Myth Busting: House Fly Control Using Coins in a Ziploc Quart Bag

Fly Bags at the Purple Feather

The door to the Purple Feather is protected by Ziploc bags with water and coins in them.

Our pest control myth for today takes us to an unlikely place on the furthest tip of Cape Cod: Provincetown, Massachusetts, affectionately known to the locals as P-town.

While exploring this fishing town turned arts community, my wife and I settled into a cafe called the Purple Feather for lunch and coffee.  (BTW – if you find yourself in P-town, we highly recommend the Purple Feather’s Bradford Street Panini and the Mac and Cheese.  And their coffee.  And their chocolate.)

Walking around town, I became intrigued that many businesses, including the Purple Feather, operated with their front doors wide open.  The weather was mild, and the city is very foot-traffic friendly, but still odd, since pests only need a small crack to get inside, and when the doors are open, well, there’s no stopping them.

So, it wasn’t until we were eating that I saw what would become the topic of today’s pest control myth and the raison d’etre for the open front door.

Strategically placed in the jam of the front door hung Ziploc quart bags, staple-gunned in placed, and filled with water and some coins.

There’s a myth that the coins and water create a prismatic affect in the eyesight of house flies (and I would assume other filth flies, like the bottle or blow fly and the flesh fly, but I’m not sure about other pests…) and this dizzying spectacle keeps them from approaching.  Hence, you can keep flies out of a building or away from an area by merely hanging up these bags.

And of course, it must work, why else would people use this technique?

Trucks in New Delhi

Fruit hangs from the front of a truck in New Delhi, India, to ward off evil.

Similarly, in India, they hang fruits off their vehicles, to keep away evil.

Well, I don’t know if the Indian technique of fending off evil has been tested, but the Ziploc bags – water – coinage has been.

An entomologist that I worked with during my tutelage at NC State University, Mike Stringham, tested this and found that it does not work.

So why does it seem to work for some people?  Well, us humans are talented at rationalizing, at coming up with reasons why something is true.  We have a tendency to explain away evidence contrary to what we want to believe, and put additional emphasis on evidence that supports our conclusions.  Most Americans would probably scoff at the Indian idea of hanging fruit from their vehicles, but surely the Indians who do this have a reason – have some sort of evidence that supports their conclusion that it works.

So what do you do about this?

Scientific principles – when academicians follow them – have safeguards built in to help avoid this tendency.  Scientific papers have to report on how the researcher protected against rationalizing, and have to be reviewed by a team of scientists prior to publication.  Assuming, of course, that there isn’t rationalizing in the team of scientists…. Ah, well, it is never ending, eh?

My conclusion, based upon the evidence, is that Ziploc bags filled with water and coinage does not work at keeping flies at bay.

Fruit, however… I may need to give this a try and see how it works on my car.

Until next time, consider the myth of Ziploc bags filled with water and coinage keeping flies away… busted!


Stringham, S.M., et al. “Suppression of house flies (Musca Domestica) in egg handling rooms of broiler breeder farms using Fly Fright bags as an optical repellent.” Proceedings of 48th Annual Livestock Insect Workers’ Conference.

2 thoughts on “Myth Busting: House Fly Control Using Coins in a Ziploc Quart Bag

  1. Joe

    Since you did not test the effect of refracted light on fly activity your presumption that it is a myth is worthless.

  2. Ted Snyder Post author

    Joe, simple observation has shown that places with water bags placed out in front are not free of fly issues, so there is no need to go further in analysis. If they are free of fly issues, such as the establishment in my post, it is due to other factors, in this case, there were no flies present. Food processors need to rely upon modern pest management technology to control flies – and a good health inspector will insist upon the same. Ted

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