What is a cluster fly?
As the temperatures drop, we are receiving more calls regarding cluster flies, but what are cluster flies? The most common species of cluster fly in the UK is Pollenia rudis. This species of cluster fly lay their eggs in soil, and once the eggs hatch, the larvae live as parasites within earthworms. The larvae develop within the earthworm, and leaves once it is fully grown having almost completely devoured the earthworm.
The adults are typical around 7mm long and can be differentiated from the common house fly by the yellow-gold hairs covering their thorax. The life cycle of cluster flies very much depends on weather conditions, however in Britain there are usually two generations each year. It has been known is warmer climates to go up to six generations in a year.
Why are they a problem?
Over the summer, cluster flies do not pose any problems. In late summer and autumn, they can often be seen sunning themselves on the south or south-west side of buildings (this being the warmest side of the building). As cold weather progresses, adults seek protected places to spend the winter. In many cases they seek shelter within walls, roof voids and attics in vast numbers. This is where the name comes from as they form clusters when hibernating. They usually return to the same building year after year.
Cluster flies are primarily seen as a household nuisance, as they do not pose any risk to human health and are not indicative of poor household hygiene. They do not cause any structural damage to buildings, but can leave staining on paintwork and paper.
What can be done?
Unfortunately, it is not possible to prevent cluster flies from entering properties. You can try to limit access by filling cracks and crevices, however it is unlikely that you will be able to fill all gaps into your attic. However, the hibernating adults are sluggish enough to be picked up with a vacuum cleaner. For large infestations, ULV (Ultra-Low Volume) treatments can provide a knock-down in lofts, but as with any loft treatment, the presence of bats and open water tanks must be checked before treatment.
If you think you may have cluster flies in your house this winter, get in touch with Pest Solution’s friendly and knowledgeable staff by clicking here.
