No Fly Zones

Speaking of arcane, useless knowledge, Friday night I was out with some friends and somehow the topic came up: "to keep flies away, hang clear plastic bags filled with water." Trying to think of some logical reason why this would work, I made a mental note to look into it. It's apparently a popular belief in Florida (so you know logic will have nothing to do with it), as well as in the Caribbean.

What's the verdict? According to some anecdotal reports, it works. According to others, well, not so much. According to this article in the Fredericksburg Free-Lance Star:

No one is sure where the idea came from, and researchers contend there's no scientific evidence to show that the bags do any good.
...
Some speculate that the sunlight reflecting through the water somehow scares or disorients flies. But Phil Kaufman, a University of Florida professor of veterinary entomology, said there is no scientific research to back up those claims.

"It's a pretty safe bet it doesn't work," he said.

On the other hand, the bags could very well worsen the fly problem, as was found in one experiment whose researcher I do not envy:

[A] North Carolina State University researcher spent 13 weeks looking at the effects of water bags on flies at an egg-packing plant. Mike Stringham, also a veterinary entomologist, meticulously counted droppings left by flies on white "spot cards."

The results were conclusive: The water bags attracted more flies.

"In the control room versus bags, the bags were consistently higher every time," he said.

The great site HowStuffWorks has a take on this, suggesting how the water bag idea might work:

The insect's head mostly consists of a pair of large complex eyes, each of which is composed of 3,000 to 6,000 simple eyes. These eyes can't move or focus on objects like human eyes, but they provide the fly with a mosaic view of the world around them. Each simple eye provides one small piece of the puzzle, much like the way a screen's pixel delivers one detail of the larger picture.

A housefly bases its sense of direction on the direction sunlight comes from. Some entomologists believe that when these complex, sensitive eyes experience refracted light, the insect becomes confused and flies away.

While some supporters claim water bags keep all kinds of flying insects away, most report success with complex-eyed insects, like houseflies.

Nice theory, but they are quick to stress that so-called evidence in favor of the water bags is more than likely a case of the "placebo effect":

In medical terms, this is when people who think they're being treated for a condition feel better, even if that treatment treats nothing at all. The same effect could occur for people who think they are treating a pest problem.

That is, some other factor might be keeping the flies away, and they correlate the lack of flies with the presence of the water bags, which may have nothing to do with it.

So for now, all we have to go on in support of this idea is anecdotal "evidence," which is rarely conclusive. (This is after all the most pressing issue in the world today, so we need this resolved ASAP.) This phenomenon was also cited on Boing Boing earlier this year, and the range of reader comments is indicative of the problem with anecdotal evidence: a few say it works, most say it doesn't. I'll stick with the scientists on this one until compelling evidence that it works is found--but then only if I move into the Amityville Horror house, since flies are the least of my problems.