing. They can also apply treatments to exterior walls before flies arrive in fall.

Infestations often return year after year. This happens because cluster flies release scents that attract other flies to the same spots. Breaking this cycle takes thorough sealing work plus well-timed treatments.

References

• Cluster Flies - Connecticut Agricultural Experiment Station
• Cluster Fly - Wikipedia
• Cluster Flies - Penn State Extension
• The Biology of Pollenia rudis - Cambridge Core