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Cluster Flies

flies, cluster fly
21 Mar

The Common Cluster fly belongs to a family of flies called Pollenia and is a medium sized, nuisance fly about 10mm in length, greyish olive in colour and adorned with golden hairs which makes them quite distinctive to the trained eye. They are also more sluggish in nature than other flies of the same size like blow flies. They get the name cluster-fly from their characteristic behaviour congregating in groups of hundreds and even thousands in lofts, crawl spaces and window frames but are also referred to as pasture flies, field flies and attic flies.


What Diseases Do They Carry?

Cluster flies have no recognised importance as carriers of disease so are not a significant pathogenic risk in most situations, and should be viewed only as an unsightly nuisance and not as a sign of poor hygiene. What is important to think about is that cluster flies will hibernate with other flies, possibly attracted by the smell created by large populations that are important carriers of human diseases like the blow flies found on faeces and rotting carrion.

Where Do they Come From?

They begin life as an egg, laid by the female during the warmer months of the year in the burrows of earthworms. Once the larvae hatch they follow the burrow until they find the worm and enter its body where they become parasites until mature, when they leave the worm to pupate. The size of cluster fly populations is very much dependant on climate and 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 in the sense you can plan to have the flies controlled in advance and take steps towards that end.

Being parasites of earth worms they are most abundant in areas with lots of grass land around especially gazing land. They are also attracted to white and light painted surfaces especially those that are south and west facing where they can bask in the sun. This creates swarms of flies in some areas causing homeowners to change the colour of their homes to more subdued tones.

Once they have mated and the outdoor temperature begins to fall in the autumn the flies seek refuge inside buildings, under bark and in the hollows of trees etc.

Do They Cause Damage?

No – not unless they are present in significant numbers. The problem facing most people who live in areas with plenty of open grassland is that their homes offer these little parasites the ideal environment to overwinter. Cluster flies like to enter window frames especially casement and sash windows where they leave significant deposits of fluid and faecal residues with an offensive sickly smell. They also like to hibernate in the warmth and shelter of our lofts and the growing popularity of ceiling halogen lights means that large numbers of flies can collect around the light fittings and potentially create a fire risk.

How To Treat A Cluster Fly Infestation In The Loft.

Cluster flies are common in the loft and attic spaces of your home and business premises. The warmth generated in these areas of the property mean that during the months of October and November flies are naturally drawn into these cavities and voids where they congregate in groups or clusters. Fly activity is often only seen 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 flies attracted to the light source you are using. Don’t make the mistake of disturbing them if you have not dealt with these insects before. It is not uncommon to find tens of thousands of flies in the lofts of secluded properties and they will very often swarm you a little like a horror movie, especially if you start spraying them with a tiny can of supermarket strength fly spray.

So fly sprays can have some effect but in most cases your only other non-professional option is fumigation. The challenge is always to try and time this so you kill as many flies as possible. Do it too early and you will need to re-treat a couple of weeks later to kill the final wave of overwintering flies. So late October and November are best, unless you need control more quickly. One way to avoid having to find the right time is to invest in a SWAK dispenser. This is a unit that houses a pyrethrum based insecticidal spray which you can set to spray at intervals over a 24 hour period. The idea is that the timed spray targets cluster flies as they come in to hibernate. To be most effective it should be used between August and December. The flies can also be a problem in early spring when the weather starts to warm up, so it would be an idea to set up the unit again for then. Another option for loft spaces are synthetic fumers. These contain 13.25% permethrin. Set the fumer on a slate or metal tray and light the touch paper, (lighting the one furthest from the exit point first if you are using more than one) They work in the same way as the SWAK unit but obviously on a one-off basis. Be sure there are no bats roosting in the loft before attempting either of these eradication methods as bats are protected.

How To Control flies in Windows.

Flies in windows are very, very common. You will often find that these often, rather plump flies are quite amazingly able to get into some of the tiniest gaps imaginable. Common places you are more likely to find the flies are the internal weight cavities or shafts of sash windows and the Internal frame cavities of double glazed units. What you will likely see is a dark brown speckled covering of fly faeces where the windows close tight into the frame. The best way to tackle this is to spray a lacquer based insecticide such as Dethlac into the cavities of the windows. A brilliant product is called the Cluster Buster. It is a sleek plastic unit containing a quicksand made of exploded eggshells. The natural behaviour of the cluster fly leads them to falling into the trap where it becomes stuck in the quicksand. The more that get trapped, the more pheromone the insects emit, so attracting yet more flies. It will hold about 1000 flies and normally lasts about 2 years. For maximum effectiveness, have a unit on each window.

How Can We Get Rid Of Flies?

Proofing contaminated areas is 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 are able to enter properties is simply because your property requires ventilation to breath and without sufficient air flow 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 is in common with other insects like wasps is that the flies tend to be a little selective on the properties they infest. It seems that 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 it is important that you assess an affected space for bats as it is illegal to disturb or kill them.

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