How to Make Natural Outdoor Fly Repellent with Essential Oils - wikiHow
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Tired of fly sprays and the unwanted chemicals they contain? There is a very easy solution to keeping flies away from the outdoor dining area and you can do it yourself.

Evidently, houseflies, being highly edible and defenseless, are nervous types, and don't like to sit still when they see something moving nearby, because it could be a predator. The water bag acts a bit like a lens—try it some time—in which the movements of people in the area are reflected. Even if the fly is too far from the action to see it directly, it can see a shifting of light and dark in the water bag, which it interprets as nearby movement, and it will fly away from the bag.