We usually expect to have to swat a fly or two in the house during the summer months, when doors and windows are left open and there are plenty of flies coming in from outside. But what about the flies you find indoors during winter, when doors and windows are sealed tight, and you wouldn't think any flies could survive the cold? These tend to be big, fat flies that sit on walls and windows, and it's clear they're not ordinary house flies.
Those large flies in your home in winter are probably cluster flies, which overwinter in the protected areas between the inside and outside walls of your home, or in the attic or basement. Overwintering insects generally stay in secluded areas until the warming and lengthening days of spring pull them from hiding. But all too often, overwintering cluster flies are drawn into the warmth of the home's interior, finding passage through cracks and openings.
Though cluster flies are a clear indoor nuisance, you can relax a bit, since these flies are not breeding in your walls, nor are they a health hazard in the same way as houseflies. And unlike horseflies, they have no interest in biting. In fact, if they were outdoors, cluster flies would not even bother to visit your picnics and social gatherings, preferring instead to spend their time sipping plant nectar.
Identifying Cluster Flies
How do you know if the winter fly is a cluster fly (Pollenia rudis) or some other large fly? The cluster fly can be distinguished from the house fly in several ways:
- Body characteristics: A cluster fly is a bit larger than a housefly and has a black/silvery-black checkered body. Young, newly emerged cluster flies have short light-brown/yellowish hairs on their lower bodies.
- Sluggish movement: The cluster fly will fly around the home but at a less frantic pace than that of the house fly.
- Overlapped wings: When at rest, the cluster fly will overlap its wings; the house fly's wings remain separate.
- Clustering at windows: If there is a large population of cluster flies, they tend to cluster along windows or inside attics and usually in little-used areas on warm, sunny days.
These flies normally live outdoors and are most likely to appear in the spring and fall. But overwintering cluster flies can emerge into homes throughout the winter.
Cluster flies don't cause much of a problem beyond annoyance. They are not known to transmit disease or damage structures, and they do not reproduce inside the home. However, they can leave tiny dots of excrement where they cluster, and large numbers can be a significant nuisance within a home.
4 Ways to Get Rid of Cluster Flies
Swat or Vacuum the Visible Flies
Once cluster flies enter a building to harbor within the walls or buzz around rooms, control methods are limited. Although insecticides can kill the flies harboring within walls, it's best to avoid chemicals because large numbers of dead or dying flies can attract secondary pests, such as beetles and rodents.
Because the flies are sluggish, it is usually fairly easy to swat or vacuum those that find their way indoors. But, unfortunately, just as you get rid of these, more are likely to emerge, causing an ongoing process of swatting, vacuuming, and exasperation.
Use Traps or Flypaper
Cluster flies are fairly easy to trap using flypaper or sticky traps, or by mixing a sweet water solution in a jar with a lid opening just large enough for the flies to enter. Like any flies, cluster flies are attracted to sweets. Make sure to empty the jar regularly.
Use an Approved Indoor Pesticide
For indoor use, an insect spray containing pyrethrum, which is based on an extract from chrysanthemum plants, will kill cluster flies on contact. Pyrethrum is found in a number of commercial products, including CB-80 and Bonide. These pesticides kill upon contact, so use it lightly, aiming directly at the flies.
Pyrethrum isn't completely harmless—it is toxic to fish and amphibians, so try to keep it out of water supplies—but it is non-toxic to humans and pets. Make sure you are buying a product listed as pyrethrum, not pyrethroid, which is a synthetic version that is more toxic and not considered an organic product.
While pyrethrum is one of the least toxic pesticides around, at least where humans and most animals are concerned, remember that it will kill all insects, including beneficial insects such as pollinator honeybees. Therefore, it is best used to target specific visible pests, not as a broadcast spray.
Read labels carefully, as some products include additional ingredients, such as piperonyl butoxide (PBO), a mildly toxic, non-organic chemical. These products will need to be used even more carefully.
Seal Interior Cracks and Gaps
The flies that have found shelter in walls by entering through exterior cracks and crevices can't become an indoor annoyance unless there are also gaps on the interior walls. The smallest cracks along window and door frames and along baseboards offer an avenue through which the flies can enter indoor rooms.
Continue your elimination efforts by caulking or otherwise fill all cracks and crevices in the home's interior wall surfaces, including:
- Around doors and windows
- Around electrical outlets
- Around vent registers.
- At the joints between baseboards and flooring
These methods will prevent new flies from emerging as you kill the ones already in your home.
7 Tips for Controlling Cluster Flies
What Causes Cluster Flies?
Cluster flies enter your home for a very simple reason: They are seeking warmth in order to survive the cold winter months. These are mostly outdoor insects that lay eggs in the soil, which hatch into larvae that feed on earthworms. They enter homes and other warm buildings as fall begins to turn cold, simply to survive the winter. These insects don't reproduce in your walls or in your home, nor are they specifically seeking food, as are standard houseflies.
Homes that are tightly weather-sealed with good insect screens on windows and screen doors have minimal trouble with cluster flies, and these insects are generally not an indoor problem in warm southern climates, where the flies remain outdoors year-round.
How to Prevent Cluster Flies
Preventing cluster flies is largely a matter of sealing gaps and cracks on the walls and foundation of your home so that the flies won't have access to warmth hiding places. The west and south sides of a building are where cluster flies most often enter. Make sure to seal gaps around exterior vents, along rooflines, water spigots and other pipe entries, and along foundation sill plates.
Make sure windows you use for ventilation are fitted with insect screens in good condition, that the windows and doors are tight-fitting, and that exterior moldings are well caulked.
If your infestations are severe, you can have a residual pyrethroid-based insecticide (such as Suspend SC or Demon WP) applied on the outside walls in late summer and early fall, as cluster flies are preparing to hibernate. This is usually a job for a professional exterminator.
Cluster Flies vs. House Flies
Though similar in appearance, cluster flies (Pollenia rudis) are slightly larger than standard houseflies (Musca domestica)—up to 1/2 inch in length vs. the 1/4 inch for a housefly. Cluster flies will usually be louder insects, with an audible buzzing sound, and they very often cluster on glass windows or walls heated by the sun. Compared to a housefly, which flies much of the time, cluster flies are relatively sluggish, often found buzzing as they creep along wall or window surfaces.
Cluster flies are most likely to appear in the early spring as the sun begins to heat western and southern walls and tend to disappear to the outdoors as warm summer weather appears and they seek plants to feed on and outdoor soil in which to lay eggs. Houseflies, on the other hand, reach their peak of indoor activity during the warmest part of summer.
Do Cluster Flies Carry Disease?
Cluster flies are most likely to feed on plant nectars and juices, turning to decaying organic material only when nothing else is available. Thus, they are less likely to spread diseases than houseflies, which feed almost exclusively on dead, decaying organic material that can harbor a variety of easily-spread pathogens.
How Long Do Cluster Flies Live?
Cluster flies are a longer-lived species, with adults living as long as two years under ideal circumstances. Houseflies, on the other hand, typically go through their entire lifecycle in as little as 15 days, up to a maximum of about 60 days.
Do Cluster Flies Bite?
Although cluster flies are similar in size to horseflies and are sometimes confused with them, they do not inflict painful bites in the way that horseflies do. Cluster flies are not blood-feeders, but rather prefer to feed on plant nectars.