Cluster Flies What Are They And What Can You Do About Them?

Cluster Flies

Cluster Flies

What Are They And What Can You Do About Them?

The large, black, pesky flies that show up in bedrooms and on window sills from late autumn through to early spring are a common household pest. These flies are known as cluster flies (Pollenia rudis), a name that describes their habit of clustering in large numbers inside attics. Their abundance varies from year to year, possibly in relation to the amount of rainfall through the summer. In a wet summer they will be more plentiful.

Cluster flies do not reproduce indoors, and home owners bothered by these pests do not need to fear the flies are “hatching” from maggots feeding on a dead animal or other unpleasant material within the attic or walls. Cluster flies develop as parasites inside the bodies of earthworms. There are three generations of flies produced each summer, and the final generation of the season migrates to houses and other buildings during mid to late September. Casual observation of client reports suggests houses located on an exposed hill top or high ground are most attractive to these migrating flies.

The flies cluster on the warm sides of buildings in late summer during the day. When the sun goes down and the temperatures cool, these flies crawl into the building through cracks under the eaves and around windows or through gaps in the siding. Once inside and secured in a protected location, they remain in hibernation until warmed by heat from the heating system or the sun.

As the flies warm throughout the winter, and especially in the early spring, they come out of their cold temperature dormancy and begin sluggishly moving around. Their random crawling brings them into the house by way of electrical outlets, window pulley holes, and small openings around windows, mouldings and skirting boards.

Cluster flies hibernate in inaccessible places, making them difficult to control. Hidden within walls or under insulation, they are protected from most treatments until they appear within the living spaces of the house. Preventing attic flies is a job for the summer and autumn. As much as possible, seal cracks and openings around the outside of the house, especially under the eaves, as you would for energy conservation. Insecticides can be used on the outside of the house in mid-September if you have a persistent problem with attic flies. Remember the problem varies greatly from year to year and is worse than average this year and tends to be worse following a wet summer.

Cluster flies ‘mark’ their territory with a pheromone which means that infestations tend to become an annual event.

Once you have a experienced a cluster fly infestation it is likely that it will be repeated on a yearly basis because of their pheromone and in truth there is no total cure for the problem as the flies are not breeding inside the house and in most cases not even on your property.

There is no treatment which can be carried out to prevent the flies breeding and only so much that can be done to prevent the flies getting into the house.

Lofts and attics are often very porous to insects and designed to allow air to flow freely so sealing holes and gaps is not always possible.

In severe cases a pest controller can spray the upper walls of the building around the fascia boards to kill the flies as they walk up the wall but the effectiveness of this is limited and depends very much on the weather.

Flies inside the loft can be killed by a pest controller using a smoke or more likely now a fogging machine but this has no residual action and may need to be repeated several times in the autumn.

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