I see you have the same sort of cluster flies in the US as we have, Pollenia sp. I don't see many, when they are about the wasps come along and take them to their nests. I have Mellinus arvensis, a wasp which appears around August and makes nests in the ground where they put the paralysed flies for their young to develop on, they take other flies too but Pollenia seem to be easier to catch. For every pest there is a predator, the amazing food chain which given the chance can solve all your pest problems.
