Cluster Flies

What Are Cluster Flies?

Cluster flies, scientifically known as "Pollenia" or "Pollenia rudis," are a type of fly that is commonly found in many parts of the world. They are nuisance pests, especially during the autumn and winter months, when they tend to invade homes and other structures in large numbers. Here is an overview of cluster flies:

• Identification: Cluster flies are medium-sized flies, typically measuring around 8-10 millimeters in length. They have a distinctive appearance characterized by their dark gray to olive-colored thorax, golden-yellow hairs on their thorax and back, and a slightly hairy body. When at rest, cluster flies tend to overlap their wings, creating a noticeable checkerboard pattern.
• Lifecycle: The life cycle of cluster flies consists of four stages: egg, larva, pupa, and adult. Adult cluster flies lay their eggs in the soil, typically near earthworm burrows, where they can access the larvae of earthworms. The developing larvae, known as "maggots," parasitize earthworms by feeding on them. After pupating in the soil, adult cluster flies emerge and seek shelter for the winter months.
• Behavior: Cluster flies are known for their tendency to cluster together in large numbers, especially in the late summer and early autumn. They often seek refuge in buildings and homes to overwinter, hence their name. Unlike house flies, cluster flies are not attracted to food or garbage; they are primarily seeking warmth and shelter during the colder months. They may enter structures through cracks, gaps, and openings, often congregating in attics, wall voids, and other protected areas.
• Impact: Cluster flies are generally considered a nuisance pest rather than a health hazard. They do not transmit diseases like some other fly species. However, their sheer numbers and presence in homes can be unsettling and inconvenient for homeowners. When disturbed or crushed, they can emit a musty odor, which adds to their nuisance factor.

Cluster flies are a common household nuisance during the cooler months, as they seek shelter in homes and buildings. Understanding their behavior, life cycle, and implementing preventive measures can help homeowners mitigate infestations and keep their living spaces cluster-fly-free. If the problem persists, seeking professional pest control assistance may be necessary to manage these pests effectively.

Are Cluster Flies Harmful?

Cluster flies (Pollenia spp.) are not directly harmful or dangerous in the same way as some other pests, but they can still pose several issues, particularly as a nuisance. Here are the primary concerns associated with cluster flies:

• Home Infestation & Nuisance – Cluster flies invade homes in large numbers, typically seeking shelter in attics, wall voids, and other secluded areas during the colder months. Their sheer volume can make them a major annoyance, especially when they emerge indoors on warm days.
• Odor Issues – When crushed, cluster flies release a distinctive, unpleasant smell. Additionally, their dead bodies can accumulate in wall voids or attics, leading to decay and odor issues.
• Attraction of Other Pests – Dead cluster flies can attract secondary pests such as carpet beetles, which may then move on to feed on other natural fibers in the home, including clothing and furniture upholstery.
• Allergen & Respiratory Concerns – While not known to bite or spread diseases, the presence of large numbers of cluster flies indoors can contribute to indoor air quality issues. Their shed skins and fecal matter may become airborne allergens, potentially irritating individuals with respiratory sensitivities.
• Electrical & Fire Hazards – Like other insects, cluster flies can sometimes accumulate around light fixtures, electrical outlets, and other wiring areas, posing a minor but potential risk of short circuits, or even fire hazards in extreme cases.
• Crop Impact (Minor Concern) – Unlike houseflies, cluster flies do not feed on garbage or decaying matter, nor do they transmit pathogens. However, in large numbers, they can occasionally impact agricultural settings where they lay eggs in soil and develop as parasites of earthworms, although this is rarely a significant economic issue.

While cluster flies do not pose serious health threats, their persistent infestations can be frustrating and challenging to manage without proper exclusion and pest control measures.

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.

Cluster Fly Infestations

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.