Cluster Flies

What Do Cluster Flies Look Like?

Cluster flies (Pollenia spp.) are often mistaken for houseflies, but they have distinct characteristics that set them apart. Here’s how to identify them:

• Size & Shape – Cluster flies are slightly larger than houseflies, typically measuring about 8–10 mm (1/4 to 3/8 inch) in length. They have a more robust, oval-shaped body.
• Color & Markings – Their bodies are dark gray to black with a non-metallic, dull appearance. They have golden or yellowish hairs on their thorax, giving them a slightly fuzzy look, which distinguishes them from houseflies. These hairs may wear off over time.
• Wings – Their wings overlap when at rest, forming a distinct roof-like shape over their body. Houseflies, in contrast, hold their wings more spread apart.
• Slow Movement – Cluster flies are sluggish compared to houseflies, often moving lazily when disturbed. They may gather in large groups around windows or walls, especially on warm, sunny days.
• Eyes & Head – Their reddish-brown eyes and lack of metallic coloring on their body help differentiate them from blowflies or green/blue bottle flies.

Where Are Cluster Flies Found?

Cluster flies are most commonly encountered in homes, buildings, and outdoor spaces with nearby soil and vegetation. Here are the specific places where you are most likely to find them:

Inside Homes & Buildings

• Attics & Wall Voids – Cluster flies seek out warm, dry places for overwintering. They often hide in attics, crawl spaces, behind walls, and in ceiling voids.
• Windows & Light Fixtures – On warm days, overwintering cluster flies become active and try to exit, often gathering around windows, skylights, and light fixtures.
• Basements & Utility Rooms – If they enter through cracks or gaps, they may be found in basements or near HVAC systems.

Outside Structures

• South- and West-Facing Walls – In fall, cluster flies gather in large numbers on sunlit exterior walls, especially on warm days.
• Cracks & Gaps in Siding, Rooflines, and Vents – They enter homes through small openings around windows, doors, roof vents, and siding.

Gardens, Lawns, and Fields

• Soil-Rich Areas with Earthworms – Cluster fly larvae develop as parasites of earthworms, so they thrive in areas with moist, well-aerated soil, such as lawns, farms, and gardens.
• Wooded & Rural Areas – Homes near wooded areas or fields often experience more cluster fly infestations due to favorable breeding conditions.

What Do Cluster Flies Eat?

Cluster flies have different feeding habits during their larval and adult stages:

Larval Stage (Maggots)

• Cluster fly larvae are parasitic to earthworms. After hatching from eggs laid in the soil, the larvae burrow into earthworms and feed on their bodily fluids until they mature. Unlike other fly species, they do not develop in garbage, feces, or decaying organic matter.

Adult Stage

• Adult cluster flies primarily feed on nectar, plant sap, and other natural sugars from flowers and fruits.
• They do not scavenge human food, garbage, or decaying matter like houseflies.
• Indoors, they do not feed at all; they simply seek shelter for hibernation during colder months.

Since they do not consume waste or rotting materials, they are not known to spread diseases like houseflies. However, their presence in homes can still be a nuisance.

Cluster Fly Life Cycle

The life cycle of cluster flies (Pollenia spp.) consists of four stages: egg, larva (maggot), pupa, and adult, and typically takes 30 to 50 days to complete, depending on environmental conditions.

Egg Stage

• Female cluster flies lay their eggs in soil, usually near earthworm burrows, during the summer or early fall.
• Each female can lay several hundred eggs.
• The eggs are small, white, and hatch within a few days (usually 3–7 days).

Larval Stage (Maggot)

• Once hatched, the larvae (maggots) immediately seek out and parasitize earthworms, burrowing into their bodies and feeding on them.
• The larval stage lasts about 13–22 days as the maggots develop inside the host.
• Unlike typical fly larvae that consume decaying organic matter, cluster fly larvae are parasitoids of earthworms.

Pupal Stage

• After feeding and growing, the larvae exit the earthworm and burrow into the soil to pupate.
• The pupal stage lasts about 10–14 days, during which the larvae transform into adult flies.
• Pupae are reddish-brown and remain in the soil.

Adult Stage

• Fully developed adult cluster flies emerge from the soil and begin their search for mates.
• They are sluggish flyers and resemble house flies but are slightly larger, with a dull grayish body and golden hairs on their thorax.
• Unlike house flies, cluster flies do not breed indoors or feed on garbage or feces.
• During late summer and fall, cluster flies seek shelter inside homes and buildings, often clustering in attics, wall voids, or other protected areas to overwinter.
• In spring, they emerge to mate and lay eggs, restarting the cycle.

Key Characteristics of Cluster Fly Life Cycle

• Overwinters as an adult in homes and buildings.
• Reproduces in soil with larvae parasitizing earthworms.
• Does not breed indoors, unlike house flies or blow flies.
• Life cycle takes about 30–50 days, with multiple generations per year.
• Most active in late summer and fall, when they invade structures in large numbers.