Can a shiny penny or a plastic bag filled with water really keep flies away? It is an old-fashioned home remedy that has been passed down through generations. You may have seen clear plastic bags filled with water and a few copper-colored pennies hanging near restaurant entrances, backyard patios, porches, barns, and picnic tables.
Supporters claim that sunlight reflecting through the water and shiny pennies confuses flies, disrupts their vision, or frightens them away. Some people swear by the trick. Others say it does absolutely nothing.
But what happens when scientists actually test it?
At D & D Pest Control Co., serving Kinston, Greenville, Goldsboro, New Bern, Jacksonville, and surrounding Eastern North Carolina communities, we believe pest control advice should be based on evidence rather than folklore.
Fortunately, researchers and experimenters have investigated whether hanging water-filled bags, with or without pennies, actually repels house flies. The results are interesting, occasionally contradictory, and not nearly as convincing as the popular myth suggests.
The short answer: There is no reliable scientific evidence that shiny pennies placed inside a bag of water will consistently repel house flies. Some laboratory observations suggest water bags may change fly behavior, but other experiments have found no measurable protection. For dependable fly prevention, sanitation, exclusion, and eliminating breeding sources are more effective approaches.
Where Did the Penny and Water Bag Fly Repellent Myth Come From?
The idea is simple: fill a clear plastic sandwich bag with water, drop several shiny pennies inside, seal the bag, and hang it where flies tend to gather.
Common locations include:
- Front porches and exterior doorways
- Restaurant patios and outdoor dining areas
- Backyard decks and picnic tables
- Horse barns and livestock buildings
- Garages, workshops, and outdoor kitchens
- Open windows and covered entrances
The practice has been observed in several countries, particularly in warm climates where flies can be a persistent nuisance.
A scientific review published in the journal Insects in 2021 documented how the original water-bag technique developed variations involving pennies, nickels, different bag shapes, and different hanging positions.
Interestingly, the review described water bags hanging in trees around an outdoor restaurant in Petra, Jordan, as early as 1997.
The continuing popularity of the practice demonstrates how a simple household remedy can spread even without convincing scientific evidence supporting its effectiveness.
Source: A Review of Alternative Controls for House Flies, Insects (2021).
Why Are Shiny Pennies Supposed to Repel Flies?
Several explanations are commonly offered for the homemade fly repellent. Although they sound plausible, none has been conclusively demonstrated as an effective fly-control mechanism.
The Light Refraction Theory
According to the most popular theory, sunlight passes through the transparent plastic bag and the water inside it. As the light moves between air, plastic, and water, it changes direction through a process known as refraction.
Supporters suggest that this altered light produces confusing visual patterns that make flies avoid the area.
Water does refract light. That part is scientifically correct.
However, demonstrating that water bends light is not the same as proving that the resulting optical effects reliably repel flies.
The Compound Eye Theory
House flies have compound eyes made up of numerous visual units called ommatidia. Their visual systems are especially effective at detecting motion and changes in their surroundings.
Some homeowners believe that the reflections and distortions created by a hanging water bag overwhelm the fly’s vision.
While flies may respond to visual stimuli, there is insufficient evidence that the particular reflections created by a plastic bag and pennies provide dependable protection against fly activity.
The Giant Predator Reflection Theory
Another version claims that flies see a distorted or enlarged reflection of themselves in the water bag, mistake that reflection for a larger predator, and immediately leave.
This explanation is popular, but scientific research has not established that house flies interpret reflections in this way.
Do Copper Pennies Produce a Repellent Effect?
Some people believe that copper is responsible for repelling flies.
However, a penny sitting inside a sealed bag does not create a demonstrated protective zone against flying insects. Copper’s antimicrobial properties under certain conditions should not be confused with its ability to repel adult house flies.
There is no convincing controlled evidence that adding pennies makes a water bag an effective house fly repellent.
Scientific Experiments: Do Water Bags and Pennies Actually Repel Flies?
Instead of relying on testimonials, let us examine several experiments and published investigations into this unusual household remedy.
Experiment 1: Brazilian Laboratory Study — Some Repellent Effects Observed
Researchers: A. D. Grützmacher and O. Nakano
Year: 1997
Subject: House flies (Musca domestica)
Research: Behavior of house flies in relation to transparent plastic bags containing water.
