The Truth Behind Hanging Water Bags: Do They Really Keep Flies Away?
In warm months, when flies and other insects seem to invade every corner of outdoor spaces, people often turn to natural or DIY remedies in hopes of avoiding chemical repellents. One curious method that has persisted for decades is hanging clear plastic bags filled with water—sometimes with coins or foil added—above doors and outdoor seating areas. It’s a popular technique in many households and restaurants, especially in the southern U.S. and Mexico.
But the question remains: does this trick actually work, or is it just another household myth passed down through generations? Let’s unpack the origins of this practice, the logic behind it, and what science has to say.
Where It All Started
The use of water-filled bags to ward off flies has been seen across various regions and cultures, most notably in outdoor settings like patios or food stands. It’s considered an old-school, low-cost solution that doesn’t rely on sprays or traps. Despite its widespread use, few people stop to ask why it’s supposed to work—or if it even does.
Theories Behind the Method
Though there’s no single explanation, several theories attempt to justify why hanging water bags might repel insects:
- Light Refraction and Reflection: The dominant theory is that water inside the bags bends and reflects light in unusual patterns. Since flies have compound eyes made of multiple lenses, these refracted lights may confuse or deter them. Adding shiny items like “pennies or tin foil” is thought to boost the disorienting reflections.
- Magnified Reflections: Another idea is that flies see their own reflections—enlarged and distorted—in the water bag. This might trick them into thinking a larger predator is nearby.
- Imitation of Water Bodies: A third explanation suggests that flies may see the bags as large pools of water. Because flies typically avoid flying over open water to reduce the risk of drowning, this illusion might keep them away.
These theories are intriguing, but theories alone don’t guarantee effectiveness. That’s where science comes in.
What Does Research Say?
Despite countless anecdotes from people who swear by this method, scientific studies offer a different perspective:
- University Study: A 13-week experiment led by entomologist Mike Stringham at North Carolina State University tested the water bag method on egg farms. The results showed “no significant reduction in fly numbers”—in fact, some areas saw more flies. Importantly, the study was conducted indoors, and supporters of the method argue that sunlight is key to its effectiveness.
- “MythBusters” Test: The TV show “MythBusters” also put this theory to the test. Their experiment didn’t show any real benefit from hanging water bags, and they labeled the method “busted.”
What Experts Think
Pest control specialists and entomologists are generally skeptical of the technique. Here’s why:
- “Lack of Scientific Basis”: According to experts, the way flies see light doesn’t make them particularly vulnerable to being confused by refracted images from water bags. Their eyes are highly adaptive and can process chaotic light patterns efficiently.
- “Placebo Effect”: Some believe that people who report success with the method may also be keeping cleaner spaces or are more alert to fly presence, creating the illusion that the bags are working.
What Actually Works?
If you’re serious about keeping flies away, here are methods that have been tested and verified:
- Good Hygiene: Take out the trash regularly and clean food-prep areas to remove any sources of attraction.
- Screens and Barriers: Fit doors and windows with mesh screens to block insects from entering.
- Fly Traps: Use store-bought fly traps, which are engineered to lure and capture flies efficiently.
- Repellent Plants: Keep herbs like basil, lavender, and marigolds near entrances. These plants emit scents known to repel flies.
Final Thoughts
The water bag method for fly control is certainly a creative and widely used home remedy. But when subjected to scientific testing, it simply doesn’t hold up. While the theories sound plausible and many people report anecdotal success, the evidence doesn’t support it as a reliable form of pest control.
If you’re looking for proven ways to manage insects in and around your home, stick to practices backed by research and practical experience. And if the water bags bring peace of mind—or even look decorative—there’s no harm in using them. Just don’t count on them to do all the work.
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