Cluster flies

Cluster Fly Control

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Cluster Fly Pest Control – FAQ’s

Pest Control Advice for Cluster Fly Infestations

Infestations of adult flies clustering inside our homes in the autumn are often clusters of slow-moving cluster flies. These flies don’t carry diseases but can cause cosmetic damage to properties and items stored in affected loft spaces and are considered a nuisance pest problem.

They enter buildings in large numbers from late August (late summer). In most cases, cluster flies stay in homes throughout the winter. Finally, the first warm days of the coming spring stimulate the flies to awaken and leave for the summer.

These unsightly flying insect pests are usually Cluster Flies belonging to the family of flies we know as Pollenia.

Cluster fly infestations are nothing to do with the smaller houseflies or slightly larger blowflies. They might end up overwintering in the same places, but only in small numbers. The frames of doors and windows are common hibernation points as well as wall voids, crawl spaces and roof voids (loft space & attic spaces).

Cluster Flies – the main facts and features:

The most common cluster fly is a medium-sized, nuisance fly, 8mm – 10mm in length, greyish olive in colour, adorned with golden hairs, making them quite distinctive to the trained eye.

They are also more sluggish than other similar flies, like Blow Flies.

Cluster flies infographic

cluster flies diary of activity

Cluster Fly Calendar

Cluster Flies Found in Bristol

Bristol is home to eight commonly encountered cluster fly species that get the name, from their clustering behaviour, sometimes in large numbers or groups of 10,000 or more resting on joists, beams and walls. The four most common species in and around Bristol Include:

Common Cluster Fly – Pollenia rudis: Olive grey with yellowish golden hairs on the abdomen.

Autumn or Face Fly – Musca autumalis: Like house flies but with a yellow/orange abdomen.

Green Cluster Fly – Dasyhora caynella: Like a house fly but shiny green/blue in colour.

Yellow Swarming Fly –Thaumatomya notata: A small fly a little like a fruit fly but with dark eyes.

What Diseases Do Cluster Flies Carry?

Cluster Flies – Not Important Carriers of Disease.

These are not interested in your food in the same way that house flies will be!

In “most” situations they are not a significant pathogenic (disease-causing or disease-carrying) risk to humans. They should be viewed only as an unsightly nuisance with a life-cycle that takes place externally, and not be seen as a sign of poor hygiene or health concern unless health professionals make assertions to contrary.

Infestations deposit large quantities of excrement or droppings, and these might increase the likelihood of disease transmission and development.

Hibernation often takes place in waves that coincide with the final warm days of early fall. The adults first gather on walls in large numbers and rest there until the temperature falls in the late afternoon. They then leave the disappearing warmth of external walls to enter hibernation for the winter in cracks, crevices and other points around a suitable structure where they gather in clusters.

These late influxes mean any control treatments need to be carried out as late into the year or as close to winter as possible to have the very best effect.

They also overwinter with, “and can encourage” other populations that are important carriers of human illness like the blowflies found on faeces and rotting carrion.

An infestation will create a very unpleasant smell, offering us just one possible reason that other, potentially more dangerous species of fly might want to overwinter with them.

Blowflies are often present in the dozens or hundreds, but never any more than that, and may very well relate to animals or birds that have recently died within or close to the property in question.

Where Do Cluster Flies Come From?

Any Grassed Areas – including lawns, pastures, parks and greens, etc. Even if you have no grassed areas immediately around you or you live in a block of flats, they will regularly settle in the highest roof voids, wall cavities or false ceilings of a building. They enter the building through any opening, including the smallest cracks and gaps around door frames.

The flies are attracted to bright surfaces, especially outside walls that have been painted white and thus reflect UV light that these insects can’t seem to resist.

What is The Cluster Flies Life-Cycle?

Cluster flies begin life as eggs. The females lay eggs during the warm, sunny days of the year, in the soil burrows of earthworms.

Maggots/larvae quickly hatch and follow the burrow until they find a worm, enter its body, and begin to feed.

These parasites mature, before leaving the dead or dying worm to pupate and develop into new flies, ready to mate.

Populations vary in number, depending heavily on climate. It has been asserted that the warmer the summer and autumn, the greater the overwintering populations will be.

This is bad news in terms of numbers, but good news, because you can plan to have them controlled in advance and take steps towards that end.

Being parasites of earthworms, they are most abundant in areas with lots of grassland, parks, and grazed pasture.

White and cream painted surfaces, especially those that are south and west facing where they can rest or bask in the sun can create swarm conditions. Over many years this nuisance forces some homeowners to seal up all available cracks and even change the colour of their homes to more subdued tones that discourage them.

Once they have mated and the outdoor temperature begins to decline, autumn signals them to seek refuge inside buildings, under bark and in the hollows of trees, etc. There they will spend the winter months.

