Claim: Hanging plastic bags filled with water will repel flies.
Example: [Collected via e-mail, June 2009]
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
While the Internet-circulated how-to on repelling flies mandates the inclusion of four pennies 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
Theories abound as to why such a practice might work. Some practitioners claim that flies perceive such bags as the surface of a large body of water and therefore avoid them. (This is a somewhat puzzling explanation, as flies are no strangers to ponds, puddles, lakes, and the like). Others maintain that the flies are scared off at seeing their own magnified reflections in the water bags. (Again, though, flies don't seem to shy away from other types of surfaces that reflect and/or magnify their images.) The most common theory is that flies bases their sense of direction by where 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.
Science has yet to provide a definitive answer either way. In 2007, Mike Stringham, a North Carolina State University (NCSU) researcher, reported on his findings after spending
A December 2010 episode of Mythbusters found no significant difference in their 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 an artifact of the particular setup used rather than proof of a general truism.
San Francisco Examiner columnist Richard Fagerlund (aka "Bugman") has tackled this question (in a decidedly unscientific way) several times over the course of years:
She was familiar with its use last summer and was convinced that it is a valid theory. I was motivated to look it up on the Internet. What can you tell me about it?
A: This is a result of a column I wrote a number of years ago and is worth repeating.
Fill a sandwich-size Ziploc bag about half full of water. Tape the top of the bag to the outside of your door. Replenish the water as needed.
It is not clear why these water bags repel flies, but they do. I have gotten a ton of mail from all over the world when this column originally ran and the mail ran 9 to 1 saying this method worked.
You may also get some questions from visitors as to why you have a Ziploc bag of water on your front door.
Some possible answers are: It attracts flies. It scares flies. If it freezes, we know it's cold outside. If it boils, we know it is hot outside. I don't know, ask the Bug Man, it was his idea.
When you drive around town, look for Ziploc bags on doors and you will know who reads this column.
Barbara "light housekeeping" Mikkelson
Last updated: 22 May 2015
Fagerlund, Richard. "Baggies Bid Flies Farewell." Albuquerque Journal. 13 July 2002 (p. B1). Fagerlund, Richard. "Odd Method Persuades Flies to Fly Away." San Francisco Examiner. 12 July 2003. Perez, Robert. "Does Theory for Avoiding Flies at Outside Meals Hold Water?" Orlando Sentinel. 25 August 2006. Perez, Robert. "Despite Restaurants' Idea, Water Bags Don't Deter Flies." Times-Picayune. 13 October 2006 (p. D4).