Cluster Flies

When large numbers of flies, approx. 8mm long, are found entering your property in Autumn the chances are they will be cluster flies.

Whilst these flies are generally harmless their presence, often in large numbers inside buildings can create a significant nuisance

What are cluster flies?

There are a number of species referred to as cluster flies. This common name refers to their behaviour of aggregating or clustering within buildings during the winter to hibernate.

The most significant of these Cluster Flies are listed below. All have similar biology and control methodsWestgate Pest Control Cluster Fly

These species are

  • Cluster fly (Pollenia rudis) (Right)
  • Autumn Fly (Musca autumnalis)
  • Green Cluster fly (Dasyphora cyanella)
  • Yellow Swarming fly (Thaumatomya notata)

Biology

These flies only become significant in urban situations during the adult stage of the life cycle.

Eggs of the Cluster Fly (Pollenia rudis) are laid in earth and once hatched, after about a week, the larvae seek out and become parasitic on earthworms. The larvae live and feed from inside the earthworm until they are ready to pupate at which point the larvae leaves the worm. Larvae of the Green Cluster fly (Dasyphora cyanella) and Autumn Fly (Musca autumnalis develop within animal dung and those of the Yellow Swarming fly (Thaumatomya notata) are parasitic of root aphids.

Once the larvae have metamorphosed into adult flies in late summer/ autumn they seek a safe place to hibernate. Traditionally sites such as behind the bark on trees were used however in our modern world a more likely and superior location is provided through roof spaces and attics. It is these areas adult cluster flies tend to be found, as pictured.Westgate Pest Control Cluster Fly  in attcic

The adult flies use an aggregation pheromone to communicate with other members of the same species once suitable harbourage is found. It is these pheromones that enable the flies to build up in large numbers, colonies of over 10,000 are not uncommon.

Once these flies have found a suitable location this pheromone remains in place and will encourage flies back to the same location the following autumn.

Problems caused

These flies can

  • Be a nuisance in properties
  • Give an impression of poor hygiene
  • Create a sickly smell
  • Provide a food source for other pests inside buildings

Control

No direct control on the larvae can be successfully be undertaken. Options are therefore limited to the adult fly. This is not ideal in fly management as control of larval populations would bring greater success.

Control – Non -Toxic Options

As with any pest control program the first point of control would be to exclude the flies. This is often unachievable due to their relative small size and mobility in areas which would be inaccessible to humans. Where food products or high risk areas are subjected to fly ingress efforts should be made to screen off windows and undertake proofing around doors, windows and other accessible points of ingress.Westgate Pest Control Cluster Buster

If flies are detected internally, physical measures can be applied to control the flies. This can be undertaken in the form of vacuuming of the flies or through the safe use of Electronic Fly Killer Units. Window traps such as cluster buster traps may be effective for use on windows where flies congregate (pictured right)

Control – Toxic Options

Where physical remove is not practicable the use of residual insecticide spray, mist or smokes can be very effective at providing control. Applications of space treatments such as ULV are particularly effective along with treatment of any identified fly alighting surfaces. Extreme care should be taken during the use of insecticides particularly in roof spaces and instructions on the product label read and complied with

When treating cluster flies with insecticides it is advisable to employ the services of a competent Pest Control professional. This will not only guarantee that an effective treatment is undertaken but help to ensure amateur users of insecticides do not put themselves or others at risk from their actionsULV Treatment

The use of insecticide treatments for these flies should only be undertaken where the absence of non-target species such as nesting birds or roosting bats has been confirmed

Westgate Pest Control are able to provide bespoke treatment packages for Domestic or Business customers experiencing problem with Cluster Flies. Get in contact today for a FREE site survey and proposal

Photographs kindly supplied by Prof. Moray Anderson of Killgerm Chemicals Ltd.

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