In 1997, researchers Grützmacher and Nakano investigated how house flies responded to transparent plastic bags containing water.
This experiment is important because it provides evidence that the idea cannot simply be dismissed without considering the conditions under which flies were tested.
According to a 2021 peer-reviewed scientific review of fly-control methods, the Brazilian study found that:
- Transparent bags containing clear water produced a repellent response in more than 30% of the house flies tested.
- Transparent bags containing yellow-colored water attracted house flies.
- The flies’ responses differed depending on the visual characteristics of the water.
What does this mean?
The laboratory results suggest that visual characteristics, including color and light interaction, may influence fly behavior.
However, a behavioral response under controlled laboratory conditions does not establish that hanging a bag above a restaurant table, doorway, or backyard patio will prevent flies from entering.
The study also does not demonstrate that shiny pennies are necessary or that adding pennies improves the result.
Verdict: Limited evidence of a behavioral effect, but not proof of dependable real-world fly control.
Original study: Grützmacher and Nakano (1997), Anais da Sociedade Entomológica do Brasil.
Experiment 2: North Carolina State University Research — Water Bags Did Not Provide Reliable Control
Researcher: Mike Stringham, associated with North Carolina State University
Research setting: Poultry-related facilities
Subject: House fly activity and water-based optical fly repellents.
This investigation is particularly relevant to Eastern North Carolina because it involved research associated with NC State University.
In a reported 13-week field trial involving egg-farm facilities, Stringham investigated whether commercial water-based optical fly repellents could reduce house fly activity.
Fly activity was assessed using indicators such as fly spots, which are deposits flies leave on surfaces.
Instead of confirming the popular repellent theory, the reported experiment found higher fly activity in areas where water-based optical repellents were installed.
That result challenges the idea that a water bag automatically discourages flies.
There is an important limitation: the experiment was conducted under indoor lighting conditions rather than direct outdoor sunlight. Therefore, it cannot answer every question about whether sunlight might produce a different behavioral response.
The 2021 scientific review also cites Stringham and colleagues’ research presented at professional meetings in 2004 and 2007, noting that the documented field investigations did not convincingly establish water bags as effective repellents.
Verdict: Research did not establish reliable fly repellency and, under some tested conditions, fly activity was higher.
References: Peer-reviewed review of alternative fly controls and Discussion of Stringham’s reported 13-week experiment.
Experiment 3: Texas Barn Fly Experiment — The Penny Trick Failed
Researcher: Christen Harris
Study: A Common Method of Barn Fly Control: Debunked
Research setting: Barn fly-control experiment.
A Texas-based investigation specifically examined the practice of hanging water-filled plastic bags containing pennies in livestock environments.
The study was motivated by the widespread belief among livestock owners that the bags discourage flies through reflections or other optical effects.
The experiment compared fly captures to evaluate whether the presence of the water-bag arrangement was associated with fewer flies.
The reported findings did not demonstrate a correlation between use of the penny-and-water-bag method and decreased numbers of captured flies.
This result is important because barns often contain manure, organic material, moisture, and other conditions attractive to flies.
A remedy that does not remove those attractions may have little impact on the insects’ activity.
Unlike a large, independently replicated field trial, this was a limited investigation, so its findings should be considered alongside other research rather than treated as the final scientific answer.
Verdict: The tested penny-and-water-bag arrangement did not demonstrate a fly-control benefit.
Study: Christen Harris, A Common Method of Barn Fly Control: Debunked.
Experiment 4: MythBusters Tested More Than 5,000 Flies
Program: MythBusters
Episode: Bug Special (2010)
Experiment: Fly vs. Water.
The television program MythBusters also tested the theory that hanging bags of water can repel house flies.
The team constructed an experimental setup containing connected chambers. Flies could move toward two areas containing the same attractive material: rotting meat.
One chamber also contained a hanging bag of water, while the other did not.
More than 5,000 flies were released into the apparatus. After the experiment, the team compared the flies collected from the two test chambers.
According to published summaries of the episode:
- The chamber containing the water bag held approximately 35 grams of flies.
- The chamber without the water bag held approximately 20 grams of flies.
