Cluster flies, also known as Pollenia rudis or attic flies, are a common nuisance in rural and semi-rural areas of the UK. These flies are most noticeable during the autumn and winter months, when they invade homes in large numbers, seeking a warm place to hibernate. Though they do not pose a health risk, their sheer numbers can make them a bothersome pest.
Identification of Cluster Flies
Cluster flies are slightly larger than the common housefly, measuring between 6 to 10 mm in length. They are often confused with other types of flies, such as the autumn fly (Musca autumnalis). However, cluster flies can be distinguished by their sluggish movement, stripes behind the head, and the presence of short golden hairs on their thorax. Their mottled dark grey and light abdomen also makes them stand out.
In addition to Pollenia rudis, other species of cluster flies, such as Dasyphora cyanella, may also be present. These flies resemble greenbottles but are less shiny. Despite their appearance, cluster flies are not attracted to human food and do not lay eggs in it, which sets them apart from more problematic species like houseflies or blowflies.
Where Cluster Flies Live and Are Commonly Found
Cluster flies are widespread across the UK, with infestations occurring predominantly in rural areas, particularly in homes close to fields or open spaces. These flies live outdoors during the summer, laying their eggs in the soil or leaf litter. Their larvae are parasitic on earthworms, which they invade and feed upon. Once the larvae mature, they pupate in the soil, emerging as adults in late summer.
As temperatures begin to drop in late August through to November, cluster flies start looking for a warm place to hibernate. They are particularly drawn to lofts, roof spaces, and wall voids of homes. They enter buildings by crawling through small gaps and cracks around windows, roof tiles, or eaves. Once inside, they tend to gather in large numbers, forming dense clusters in attics, behind walls, and around sash windows. These flies release a pheromone that attracts other cluster flies, meaning they often return to the same location year after year.
Cluster fly infestations are typically first noticed on warm, sunny days during autumn or winter, when the flies become active and congregate around windows or light sources in large numbers. While they do not cause structural damage, they can leave unsightly stains from excrement on windows and walls.
Life Cycle of Cluster Flies
Cluster flies have a unique life cycle tied to their parasitic relationship with earthworms. In the spring and summer, the adult females lay eggs in the soil, often near earthworm burrows. Once the eggs hatch, the larvae penetrate the earthworms, feeding on them for 11 to 14 days before pupating. After a few days, adult cluster flies emerge from the soil to repeat the cycle.
In the autumn, the adult flies seek out sheltered spots, such as homes and buildings, where they can hibernate. While they would naturally hibernate under loose bark or in hollow trees, buildings provide an ideal alternative. The flies remain dormant throughout the winter and re-emerge in the spring, flying back outdoors to continue their life cycle.
How to Prevent and Control Cluster Flies
Preventing cluster flies from entering a home can be challenging due to their small size and ability to crawl through even the tiniest gaps. Sealing cracks and gaps around windows, doors, and roof tiles with caulk or other sealants can provide some protection but is rarely 100% effective. Cluster flies are persistent and will exploit any small entry point they can find.
Mechanical Exclusion
One of the best ways to manage cluster flies is to prevent them from entering the building in the first place. Regular inspections to identify and seal entry points are recommended. Pay attention to gaps around windows, doors, roof tiles, and eaves. Installing fine mesh screens over attic vents and other openings can also help keep the flies out.
Insecticides and Fumigation
For larger infestations, insecticides are often the most effective form of control. Cypermethrin-based residual insecticides can be applied to surfaces where the flies tend to rest, such as window frames and eaves. In more severe cases, fumigation with smoke bombs or foggers may be necessary. These methods release insecticidal vapours into the affected area, killing large numbers of flies. Fumigation is particularly useful in loft spaces and attics, where the flies tend to cluster in high numbers.
It is important to note that fumigation or insecticide treatments may need to be repeated. A follow-up treatment in the spring can help eliminate any flies that survived the winter and are emerging from their hibernation.
Vacuuming and Physical Removal
For smaller infestations, simple methods like vacuuming can help manage the problem. Cluster flies tend to be sluggish and are easy to vacuum up, particularly when they gather around windows. However, this method only addresses the visible flies and will not eliminate those hiding in cracks and crevices. Similarly, sticky fly strips can be hung in areas where the flies are most active, though this will only capture a limited number.
Where Cluster Flies Occur in the UK
Cluster flies are found throughout the UK, but they are more common in rural and semi-rural areas. Homes located near fields, gardens, or other open spaces are particularly vulnerable to infestations, as these environments provide ideal conditions for earthworms, which are essential to the cluster fly’s life cycle.
Cluster flies tend to favour homes with sunny, south-east facing walls, as these provide warmth during the autumn months. Buildings with light-coloured exteriors are also more attractive to the flies, though it is not entirely clear why some homes are more susceptible to infestations than others. Even neighbouring properties may experience significantly different levels of infestation.
Managing Cluster Fly Infestations
Controlling cluster flies often requires a combination of prevention and active management. Sealing entry points can help reduce the number of flies entering the home, while insecticides, foggers, or smoke bombs can be used to eliminate large infestations. In severe cases, or when dealing with large buildings, professional pest control services may be necessary to fully address the problem.
Cluster flies do not reproduce indoors, and while they are not a direct threat to health, their presence in large numbers can be unpleasant. Additionally, dead flies can attract other pests, such as carpet beetles, which may feed on the remains and cause further problems. Regular cleaning, vacuuming, and appropriate insecticide treatments can help keep cluster fly populations under control and prevent re-infestation year after year.