What are Cluster Flies?

Cluster Flies (Pollenia Rudis), also known as Attic Flies, are those large, sluggish and noisy flies you find clustered in large numbers around your windows on warm sunny days in the winter and spring.

The cluster fly is a parasite of earthworms, and in the spring and summer they breed outdoors in fields and lawns. They are usually a larger problem for those living in rural and suburban areas due to larger grassy areas which favor higher earthworm populations. Cluster flies eat flower nectar and not interested in human food, garbage, feces, animal carcases, livestock or manure. They are not a sign of unsanitary conditions if you discover them in your home or business.

Cluster flies, like the Asian lady beetle, western conifer seed bug and the boxelder bug, overwinter (hibernate).  And while most spend the winter outside in and on fence posts, under stones, and in other protected places, they will overwinter in homes and buildings from late fall through early spring. They prefer warm secluded parts like attics and wall voids. On warm sunny winter days these insects may awaken in a confused state thinking it is spring. Once active in the winter or spring, they are a nuisance day and night, found buzzing and congregating at windows and lamps as they try to return outdoors to nature.


What do Cluster Flies Looks like?

The average adult cluster fly is 8 to 10mm long. They are larger than the common house fly. They have a dark grey-checkered abdomen, and the yellow or golden coloured hairs on their thorax, which give the appearance of a golden sheen. When resting, both of their wings overlap across the abdomen. They are slow and sluggish in flight, making them an easy target to swat or capture. They often appear to have no sense of direction and at times, even appear to be dead.

                                   Image of a Cluster Fly


The Life Cycle of a Cluster Fly

A cluster fly’s development time from egg to adult averages 27 to 39 days.

Cluster flies mate outside during the spring. Females lay each egg in its own soil crevice near an earthworm. The egg and larva (maggots) are seldom seen as the eggs are deposited on the soil and once hatched, the cream coloured parasitic larva burrow into earthworms, which they feed on as their main food source. They will slowly consume the worm from the inside for about 3 weeks. They then molt and pupae in the soil for up to 2 weeks, finally emerging as an adult cluster fly. The adult females stop egg laying in late August and September and start to seek hiding warm places in homes and buildings to overwinter. The number of generations of flies varies per region and usually ranges from 4-6 generations each summer.


Do Cluster Flies Bite?

Cluster flies do not bite animals or humans. Their mouthparts are similar to a trunk and used for sucking. They do not carry or transmit disease. While they do not pose a serious health risk to humans, People with asthma or breathing sensitivies may be bothered by their presence.


Do Cluster Flies Cause Damage?

Cluster flies are a nuisance pest. They cause annoyance by their presence, disorientation and constant buzzing in human homes and buildings. People with asthma may be bothered by their presence. If cluster flies die in wall voids, they may attract larder beetles, who will feed off their carcasses. They may occasionally leave small dark-colored spots of excrement on windows and walls. They will leave greasy stains in squashed on windowsills, walls, curtains or carpets, along with a sickly sweet smell.


Why do Cluster Flies Infest Homes and Buildings?

In late August cluster flies enter homes and buildings in large numbers in search for a place to over-winter. They, unlike house flies, are not in search of food, water or a place to mate. While many find warm protective cover outdoors to hibernate until the spring, many locate protective cracks for shelter in homes and buildings.


How Does a Cluster Fly Infestation Happen?

In late summer and fall when the weather cools, cluster flies begin to congregate on sunny walls of homes and structures during the day. When it cools in the evening, they seek protective cover to begin hibernation. They will slip into homes and structures through cracks near windows or door frames, gaps between siding, under the eaves or through open or unscreened windows and vents.

Once in the home, they seek isolated locations to hibernate. They most often cluster and infest attics, false ceilings, wall voids, under baseboards, in cracks, under carpets and rugs, under clothing in closets, beneath curtains and drapes, behind furniture and pictures, and even in dark corners of your home. In multi-story buildings they are often found in the upper two or three floors of the south and west sides of the building.

Cluster flies will remain dormant until the temperature reaches 12 degree Celsius and above. They will then become active (although very sluggish and clumsy), and think it is spring, and therefore time to return to the outdoors. In the winter months, those above temperature warm sunny days that we love, can unexpectedly trigger a cluster fly infestation to become active prematurely. This is why it can seem that a cluster fly infestation in your home or business suddenly and rapidly appears in the winter.


How to Get Rid of Cluster Flies Effectively

Prevention in both the spring and fall is the best approach to keep cluster flies from entering your home or building. Pest proofing your home or building helps minimize the chances and number of cluster flies that gain entry. Here are some things that you can do to help prevent cluster fly infestation:

  • Seal exterior gaps and cracks around windows, doors, eaves, roofs, siding, utility pipes, behind chimneys, underneath fascia and other points of access with high good quality silicone or silicone-latex caulk.
  • Repair or replace all damaged screens on doors or windows.
  • Install insect screening or mosquito nets over air vents in soffits.

The most effective prevention action you can take to prevent a cluster fly infestation is to have the perimeter of your home and building treated by a licensed pest control professional with a residual spray, especially the sunny side, prior to their migration indoors in spring and fall.

In some situations, comprehensive pest proofing is too time-consuming, impractical and impossible. For large infestations with intolerable numbers, spraying pyrethroid insecticides such as permethrin to the outside of the building when the cluster flies first appear will help prevent pest entry. Exterior applications of insecticides may offer some relief from infestations if the applications consist of a synthetic pyrethroid (i.e. bifenthrin, cyfluthrin, lambda-cyhalothrin, cypermethrin, deltamethrin, sumithrin or tralomethrin) or a neonicotinoid (dinotefuran). These should be applied by a licensed pest control operator in the fall just prior to fly congregation.


How to Control a Cluster Fly Infestation?

If large amount of cluster flies are invading your living spaces try to locate the sources they are coming from. Examine baseboards, windows and door trim, exhaust fans and ceiling lights. Next seal any openings with caulk or other suitable materials to prevent any more cluster flies from crawling out. 

As cluster flies are sluggish and do congregate in large volumes around windows and lights, you can vacuum them up to help minimize the noisy nuisance they cause. You can also swat them with a fly swatter. Keep in mind that when squished they will emit an unpleasant sickly sweet odour, and may leave a stain. 

Contact a pest management professional with expertise in cluster flies and outdoor fly control. They can help you prevent these pests from invading your home, and effectively control and eliminate those that do.

If you have issues with cluster fly issues, seek cluster fly pest control services from a true expert, because the reality is that once you have a cluster fly infestation you will experience an annual pestering from them if you do not address the issue. Cluster fly issues can and will dramatically lessen with regular fly control treatments and ongoing prevention efforts from a pest management professional. 

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