- More fly mass was recovered from the chamber containing the water bag.
Rather than demonstrating a repellent effect, the test produced the opposite of the result predicted by the myth.
The MythBusters team classified the claim as Busted.
However, this was a television demonstration, not a peer-reviewed scientific field study. It tested a particular arrangement involving a water bag and food attractant. It should not be interpreted as proving that every possible water-bag configuration will produce the same outcome.
Verdict: The water bag failed to repel flies in the televised experiment.
Reference: MythBusters 2010 season, Bug Special — Fly vs. Water.
Experiment 5: The 2021 Peer-Reviewed Scientific Review
Publication: Insects
Year: 2021
Article: A Review of Alternative Controls for House Flies.
One of the most useful scientific sources is the 2021 review published in Insects, which examined alternative methods for controlling house flies.
The researchers devoted a specific section to bags of water with and without coins.
They discussed historical use, possible visual mechanisms, laboratory results, and field investigations.
Although certain experiments suggested water bags might affect fly behavior, the review found that research had not established dependable effectiveness as a practical house fly-control method.
The authors also noted that the proposed optical explanations require additional research.
Verdict: There is insufficient scientific evidence to recommend water bags and pennies as a reliable standalone method of controlling house flies.
Read the research: A Review of Alternative Controls for House Flies (2021).
Comparing the Scientific Evidence
| Experiment or Research | What Was Tested | Reported Findings |
|---|---|---|
| Brazilian study, 1997 | Transparent bags containing clear or colored water | Clear water produced a repellent response in more than 30% of tested flies; yellow water attracted flies. |
| NC State-associated research | Water-based optical fly repellents in poultry facilities | No convincing control benefit; one reported field trial found increased fly activity. |
| Texas barn experiment | Water-filled bags with pennies | No demonstrated association with reduced fly captures. |
| MythBusters, 2010 | Water bag versus no water bag with rotting meat attractants | More fly mass was recovered from the water-bag chamber. |
| Scientific review, 2021 | Published evidence on alternative house fly controls | No definitive evidence establishing water bags as dependable fly control. |
Bottom line: A water bag may affect individual fly behavior under certain conditions, but the research does not justify treating it as a proven fly repellent. The evidence is even weaker for the specific claim that shiny pennies improve its effectiveness.
Why Do Some People Swear That Water Bags Work?
Personal observations can be sincere without proving that a treatment caused the improvement.
Suppose someone hangs a bag of water near a porch on Monday and notices fewer flies on Tuesday. It is natural to credit the water bag.
However, several other factors can affect fly activity:
- Temperature and humidity changes
- Rainfall and wind
- Changes in sunlight or shade
- Nearby garbage collection
- Removal of animal waste or food scraps
- Changes in nearby breeding conditions
- Normal fluctuations in fly populations
- Different times of day
A single observation cannot reliably separate the effect of the water bag from these other influences.
In scientific research, a control group is used to help determine whether an observed change is actually caused by the treatment being tested.
This is the difference between saying, “There were fewer flies after I hung the bag,” and establishing that “The bag caused fewer flies to appear.”
Can You Test the Penny and Water Bag Theory Yourself?
Homeowners in Eastern North Carolina can conduct a simple observation-based experiment to see whether the trick appears to make a difference on their property.
This experiment will not replace a controlled scientific study, but it can demonstrate why comparing conditions matters.
A Simple Backyard Fly Repellent Experiment
- Choose two similar outdoor locations. Select areas with comparable shade, airflow, and fly activity.
- Prepare the water bag. Fill a clear bag approximately halfway with clean water and add two or three pennies.
- Hang the bag in one area. Leave the other location without a water bag.
- Observe flies for equal periods. During a fixed observation period, count fly landings on comparable clean surfaces.
- Repeat over multiple days. Keep the observation schedule consistent and record the weather.
- Switch the bag between locations. Rotating the treatment helps reduce the possibility that one location naturally attracts more flies.
- Compare the results. Determine whether fly activity was consistently lower near the bag rather than relying on a single observation.
For a more useful comparison, test a water bag with pennies against a similar water bag without pennies on different occasions. This helps isolate whether the coins themselves appear to matter.
