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Do Water Filled Zip-Lock Bags with Added Pennies Keep Flies Away?

Outline
Long circulated tip claims that hanging a plastic bag half filled with water and containing four pennies will repel flies.

Fly

© Depositphotos.com/ Brandon Alms



Brief Analysis
There is currently not enough evidence to conclusively dismiss the technique as ineffective. Nor is there any scientific evidence to confirm that it works, or, if so, WHY it works. In fact, two scientific studies found that the technique did NOT repel flies and may have actually attracted more of the pesky critters. However, the studies have been criticized because they conducted the tests indoors under artificial light rather than in sunlight.  Many commentators swear that the tip works. Just as many others dismiss it as ineffective.

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Example

Zip Lock Baggies...........who knew?

We went with friends to a restaurant on Sunday for lunch and sat in the patio section beside the store. We happened to notice zip lock baggies pinned to a post and a wall. The bags were half filled with ...water, each contained 4 pennies, and they were zipped shut. Naturally we were curious! The owner told us that these baggies kept the flies away! So naturally we were even more curious! We actually watched some flies come in the open window, stand around on the window sill, and then fly out again. And there were no flies in the eating area! This morning I checked this out on Google.
Below are comments on this fly control idea. I'm now a believer!

Zip-lock water bags: #1 Says: I tried the zip lock bag and pennies this weekend. I have a horse trailer. The flies were bad while I was camping. I put the baggies with pennies above the door of the LQ. NOT ONE FLY came in the trailer.The horse trailer part had many. Not sure why it works but it does!

#2 Says:Fill a zip lock bag with water and 5 or 6 pennies and hang it in the problem area. In my case it was a particular window in my home. It had a slight passage way for insects. Every since I have done that, it has kept flies and wasps away. Some say that wasps and flies mistake the bag for some other insect nest and are threatened.

#3 Says:I swear by the plastic bag of water trick. I have them on porch and basement. We saw these in Northeast Mo. at an Amish grocery store& have used them since. They say it works because a fly sees a reflection& won't come around.

#4 Says:Regarding the science behind zip log bags of water? My research found that the millions of molecules of water presents its own prism effect and given that flies have a lot of eyes, to them it's like a zillion disco balls reflecting light, colors and movement in a dizzying manner. When you figure that flies are prey for many other bugs, animals, birds, etc., they simply won't take the risk of being around that much perceived action. I moved to a rural area and thought these "hillbillies" were just yanking my city boy chain but I tried it and it worked immediately! We went from hundreds of flies to seeing the occasional one, but he didn't hang around long.

Ziplock Water Bag Fly


Detailed Analysis


For decades, a simple and safe method for repelling flies from your house or business has circulated, first via word of mouth and print and – more lately – via email and social media as well. The story goes that, if you hang plastic bags half-filled with water and containing four pennies in strategic locations around your premises, pesky flies will stay away in droves. Proponents of the idea suggest that colourful reflections from the bags of water frighten or disturb the flies and they therefore avoid the area and go somewhere else. The theory is that flies, which have compound eyes, become confused by the reflected light from the water bags.
 
There are several variations of the story. The number of recommended coins varies. Some versions suggest placing small pieces of silver foil in the water bags rather than coins. Many recommend using just the plain bag of water and make no mention of other items such as coins or foil.

But, does the tactic really work? Or is it just another folksy tale with no truth to it? Alas, at this point, there is not enough solid evidence to conclusively confirm or deny its effectiveness. Many swear that it works. Just as many others dismiss it as nonsense.

So, lets discuss what we do know. At least two scientific studies have found that the water bag trick is not only ineffective, but may actually increase the number of flies.

An article on HowStuffWorks Science notes:
Fly Two

© Depositphotos.com/ Alexandr Pakhnyushchy

When Mike Stringham, professor of entomology at North Carolina State University, investigated the use of clear plastic water bags as a fly deterrent, he encountered just such a situation.

Stringham conducted a 13-week field trial by installing commercial, water-based optical fly repellants on two egg farms. Stringham measured the fly activity based on the spots of regurgitated material the flies left after feeding. He concluded that areas equipped with water bags actually experienced higher levels of housefly activity.

Popular hoax-debunking program MythBusters also investigated the story and declared it "busted", noting:

The Build Team made a rig consisting of three chambers separated by trap doors. The first chamber would hold the flies, the second would hold some rotten meat, and the third would hold both rotten meat and a bag of water. They then released over 5,000 flies from the first chamber and waited to see how many flies would go into each of the other two. After the chambers were sealed off, they let all the flies die and collected the corpses to weigh for comparison. The chambers with and without the water contained 35 and 20 grams of flies, respectively, busting the myth.

And a September 2010, The Times-Picayune article about the bags explains:

That argument doesn’t sway Zack Lemann, a bug expert from the Audubon Insectarium. He doesn’t buy the whole water-bag-as-insect-deterrent theory. The first time he ever saw the bags put to use was at a barbecue, which he says was besieged by flies regardless. When some of his Audubon Institute colleagues tried to test out the water bag theory to keep flies away from outdoor animal feed bowls, he said, the experiment ended in failure.

That said, many restaurant owners claim that the bag trick helps keep flies away from their outdoor eatery areas.  Many householders maintain that they have considerably less flies in their home since using the bags.

And, both the egg farm and MythBusters studies have been criticized because the experiments were conducted indoors under artificial light. Some have postulated that sunlight reflecting off the bags might make them more effective.

But, a correlation between two variables does not always mean that one causes the other.  It is in our nature that, if we are invested in an idea, our peceptions of its effectivness – or lack thereof – can be skewed. 

If we began using the bags and flies seemed to be less of a problem, then we could easily conclude it was the bags that did the trick. But, it could well be that other factors, such as the general cleanliness of the premises, weather, fly breeding cycles, and the proximity of fly attracting food were the real reasons why less flies were visiting. And, flies tend to be seasonal, and in my experience at least, some years are more fly prone than others.

It is, in fact, quite difficult to scientifically measure the effectiveness of this technique in real-world conditions.

So, pending further evidence, the jury is still out on this one. If the technique works for you, well and good. But, I'd suggest that the remedy needs to be taken with a sizable grain of salt and might actually be a tragic waste of perfectly good zip-lock bags.

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Last updated: July 10, 2013
First published: July 10, 2013
By Brett M. Christensen
About Hoax-Slayer

References
Can a bag of water keep flies away?
Bags of water hung from the ceiling can repel flies
Do those homemade fly frighteners really work?
Correlation does not imply causation