Claim: Hanging plastic bags filled with water will repel flies.
Example:[Collected via e-mail, June 2009]
We went with some out of town friends to Sweety Pies on Sunday for breakfast, and we sat in the enclosed patio section beside the house. We happened to notice a couple of zip lock baggies pinned to a post and a wall. The bags were half filled with water, each contained 4 pennies, and they were zipped shut. Naturally we were curious! Ms Sweety told us that these baggies kept the flies away! So naturally we were even more curious! We actually watched some flies come in the open window, stand around on the window sill, and then fly out again. And there were no flies in the eating area!
Origins: In the same vein as the oft-asserted claim that leaving bottles of water on one's lawn will keep dogs from using that space as their personal lavatory is the belief that see-thru plastic bags filled with water and hung where flies can see them will keep those airborne pests at bay.
No one has yet offered a confirmable explanation as to how this belief entered the canon of old wives' lore, but theories abound as to why such practice might work. The most popular one appears to be the notion that something about the way water refracts light works either to frighten flies or overwhelm their multi-eyed visual arrays. Numerous folks swear by the practice, and certainly restaurant operators from the Florida Keys to Texas number among that group.
While the Internet-circulated how-to on repelling flies mandates the inclusion of four one-cent coins in the bag, others who endorse the practice advocate placing only a single penny in each water-filled pouch, and others recommend using instead flakes of tin foil. However, the most common array of this sort
is the unadorned bag of water, with such filled pouches to be either taped to one's doors or suspended in areas where flies are a particular problem. As to whether this method works, some swear by it, while others who have tried it assert that such preventives don't deter the flies one whit. And science has yet to provide a definitive answer either way.
In 2007, Mike Stringham, a North Carolina State University researcher, reported on his findings after spending 13 weeks studying the effects of water bags on flies at an egg-packing plant. After meticulously counting droppings left by flies on white "spot cards," he concluded that the bags, rather than repelling the flying annoyances, actually attracted more of them. Said Stringham, "In the control room versus bags, the bags were consistently higher every time."
Stringham's study was conducted indoors under fluorescent and incandescent lighting, a factor which may limit its applicability: it's possible the effects of direct sunlight on the bags might produce a different result (as sunlight in outdoor environments varies and dances in ways that unwavering manmade sources of light do not).