Do not use exposed meat, garbage, or other unsanitary attractants to conduct the experiment. They may attract additional flies or introduce unnecessary contamination.
Even if one backyard trial shows fewer flies, that does not prove the technique will perform consistently at other homes or commercial properties.
What Actually Works to Keep Flies Away?
University Extension specialists consistently emphasize removing breeding material and preventing fly entry rather than relying on unsupported household tricks.
The University of Minnesota Extension recommends sanitation, securely closed garbage containers, removal of decomposing organic material, window and door screening, and physical trapping as practical fly-management strategies.
Mississippi State University Extension also specifically discusses the water-bag-and-penny myth and advises homeowners to focus on exclusion and cleaning.
Source: University of Minnesota Extension — Flies and Mississippi State University Extension — How to Control Flies in Your Home.
1. Remove Fly Breeding Sources
House flies develop in decaying organic material, including some forms of manure, garbage, and decomposing waste.
Reducing access to breeding material is one of the most important steps in preventing repeated infestations.
Homeowners should:
- Remove pet waste from lawns and dog runs regularly.
- Keep garbage inside tightly closed bags and containers.
- Wash trash cans when food residue accumulates.
- Remove spoiled produce and fallen fruit.
- Clean food spills promptly.
- Check outdoor waste-storage areas for decomposing material.
- Keep compost properly managed and avoid exposing food waste to flies.
Simply killing adult flies does not eliminate an infestation if larvae continue developing nearby.
2. Install and Maintain Window and Door Screens
Flies commonly enter homes through open doors, damaged window screens, and gaps around exterior openings.
Repair torn screening, replace damaged door sweeps, and ensure exterior doors close properly.
For businesses with frequent customer traffic, well-maintained self-closing doors and appropriately installed air curtains can help reduce fly entry.
3. Use Fans Around Outdoor Seating Areas
Moving air can make it more difficult for many flies to approach or settle in a particular location.
A properly positioned outdoor fan may provide more practical short-term protection for a patio or porch than a hanging plastic bag.
Fans do not eliminate the source of a fly infestation, but they can be a useful comfort measure when paired with sanitation.
4. Use Fly Traps Correctly
Different fly species respond to different attractants and trapping systems.
Depending on the location and type of fly, control devices may include:
- Sticky flypaper
- Outdoor baited fly traps
- Indoor insect light traps
- Window-mounted fly traps
- Species-specific monitoring traps
Outdoor baited fly traps should generally be positioned away from doors and outdoor seating areas so they do not attract additional flies toward people.
In food-service environments, insect light traps require appropriate placement to minimize contamination concerns. They should not be installed directly above exposed food or food-preparation surfaces.
5. Identify the Type of Fly
Not every small flying insect found inside a house is a common house fly.
Different species have different breeding sources, which means the proper treatment depends on identification.
- House flies: Often associated with manure, garbage, and decomposing organic waste.
- Fruit flies: Frequently associated with fermenting produce, spilled beverages, and food residue.
- Drain flies: Commonly develop in organic film inside drains and other persistently wet areas.
- Phorid flies: May originate in decaying organic matter, drains, or concealed moisture problems.
- Blow flies: May be associated with carcasses, garbage, or other decomposing animal material.
For example, hanging a water bag above a kitchen sink will not remove the organic film supporting drain fly larvae.
For detailed guidance, see our article: How to Stop Drain Flies in Your Home for Good.
Fly Problems in Eastern North Carolina: Why Local Conditions Matter
Eastern North Carolina’s warm weather, seasonal humidity, agriculture, residential development, and outdoor dining activity can contribute to recurring fly problems.
However, humidity alone does not necessarily mean house flies will reproduce. They require appropriate breeding material, and the type of material varies by species.
Properties around Greenville, Kinston, Goldsboro, New Bern, and Jacksonville can experience fly activity for different reasons.
Residential Homes and Backyard Patios
Garbage containers, pet waste, outdoor food preparation, and nearby decomposing material can attract flies.
Homeowners should start by examining where flies gather and investigating whether the source is on their own property or nearby.
Farms and Rural Properties
Livestock manure, animal-feed residue, and accumulated organic waste can support house fly development.
Proper manure management, drainage, waste handling, and monitoring are important elements of agricultural fly control.
