Cluster flies are 6-8mm long, dark or olive in colour.
Egg - Larva - Pupa - Adult
The name Cluster Fly refers to their habit of clustering together in large numbers over winter. There are several species found in the UK and they can be found in mixed swarms. These sizes vary between species to species ranging from 3mm to 8mm in length. Only adult flies hibernate over winter, they start to cluster from early September onwards.
Once they have found a suitable site to hibernate they emit a pheromone, it’s possible if the numbers clustering are larger enough too smell the pheromone, they will attract others to the site. They will return year after year to the same site in ever increasing numbers.
They emerge in early spring congregating on sunny walls, roofs and fences etc. to warm up in the sunshine. They then fly off to lay their eggs, these are laid in leaf litter, compost heaps, or muck heaps. Once hatched the larva, which are parasitic, seek out earthworms, they then bore through the worms skin and proceed to eat it alive. When the worm dies the larvae emerge and pupate in the surrounding soil, the adult flies feed on the nectar of garden and wild flowers. During the summer, cluster flies are not a nuisance, only as autumn approaches do they become a pest as they start to invade peoples house’s and offices.
Cluster Flies are unpleasant in the home, they are not a health risk however they congregate in large numbers to hibernate, if numbers are large enough a sickly sweet pheromone smell can be detected.
Cluster Flies usually look for a warm place to hibernate throughout the winter months, homes in rural areas are usually more susceptive to infestation. They usually cluster within roof spaces, window frames and sashes or any other spaces they find suitable.
Treatments: Insecticidal Control
The control of these insects is difficult. To prevent them entering the building would to be costly and rarely possible. Sealing window frames, sky lights etc. will help along with any other potential entry/ exit points. As for treatments, there is a wide range of products available for DIY /Amateur use, so you would have to think carefully about the areas to be treated… for example window sashes (box type) if you could pump an insecticidal powder having a long residual life with repellent value, in to the sash this would be better than a liquid, if not then a good quality liquid with a long residual life.
External treatments of window frames, roof lines, soffits and fascia boards etc. are a waste of time as environmental conditions soon neutralise any products applied.
If Cluster Flies hibernate in your loft space or attic and you have sufficient room to move about then there are a number of treatments available to you. Liquid sprays could be applied via a hand held sprayer similar to a Knap Sack or garden spray, apply to all surface areas including insulation and finally directly on to the flies.
Dead Cluster flies on window sill Cluster flies hibernating in loft space Cluster flies hibernating in loft space
The above pictures courtesy of Kiwicare, New Zealand www.Kiwicare.co.nz
The second method and my choice every time assuming you have the space, is a Total Release Aerosol or Fumer. The Fumer is a pyrotechnic type of fogger this product must be placed on a flat metal surface before you light the fuse as it gets very hot, it burns for about 30 seconds and fills your roof space with a thick fog, this fog carries the insecticide, it does have a bit of a smell that lingers for some time post treatment. The cleanest and most efficient of all the methods and products available, in my opinion are the Total Release Aerosols they are a twist type top, once twisted they release the total contents of the aerosol in one go, this fills your loft space with a thick fog carrying the insecticide this permeates throughout the loft, you can set it off from the loft hatch, with this product there is no fire risk. Which ever method you adopt, once treatment has been applied DO NOT ENTER LOFT SPACE FOR 48hrs MINIMUM you may have to ventilate before you enter, any dead flies you find remove buy vacuuming.
With most Cluster Fly treatments, some times it is necessary to treat twice a year and this may have to be repeated for three or four years in succession.
Warning any treatments using fumigants’ i.e. Aerosols, Fumer's, Liquids or Dusting powders you must wear a suitable face mask and gloves, Deactivate all fire alarms systems as this range of products will activate a fire alarm.
Alternatively you could bring in a professional pest controller www.epcspestcontrol.co.uk (EPCS)
Should you wish to purchase any products for a DIY treatment please see our buy section