Sitting on the back porch at a family barbecue this weekend I couldn’t help being annoyed by the numerous flies trying to sample my lunch. I felt like I was waving my arms around to keep the flies off of my food more frequently then I could consume it. That’s when my aunt chimed in and said, “We should get a zip lock bag filled with water and a penny to keep the flies away.” I looked around the table at the blank stares. I had never heard that theory and apparently neither had the other guests. It needed some investigation.
Turns out there are several versions of the same adage. One is to hang a plastic bag filled with water on the door closest to the fly population. Another suggests you might add small pieces of aluminum foil to the bag instead of a penny to make it more repelling to the pests. Logically I could not get on board with either solution. How could a bag full of water repel flies? So, I decided to do a little digging.
Upon my initial review, the believers of this method seem to think that a penny gets magnified in the liquid resembling another insect’s eye and scares the flies away. Another explanation is that the clear liquid looks like the surface of a body of water, which the flies don’t want to land near. For the later, it seems like the flies had no issue landing in my drink so I’m not sure I believe in their fear of water.
Back in science class we learned that flies have big complex eyes made up of thousands of small simple eyes that don’t move or focus. The most common theory seems to be that flies base their sense of direction on where the sunlight is coming from, and their complex eyes are overwhelmed and confused by the multi-directional refracted light produced by the bags, causing them to fly off rather than hang around. Straining the fly’s eyes, that was a new one.
Scientists have not yet been able to provide an answer to the plausibility of this wives’ tale. In 2007, Mike Stringham, a North Carolina State University (NCSU) researcher, studied the effects of water bags on flies at an egg-packing plant. He discovered that the bags, rather than repelling the flies, actually attracted more of them. However, his study was conducted indoors under fluorescent and incandescent lighting, a factor that might have affected its outcome: it’s possible that direct sunlight on the bags might produce a different result (as sunlight in outdoor environments acts in ways that static manmade sources do not).
Mythbusters even took a crack at this theory and found no significant difference between the number of flies attracted to a chamber holding both rotten meat and a plastic bag of water (without coins) and one which held only rotten meat. As with the NCSU experiment, however, the results might have been a result of the experimental factors used rather than proof of its ineffectiveness.
The most logical explanation for the “bags of water repel flies” phenomenon is simple confirmation bias. When people put up bags of water and notice a decrease in the number of flies hanging around (for reasons unrelated to the repellant qualities of the bags), they believe the method absolutely works. But when people try it and the flies still seem to linger, they continue shooing the flies away and do not share the outcome of their efforts.
It seems like the jury is still out on this one although the evidence seems to lean closer to fiction than fact. I say if you believe it works then keep doing it. If you’re like me and think this one is debunked then you can identify believers by their water filled baggies and at least be a little more informed about their superstitions. I’ll leave you with this. Imagine a salesperson offers you an irresistible bargain: she’ll sell you a ring to wear that can prevent the common cold. You wear it for a week and, sure enough, no cold. Does this mean the ring worked?