In early spring, the first warmer days will stimulate them to return to the fields and soil from which they came.

Many flies will perish as food for predators such as spiders and others will be easy prey or food sources for wasps and birds, especially when resting on external walls and surfaces.

Do Cluster Flies Cause Damage?

YES: When present in significant numbers.

The problem facing most people who live in areas with plenty of open grasslands is that their homes offer these little parasites the ideal environment to overwinter.

They also like to enter casement and sash windows or those made of UPVC where they leave significant deposits of fluid and faecal residues or excrement with an offensive, sickly smell.

These insects also like to hibernate in the warmth and shelter of our lofts and the growing popularity of ceiling halogen lights means that scores of flies can collect around the light fittings and potentially create a fire risk.

Electrical outlets are particularly attractive to the tiny yellow swarming cluster flies and this does pose the risk of fire. For this reason, we recommend that electrical outlets at risk of infestation are well insulated and protected from insect ingress.

How To Get Rid Of A Cluster Fly Infestation.

Most People Get Rid Of Cluster Flies With Fumigation & EFKs.

The cluster fly is common in loft and attic spaces of homes and business premises.

The warmth generated in these areas of the property means that during October and November these insects are naturally drawn into these cavities and voids where they congregate in groups or clusters.

Insect activity is often only discovered in roof voids when people like yourself go up to collect the Christmas decorations or camp beds etc.

You will first notice a very sickly and rich smell, and then at first, a few are attracted to the light source you are using.

Don’t make the mistake of disturbing them, especially if you’ve not dealt with these insects before as it’s not uncommon to find tens of thousands of these insect pests. Smoke bombs are a common treatment method but cold fogging is often safer.

This is especially true in the lofts of secluded properties or properties where the loft is not used.

Once disturbed, they often swarm you a little like a horror movie, especially if you start spraying them with a tiny can of supermarket strength fly spray.

You don’t want to panic and fall through a ceiling or fall through an open loft hatch – and yes it has happened!

So insect spray can have some effect but in most cases, your only other non-professional option is the use of an industrial EFK (electronic fly killing) unit with a specially adapted collection tray or plastic sack.

Good quality EFK units will set you back many hundreds of pounds, but if you use a cheap one that’s not designed for heavy use in confined spaces it may set your house on fire!

Fumigation is usually the very best form of rapid control, and the challenge is always to try and time this, so you kill as many clusters as possible. Do it too early, and you will need re-treatment a few weeks later to kill the final wave of overwintering flies.

So late October and November are the best treatment times in the UK unless you need control more quickly. Proofing with sealants around the frames or doors and other timber or UPVC is also a good idea where it is safe and accessible.

How To Exterminate Cluster Flies in Windows.

Proofing and Residual Insecticide Treatments are the most common control methods.

Flies in door and window frames are very, very common. You will often find that this rather plump fly species is amazingly able to get into some of the tiniest gaps imaginable.

Common places you are more likely to find cluster flies are the internal weight pockets or shafts of sash windows and the Internal frame cavities of both traditional timber and UPVC double glazed units.

What you will likely see is a dark brown speckled covering of faeces where the windows close tight into the frame.

The best way to tackle this is to spray a lacquer based insecticide or residual pyrethroid based insecticide spray into the cavities of the windows and wipe oil-based persistent pesticide products around the window pane perimeters to knock down the cluster fly infestation as possible that land on them.

You often first notice them when you open a window after October or in the spring you notice them emerging into your home instead of your garden.

These flies are not described as being an important carrier of disease; however, they do defecate a lot and cause unsightly and filthy faecal staining on windows and window frames.

Limitations of Cluster Fly Treatments

Nothing Provides 100% Control.

Proofing contaminated areas is often of little use, as in most situations they will be able to find a hole large enough to fit through somewhere on your property.

Another reason they can enter properties is simply that your property requires ventilation to breathe and without sufficient airflow, mould and rot can set in.

Another phenomenon is that it is believed, aggregation pheromones and other chemicals are responsible for the clustering behaviour.

What is interesting and common with other insects like wasps, is that these flies can be very selective on the properties they infest.

Once you have a problem with these flies they will keep coming back year after year and in greater numbers than adjoining or nearby properties of the same or similar age.

Before you have any treatment carried out, you must assess an affected space for bats as it is illegal to disturb or kill them.

Dead Flies

One often overlooked aspect of cluster fly treatments is the number of dead flies that will be killed. These flies will often continue to smell for a time after treatment as they desiccate.

Some piles of flies might need specialist clearance and this can be very costly. Some lofts contain hundreds of thousands of flies or bin bags of flies! For this reason, we recommend considering having attics boarded to ensure access and cleanup is safer and more effective in properties prone to annual infestations.