They seem to come from no-where and literally cover window sills. Fortunately they carry no real health risks are more an annoyance than anything else.
Cluster flies are actually parasitic insects. Don’t worry, its not humans they parasite – its earth worms. The female flies lay their eggs at the entrance to worm holes and in the leaf litter, once hatched the small maggots wriggle down into the ground looking for worms. When they find a suitable host the maggots eat their way into the worms where they stay until they burst out of the then still alive worm. Like other fly species Cluster Flies go from an egg, to a maggot, to a pupae before hatching out into adult fly form. The reason Cluster flies appear in our homes is because they need to hibernate. In late autumn they enter roof cavities and lofts where they can gather in such large numbers (hence the name ‘cluster’ fly) that they can resemble bunches of grapes stuck on the beams. When the first warm days of spring register and the flies wake from they’re hibernation they rottenly and mistakenly come down into the house rather than escaping under a roof tile.
There a number of ways to treat cluster flies. ULV or Fogging a loft cavity is a highly effective treatment for cluster flies, the airborne particles float on the air current and penetrate the crevices and cracks like a smoke whilst also giving some residual effect.
Surface Sprays are an excellent longer term solution providing residual insecticide on surfaces which will kill flies for up to three months in good conditions. The down side is that they do not penetrate all the areas the flies are likely to be hiding and the flies must actually land on the area sprayed.
Electric Fly Killer units offer an insecticide free solution to fly problems. Care must be taken when selecting a machine as it will be left on for extended periods of time unattended.