Professional Pest Controller Magazine Issue 95

28 May 2019

PestWatch: Cluster flies

Technical | PPC95 May 2019

BPCA Field Officer, Natalie Bungay, is back again with some more seasonal pest management advice. This PestWatch we’re taking a look at the tiny cluster fly.

This guide is for pest management professionals however we have a guide for home owners or businesses that need pest advice on cluster flies here.  

The typical cluster fly Pollenia rudis is about 7mm long, has short golden-coloured hairs on the thorax and irregular light and dark grey areas on the abdomen. Cluster flies are typically slow moving and pass the winter as adults so are very common in early spring.


  • There are over 20 Pollenia species recorded in Britain, most of which can’t be identified without microscopic examination to key out complex features
  • Pollenia rudis is generally the most common species
  • In summer and autumn, it can be commonly found sunning on sheltered fences and tree trunks
  • Cluster flies are so called because they congregate, often in large numbers, to hibernate in attics and out-houses.

The time is upon us, (or maybe it has for a couple of months now) when cluster flies (Pollenia rudis) are a common phone call coming into your organisation from concerned customers. As we know, cluster flies become a serious problem and a major pest in rural homes, and other structures in the autumn when they enter buildings to overwinter, and again, more relevantly, in the spring when they seek ways to escape to the outside.

Heavily infested buildings may contain several thousands of these flies. Sometimes a few thousand of them aggregate and form a cluster which can encourage the assumptions, when not identified as cluster flies, that there is a public health concern or dirty conditions.

While cluster flies are not filth flies, they are severe nuisances when they fly in huge numbers around living quarters.

Activity, biology and behaviour

In autumn, cluster flies congregate in large numbers in upper rooms or roof spaces of houses to hibernate. They also commonly cluster around the edges and cracks of window frames and openings, escaping the cooler temperatures.

Once successfully into a structure, the flies will seek suitable overwintering spots. The flies have been documented staying in tunnels made by beetles in timber and in animal burrows. Before overwintering, cluster flies’ abdomens are full of fat globules to help them survive months of inactivity.

In spring, once the weather warms, cluster flies start to become active again. Although, because of our temperature-controlled homes, it’s not uncommon for them to emerge earlier having been tricked into thinking spring has arrived.

Once the outside conditions are stable, the adult fly gets straight on with furthering its generation by laying its eggs within damp soil or rotting vegetation. The eggs hatch within about one week. The larvae then actively seek earthworms.


Earthworms are a major source of food for the larvae of Pollenia rudis. The main species of earthworm that these cluster flies infect are Aporrectoda caliginosa, Aporrectoda chlorotica, Eisenia lucens, Lumbricus rubellus, and Lumbricus terrestris. Immediately after the larvae hatch they begin looking for worms.

The first instar larvae eat their way through the integument section of the earthworm’s epidermis. While feeding, the larvae leave the spiracles outside of the earthworm. Inside the earthworm, the larvae feed until they are ready to pupate. Once ready, the larvae bores its way out of the earthworm, tunnels back nearer to the surface of the soil and pupates.

The adult flies are, in most cases, herbivores. They feed on many types of organic matter. Plant sap, fruit and flowers and so causing very little problems through the warmer months of the year.

Because of this metamorphosis, these flies are generally most common in rural environments.

It can be hard to accept sometimes that preventing cluster flies within certain properties is mostly impossible


It can be hard to accept sometimes that preventing cluster flies within certain properties is mostly impossible. If you think about it logically, preventing means to ‘break’ the cycle of one of the metamorphosis stages. Let’s consider this then:

  • The egg – can we remove the desirable areas of which the adult flies wants to lay its egg? Not really, unless we consider flattening all foliage within a radius of few miles and concrete over
  • Larvae and pupae – can we take the food source away from the larvae, the earthworms? Again, no, not unless you consider the above.

With this all in mind, prevention is determined as unrealistic. We must then turn our attention dealing with these flies as and when they decide to enter buildings.

Proofing a property will seldom be 100% effective but it should and can be considered. Caulking around window frames and sealing other possible entry points can contribute greatly.

Once the flies are inside, control is relatively simple with both physical and chemical approaches. Cluster flies can often most easily be removed physically, with a vacuum cleaner for example. Doing this whenever they occur will greatly reduce their nuisance factor.

Alternatively, or in addition, most pyrethrins/pyrethroid-based space sprays will quickly kill exposed flies, and in some situations, smoke formulations based on permethrin can be very effective. Special care must be taken when using smoke generators with a fuse lighting mechanism, particularly in the view of fire risk.

Care should be taken to check for the presence of bats before carrying out insecticidal smoke treatments, if bats are suspected or reported by the customer then you must not carry out any works but refer the customer to a body such as the Bat Conservation Trust (BCT).

In some situations, it may be reasonable to install an electric fly killer which is switched on only in the months of known cluster fly activity. Again, be sure that if one is installed in an attic, it is certain that no bats are present.

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