Can You Repel Flies With Water-Filled Bags?

By Chris Williams on March 28, 2013.

Water filled bag keep away fliesQuestion

I found some information on the Internet about how you can hang bags of water on your deck to keep flies away when you’re eating outside. Somehow they see their reflection and are scared away. Does this really work?


I’ve been wondering when I would get this question. For some reason, this method of fly control has become very popular, especially among outdoor restaurants. When traveling in the Southwest, we saw more than one restaurant that had zip-lock baggies of water hanging under each table umbrella in the outdoor seating area. Restaurant owners seem to swear by this nontoxic method, their patrons are amazed that there are no flies, and the customers take the idea home with them. There are actually Internet entrepreneurs that will sell you all the makings for your very own zip-lock bag fly repellent system.

If you search this idea on the Internet, you’ll find many different homemade versions. Some say use just a half bag of plain water. Others say the baggie has to contain a penny, or exactly 4 pennies, or bits of floating aluminum foil. There are also multiple explanations as to why this supposedly works. One is that flies are frightened away by their own reflection, or by the reflecting foil or pennies. Or that flies are afraid of drowning. A more reasonable explanation is that it’s simply light refraction through the bag and the water that keeps the flies away.

A house fly has a complex visual system consisting of two large complex eyes that are each made up of thousands of simple eyes. Each simple eye provides one small visual piece of the picture, sort of like pixels on a screen. All of the pieces from each of the eyes together make up the total picture for a fly. It’s theorized that when these complex eyes experience refracted light traveling through the water bag, the picture is scrambled and the fly becomes confused.

Entomology professor Mike Stringham of North Carolina University decided to put the water baggies to a scientific test. His sites were two egg-production farms where you can be sure there were plenty of flies. I won’t go into detail but suffice it to say that he spent 13 weeks measuring fly activity in the two sites by counting the number of fly specks that he found. His conclusion was that sites with the water bags actually had more fly activity. However, proponents of the baggie system argue that he didn’t test the bags outdoors in sunlit conditions which could make a difference.

There is no scientific reason why the baggies should work. So, why do people think they work? Maybe because there was no real fly problem in the first place. Bottom line: If it makes you feel better to hang water bags on your deck, go ahead. But the best way to control flies is good sanitation. Flies breed in garbage, animal feces, and carcasses. If it’s not there, they won’t come. I worry about a restaurant that feels it has enough of a fly problem that it needs to hang water baggies! Can’t be good for business.

Photo credit: silverfuture / / CC BY-NC-SA



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