Why would a bag of water deter a housefly from landing on this tasty slab of meat?

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Debunking the Water Bag Myth

There are plenty of people who don't think water bags can repel flies. Critics often classify this theory in the realm of old wives' tales and modern superstition. They chalk success stories up to confusion between correlation and causation.

Imagine a traveling salesman offers you an irresistible bargain: For only $19.95, he'll give you a belt buckle that can prevent shark attacks. You wear it for a week and, sure enough, no shark bites. Does this mean the magic belt buckle works? Is there an actual correlation between wearing the belt buckle and avoiding sharks? Is one the cause of the other? To properly measure this, you'd have to consider how often sharks attacked you prior to wearing the buckle, and the various other reasons sharks may be leaving you alone.

If all the factors are not taken into account, hanging water bags used to repel flies may seem to work due to the placebo effect. In medical terms, this is when people who think they're being treated for a condition feel better, even if that treatment treats nothing at all. The same effect could occur for people who think they are treating a pest problem.

But what if the situation is even worse? What if the placebo actually increases the problem being treated? 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.

However, the study was not conducted under natural lighting conditions. Its purpose was to determine whether the water bags could be used to decrease fly populations on egg farms. The study didn't explore the possibility that direct sunlight increased the water bags' efficiency.

So do bags of water lower the number of houseflies around homes and restaurants? There are reasonable explanations that argue yes and significant evidence that proves no. Regardless, you can still find water bags hanging near restaurant patios and backyard porches across the globe.

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