Someone I know hangs a zip-lock sandwich baggie full of water over their doorway. He swears that it keeps the flies out of his house and he leaves his front door open and doesn't have a screen door. I have no idea if it works or even how or why it works, but I don't see any flies in his house.
As for the flies outside of your house waiting to ambush you. I'd call a plant nursery or an exterminator and ask questions as to what's causing it to happen all of a sudden. Good luck.
Reading the comments, I have to say, this is strange; this is the first time I have had a real problem with them, too....But I have gone to an outdoor fruit/ vegetable stand in the past, where flies are usually notorious and the man had the baggies of water hanging,and not the first fly!
We are having the same problem with outside our front door. Out of nowhere they have come to ambush us! A friend of mine told me that she used an Ortho product that you add to your hose and spray. She also said that hers was due to a plant attracting the flies. For us though, this is the first time in 8 years, same house, same plants, etc. So I was looking around for any dead critters (birds) but didn't find or smell anything. So....I'm going with either the Ortho or a flystrip or maybe a natural repellent such as mothball crystals.
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