Plastic bags filled with water will repel flies.
If you have an email account, you already know that. Or, more accurately, you have been told that.
I have a cousin who is a true believer. She was ready to go ten rounds with me (though she could have decked me in one) when I had the temerity to announce my skepticism of this Internet-inspired home remedy.
Admittedly, the plastic bag nostrum does not fall into the "Windex cures liver cancer" category. But,its advocates have been unable to tell me the science behind it. Why would a bag of water scare away a fly?
The most common reason I have heard is that the light passing through the bag causes the complex eye of the fly to disorient itself. I suppose that is why you never see flying insects anywhere near large (or small) bodies of water.
Part of my skepticism is born of the various denominations of the Church of Water-induced Fly Confusion. Some communicants swear the whole thing is an exercise in futility unless four pennies are included. The trinitarians advocate three. The aluminati propose flecks of foil. The puritans believe the others are heretical. Nothing will suffice other than pure, unadulterated water.
Considering the popularity this urban (and rural) myth, you would think some sort of scientific study would have been conducted to test the efficacy of the theory. And there are some. Sorta.
A study at an egg-plant in North Carolina concluded water in plastic bags attracted flies rather than repelling them. A Mythbusters episode concluded the presence of the bags was a wash. They did nothing.
But, there is a wealth of anecdotal testimony. All conducted without scientific controls.
You can find plenty of people (you may be one) who are willing to hold up their hand and swear: "I had a lot of flies on my patio. Just one day after I put up the bags, all of the flies were gone. And the arthritis in my right foot has greatly improved."
People who make such claims (and I am amongst their number) are absolutely sincere. They report what they think they perceive. But, they may actually be seduced by confirmation bias. We often want something to be true so much that we see what we want to see.
Confirmation bias is what concerned Thomas when he challenged the assertion of the other disciples that Jesus had appeared to them.
I saw it in operation this afternoon in San Patricio. I stopped at my favorite butcher and ordered chicken breasts. While she was retrieving them, I looked up and saw three clear plastic bags filled with water. I had never noticed them before.
What I did notice today were the four flies on the bag nearest to me. By the time I had pulled out my camera, they had fled to a long string chorizo just behind the bag. I was able to capture only one before I felt self-conscious about photographing sausage.
When the butcher gave me my chicken, I asked her how effective the bags were. She smiled, and gushed: "We have not had a fly her since we put them up."
Now, she was either more sardonic than your correspondent or something else was going on. I realized all of the flies were on my side of the counter where I could see them, and she could not. At least, that was my confirmation bias for the afternoon.
I considered telling her about the magical powers of washing her counters with a 3 to 1 solution of Windex and WD-40, but I let the moment pass.
She has an email account. Undoubtedly, she already knows that helpful tip.
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