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Question of the Week © 2010 Mike McGrath Note: The 'Search' box and categories to the left are for Gardens Alive products, not articles. To search for Mike's answers to your garden problems, Click here for a complete alphabetical archive of Questions of the Week.
Can Bags Full of Water Really Repel Flies?
Q. Do pennies in a zip lock bag of water actually keep flies and mosquitoes away? Is this a proven natural method of repelling pests?
A. Some of you may remember this question being asked in a recent phone call on the show—and my response that I had never heard of such a thing, and could not imagine how such a trick would actually work. Well, many of you expressed interest, and then our unparalleled unpaid fact finding fanatic Charles Younger sent me a link to an article about this very topic on Snopes dot com—the famous myth busting (or, in some cases, myth confirming) website.
Snopes ruled it neither true nor false; instead letting it fall into their weasely ‘undetermined’ category. They note that the rumor to which Skip refers—about using some type of hanging plastic bag containing water to repel flies—is very prevalent down South, but in most variations the bags contain only water. A lesser percentage of the time, the bag user is instructed to add either a specific number of pennies (generally four), a single penny, or bits of shredded tinfoil to the water in the bags. Presumably the distortion of the water, the flashes of light from the shiny things and the possibility of seeing their own reflection in a distorted manner frightens or confuses the flies.
The Snopes article—posted on the site in August of last year and credited to Barbara Mikkelson—goes on to mention that a researcher in North Carolina actually tested the premise scientifically in a building where chicken eggs were sorted and readied for market. That researcher, Mike Stringham, an entomologist at North Carolina State University, was able to fill me in on the details.
Mike explained that during his many years on the job, the topic of hanging water filled bags to repel flies in barnyard and packing situations would come up every once in a while, with many people swearing by its effectiveness. Oddly, he explains, in one version of the ‘secret origin’ of this story, the idea begins with a fisherman who noticed a lack of biting flies attacking him on his boat after he stacked some water-filled plastic bags containing live fish on top of his cooler.
“It wasn’t long before stacked bags filled with fish and water repelling biting flies in a boat became hanging bags of water alone repelling flies in general,” explains Mike.
The idea remained just another conversational curiosity until Mike came across someone who had begun a small business selling specially made plastic bags designed specifically to hold water for indoor fly control—so he decided to test them.
“The bags were in the shape of small cylinders,” he explains; “we filled them half way with water, tied the tops and bottoms closed and hung them head-high, ten bags to an egg room. As a ‘control group’ we left some rooms in the egg house un-hung. Then we spread what are called ‘speck cards’ or ‘spot cards’ throughout the facility.
“These are small white pieces of paper—we used standard index cards—stapled to wall studs that measure the amount of fly excrement in a given area. We then use the information {ahem} ‘collected’ on the cards to estimate the fly population; it’s a system that works pretty well.
These egg rooms can be very dusty environments, so we followed the designer’s recommendations and replaced the bags with fresh ones about once every ten days to make sure that they stayed clean and the water could continue to do whatever reflecting of light is supposed to provide the repellant action. We ran the experiment for 13 weeks.
And?
“At the end of the experiment, there was actually evidence of a higher number of flies in the rooms with the hanging bags, not the expected lower number. The control rooms that had no hanging bags actually had fewer flies.
“In a professional poultry situation like this, fly control is all about keeping the flies out to begin with and then managing chicken manure and indoor moisture levels. The more efficient a poultry house is at managing its manure and keeping moisture levels low, the fewer fly problems they’re going to have.”
And in a home situation?
“Keep your window screens in good repair, and don’t leave unscreened doors or windows open, of course. Old-fashioned sticky flypaper works very well to capture any flies that do sneak inside. Unroll and hang it just before you go to bed in the evening. In the morning, cover your hands with old newspapers, pull it down and throw it away.”
I mentioned to Mike that when my wife and I enjoy a summer meal in an outdoor area that’s not screened, I set up an oscillating fan to create a breeze that seems to blow most flies and mosquitoes away from us.
“That can work well to protect a picnic table”, he agreed. “You should also pay attention to the basics: keep trash cans on your property well sealed, clean up the dog run frequently, and do all the other basic things you can not to lure flies to your house.”
But, alas, it would appear that doesn’t include hanging water-filled bags.
Here’s a
link to the Snopes article.
And to
Mike’s staff page at NC State.
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