A hanging bag of water cannot substitute for these practices.
Rental Properties and Apartment Complexes
Shared dumpsters, poorly sealed trash containers, damaged screens, and common-area sanitation problems may contribute to recurring fly complaints.
Property managers should inspect waste-storage locations and maintain sanitation procedures rather than relying solely on fly traps or repellents.
Restaurants and Commercial Kitchens
Restaurants face particular challenges because food waste, dumpsters, floor drains, deliveries, and frequently opened doors can provide opportunities for flies.
Outdoor dining areas may experience seasonal fly pressure even when indoor sanitation is maintained.
A hanging water bag should never replace a documented fly-prevention and sanitation program.
Learn more about our commercial pest control services in Eastern North Carolina.
Can a Bag of Water Actually Make a Fly Problem Worse?
There is no proof that every hanging water bag attracts flies. However, certain experiments have produced increased activity around water-based optical repellents.
A water bag also leaves the original fly attractants untouched.
If the property has overflowing garbage, exposed manure, decaying produce, or a contaminated drain, the problem may continue regardless of whether a bag is hanging nearby.
The greatest concern is not necessarily that a bag of water will cause an infestation. It is that homeowners or business owners may develop a false sense of protection while overlooking actual breeding sources.
For restaurants, rental properties, and homes with substantial fly activity, identifying and correcting the source is more important than experimenting with unverified repellents.
When Should You Contact a Professional Pest Control Company?
Seeing an occasional fly does not necessarily require professional treatment. However, recurring or unusually high numbers may indicate a breeding source that needs investigation.
Consider professional assistance when:
- Flies repeatedly appear indoors despite regular cleaning.
- Large numbers gather around windows, doors, or trash-storage areas.
- Flies return shortly after traps or sprays are used.
- Small flies continually emerge around drains or plumbing fixtures.
- Restaurant fly activity creates sanitation concerns.
- A hidden animal carcass or concealed organic material may be involved.
- Fly activity is affecting customers, tenants, or commercial operations.
A professional evaluation may help identify the fly species, locate breeding material, evaluate entry points, and determine whether targeted control is appropriate.
D & D Pest Control Co. provides residential and commercial pest control services throughout Eastern North Carolina.
Frequently Asked Questions About Pennies, Water Bags, and Fly Repellents
Do shiny pennies really keep flies away?
There is no convincing scientific evidence that shiny pennies reliably repel house flies. The belief is usually connected to reflections and light refraction, but researchers have not established that adding pennies to a bag of water produces effective fly control. Removing breeding sources and blocking entry points are more dependable prevention strategies.
Does hanging a plastic bag filled with water repel flies?
Research has produced mixed results. One laboratory study reported a repellent response from some house flies exposed to bags containing clear water. Other experiments failed to demonstrate reliable protection. A 2021 scientific review concluded that the effectiveness of water bags as a practical fly-control method remains unproven.
Why do people put pennies in bags of water?
The traditional explanation is that shiny coins reflect light through the water and confuse flies’ compound eyes. Other versions suggest that reflected images resemble predators. These explanations are theories rather than established mechanisms. There is no strong evidence that pennies improve a water bag’s fly-repellent effectiveness.
How many pennies should you put in a bag of water to repel flies?
Some versions of the folk remedy recommend two or three pennies, while others suggest more coins. However, no scientifically established number of pennies has been shown to reliably repel flies. Changing the number of coins does not address the underlying lack of evidence for the method.
Do copper pennies repel flies better than modern pennies?
There is no reliable comparative research demonstrating that older copper-rich pennies repel flies more effectively than modern copper-plated pennies. Claims involving copper’s antimicrobial activity do not establish insect repellency. A coin’s metal composition should not be treated as a proven fly-control method.
Does sunlight make water bags more effective against flies?
Sunlight can change the reflections and refraction produced by a water-filled bag. However, the scientific evidence does not establish that direct sunlight consistently makes these bags effective fly repellents. Certain studies used indoor lighting, so additional controlled outdoor research would be needed to evaluate this possibility.
Did MythBusters prove that water bags do not repel flies?
