Cluster Flies

One day I visited friends in the next valley and was surprised by the level of buzzing coming from the vicinity of their front door. At first I assumed it was late season bees in the Russian Sage but then I realised it was dozens of golden haired flies.

Female cluster fly.

The identity of these flies wasn't much of a mystery. The golden hairs make them unmistakable as cluster flies Pollenia sp. They are parasites of earthworms and will have mass emergences from time to time, especially if there is well rotted cow manure around to attract earthworms and therefore the flies. That was the case here, as a few days earlier the neighbouring farmer had been muck spreading.

Male cluster fly.

The flies can be a bit of a nuisance, in that they come indoors to hibernate in nooks and crannies, and where they will die in large numbers on windowsills and in attics. They are not attracted to human food so do not represent a health hasard, but once they are present in a house you will likely never get rid of them, as each new generation seems to return to a favoured location. I can remember housesitting at Tyntesfield in England before the National Trust opened the property to the public. Crossing the floor in the attic was to crunch across a carpet of cluster flies that had accumulated over several years.

Cluster flies on the exterior of the house.

There are 18 species of Pollenia in France, not very easy to identify to species level. These are probably the most abundant species, the Common Cluster Fly P. rudis (Fr. Mouche des greniers).

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