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!
Hoax or Fact:
Circulating messages suggest that when you hang plastic zip lock bags half-filled with water, adding few pennies to it, it will drive away all pesky flies naturally. This insect repelling technique has been in practice since decades now, as many people use it in their households, and even some modern restaurants hang these plastic bags on the doors or fix them in outdoor dining areas. However, there is no proper scientific evidence validating the effectiveness of this technique, and then there are also few studies which contradict these claims.
This age old concept of naturally driving away flies with simple plastic water bags is now circulating in various versions. Some say only a simple plastic water bag will do the trick, others say you need to add one or more pennies to it, while few others say you have to add small pieces of silver foil to make it work! Using these plastic bags as insect repellents has become so popular that there are couple of industrious websites now that sell specially designed water bags for this purpose.
How can it Work
As far as the effectiveness of this trick is concerned, there are two versions to it. Many people swear that it works great, while as many simply dismiss it as nonsense. Many scientists and entomologists have come up with various theories and explanations as to why this technique can possibly work, the best possible truth among them being the refraction of light from the plastic bags holding water.
Refraction is the change in course and velocity of light when it passes through a clear or opaque object, the plastic bag of water here. This refraction of sunlight reflecting off the bags can result in a number of optical illusions, which is believed to confuse the flies, driving them to fly away. This is attributed to the fact that most insects' heads consist of a pair of large complex eyes, which are sensitive and can't move or focus on objects like we humans can do. It was observed that most of the successful reports of this insect repelling trick happened to be in case of complex-eyed insects, like houseflies. Few theories also claim that the insects fly away at the sight of their own magnified reflection. But then there is other side to these claims.
Mike Stringham Study
Mike Stringham, a professor of entomology at North Carolina State University, investigated the use of these clear plastic water bags as a fly deterrent. Stringham installed commercial, water-based optical fly repellents on two egg farms and studied the effect on fly activity for a period of 13 weeks. Based on the spots of regurgitated material the flies left after feeding, the professor concluded that the water bags actually increased the housefly activity.
MythBusters Bug Special Study
The famous MythBusters television program on the Discovery Channel also investigated into these claims of hanging water bags from ceiling to repel flies. The team made a rig having three chambers separated by trap doors, the first one holding the flies, the second having some rotten meat, and the third chamber had both the rotten meat and a bag of water. Then they released over 5000 flies from the first chamber and studied the fly activity in other two chambers, until they died after sealing the chambers. By weighing the corpses of flies in both chambers, it was observed that the ones with and without the water contained 35 and 20 grams of flies respectively, busting the myth.
Difficult to Study Practically
The above two studies are criticized by many people, for the fact that they were not conducted under natural lighting conditions, and that it could not explore the role of direct sunlight in the water bags' efficiency. It is also a fact that scientifically measuring the effectiveness of such a technique would be practically difficult, as there would be several variables that should be taken into account, like the cleanliness of the area in question, the weather, the types of flies, their breeding cycles etc. If we do not take all these factors into account, then it is also possible that the effectiveness of this natural insect repelling technique reported by many people is just a 'placebo effect'.
So, with the available scientific studies, it would be difficult to judge the effectiveness of these plastic water bags driving away flies. If it is working for you, it is well and good, and if it is not working, then you should look for other effective ways to ward off these flies, as mentioned in the reference section below.