In the 2010 Bug Special, MythBusters tested a hanging water bag using an experimental apparatus involving more than 5,000 flies and rotting meat. More fly mass was recovered from the chamber containing the water bag. The team declared the myth busted. This strongly challenged the popular claim under those conditions, although it was not a comprehensive scientific field study.
Will hanging water bags keep flies away from restaurant patios?
There is insufficient scientific evidence to rely on water bags as protection for outdoor dining areas. Restaurants should prioritize covered waste containers, prompt spill cleanup, regular sanitation, properly maintained doors, and appropriate fly-monitoring systems. Fly-control plans should also consider local environmental conditions and food-safety requirements.
What is the best natural way to keep flies away without spraying pesticides?
Reducing fly attractants is one of the most dependable pesticide-free strategies. Remove animal waste, clean garbage containers, dispose of decomposing food, maintain screens, and use physical trapping where appropriate. Fans may also help discourage flies from settling in outdoor seating areas. The best combination depends on the fly species and the source of the problem.
Why do flies keep returning after I kill them?
Repeated fly activity may mean new adults are emerging from breeding material nearby or additional flies are entering from outdoors. Swatting or trapping visible adults only addresses part of the problem. Identifying the fly species, locating breeding sources, and correcting sanitation or exclusion problems are essential for longer-lasting control.
Final Verdict: Shiny Penny and Water Bag Fly Repellent — Mostly Myth, Not Proven Fact
So, does a shiny penny or a bag of water keep flies away?
The scientific evidence does not support relying on the method for dependable fly control.
One laboratory study suggests that water bags may influence house fly behavior under specific conditions. However, research associated with NC State University, a Texas barn experiment, and the well-known MythBusters demonstration failed to establish the practical protection claimed by supporters of the remedy.
Most importantly, none of these findings provides convincing evidence that adding shiny pennies creates a reliable fly-repellent barrier.
For homeowners in Eastern North Carolina, the better strategy is simple: eliminate what attracts flies, prevent them from entering, and address breeding sites before another generation develops.
A few pennies might be inexpensive, but sanitation and prevention are far more valuable investments.
Professional Fly Control in Eastern North Carolina
If flies are becoming a persistent problem around your home, restaurant, rental property, or commercial building, D & D Pest Control Co. can help you evaluate the situation and identify appropriate control options.
Serving Eastern North Carolina since 1994, D & D Pest Control Co. offers residential and commercial pest control services, including assistance with recurring fly problems.
Our service area includes Kinston, Greenville, Goldsboro, New Bern, Jacksonville, Snow Hill, Trenton, Ayden, Winterville, and surrounding communities.
Call D & D Pest Control Co. at (252) 523-8255 to discuss your fly-control concerns.
Visit www.ddpestcontrol.com to learn more about our services.
NC Structural Pest Control License #1274 PW
Research References and Scientific Sources
- A Review of Alternative Controls for House Flies. Insects, 2021, 12(11), 1042. Reviews the scientific literature concerning water bags, coins, and other alternative fly-control approaches.
Read the peer-reviewed article. - Grützmacher, A. D., and Nakano, O. (1997). Behavior of house fly, Musca domestica L., in relation to transparent plastic bags with water. Anais da Sociedade Entomológica do Brasil, 26, 455–461.
View the original laboratory study. - Stringham, S. M., Watson, D. W., Parsons, J., Carver, D. K., and Wineland, M. (2004). Suppression of house flies in egg-handling rooms using Fly Fright bags as an optical repellent. Conference proceedings referenced in the 2021 review.
Review containing the research citation. - Stringham, M. (2007). Water Bags: Optical Fly Repellent? North Carolina Mosquito and Vector Control Annual Meeting. Conference research cited in the 2021 scientific review.
Review containing the conference reference. - Harris, Christen. A Common Method of Barn Fly Control: Debunked. Texas-based investigation examining penny-filled water bags and captured fly numbers.
View the reported study. - MythBusters (2010). Bug Special — Fly vs. Water. Televised experimental demonstration using more than 5,000 flies.
View the episode summary. - University of Minnesota Extension. Flies — Identification, prevention, sanitation, and control.
Read Extension recommendations. - Mississippi State University Extension. How to Control Flies in Your Home — Practical fly prevention and discussion of ineffective home remedies.
Read the Extension article.
