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Flies - and how to repel them?

Hi, My parents live near a cow farm, and in summer, there is a major problem with flies. Over the years, the house has had most of it's orifices closed with fly & mosquito screens, but even so when eating outside, or playing in the pool, the little buzzers are a real pain. For the moment, nothing really seems to work on repelling flies. What works for mosquitoes does not apply to a fly, a part from fly swats & screens... Does any one have any natural or electronic fly repeller that really works? Cheers, Bpfh