What I mean here is, filling a ziplock baggie with water and hanging it up on the patio to deter houseflies.
This is crazy, but I know a lot of people who swear by them. Do they work? Why on earth would they?
I don’t see how it could hurt.
I notice there’s a lot of stuff on the Internet, most of which says stuff like “This really works, no one knows why.” Even SNOPES doesn’t know why (or if) they work.
This is a greater mystery than whether the plane can take off, and more useful, too.
Suppose there were a housefly on a treadmill . . .
Mythbusters busted this. Their results matched the research mentioned in snopes: if anything, the flies were attracted to the bags.
perhaps sugar water? my grandfather used to hang a gallon jug in a tree about 1/2 full with some kind of trap “door” in the neck that let them in but not out(don’t ask details because I don’t know). Worked well for yellow jackets, hornets, and wasps as well as flys. As I said, not really sure of the mechanics because I only saw it from ground level and it was always hanging 10-12 up in the tree
aaand then I saw tellyworth’s post and followed the link…:smack:
A few years ago, a checkpoint in Baghdad had plastic bags hanging up that were filled with flies. I don’t know what was in them (it looked like water).
But those suckers were lining up to get in those bags.
It might seem reasonable that flies, like any life, would be drawn to water in a water-scarce enviroment. But Baghdad, while it ain’t Seattle, isn’t a desert.
Never noticed if there were coins or other items in the bags.
I found it slow going at first, but as the bodycount increased flies were attracted to the fly corpses, the corpses formed an island of death, and nothing was drowning anymore. By the end of summer I had generations of flies that had never been out of that bag. About two pounds of flies, total–the wall connection couldn’t take the weight and broke.
I would say that you should bait the water with something dead for faster results. Pennies wouldn’t work.
Several years ago I had an unexplained fly invasion in my back yard…to this dsay I have no idea what attracted them but I had a swirling vortex of flies on my back deck every day. It was gross.
At one point, I bought several of those bags that claim to attract and kill flies. They worked, sort of, in that they killed lots and lots of flies. However, they also attracted flies - so I was luring hordes of flies into my yard, then killing them. The luring and killing parts sort of cancelled each other out, I think.
Mythbusters did NOT bust this, as they did not set it up like any normal person’s patio/deck etc. Of course Mythbusters has never set up anything the way real people would do it, Mythbusters is entertainment not science, and I’m not trusting Mythbusters.
Of course, I’m also not formulating any experiments of my own.
Except: last night, before barbecuing, we sprayed Raid, and yet we swatted seven flies and three or four more got away.
Today we have a hanging bag with water in it, and a penny. (God, the Internet. Some people say just one penny. Others say you have to have three. My original source said nothing about a penny–he, by the way, owned a restaurant with outdoor dining.) I will update with the body count. Maybe. If I happen to feel like it.
But…I don’t want an island of death on my patio. I don’t want two pounds of flies. I want NO flies. Did my OP not say “deter”? Does “deter” mean something I’m unaware of?
“Repel” is probably a better term. I mean repelling the flies, not my dinner guests (and no, the flies are not my dinner guests).
But it was SO COOL! Guests who cannot appreciate its coolness were invited by your spouse. Not my wife, because she also thought it was cool. “Ooh, there are little maggots feasting on the dead flies.”
I vote for myth here. I have never seen evidence (in restaurants, or anyplace else) that movement makes flies nervous (the argument in the SD column is that bags of water supposedly creat a sense of movement that repels flies). Sudden swatting motions, sure - but vague reflections and slow movements aren’t going to repel flies when they sense tasty food/humans/pets/waste nearby.
This is like lots of folklore - in the absence of controlled experiments there are always going to be circumstances where the cure seems to work merely by chance or undetected factors (maybe the flies decided that the pickings at the water bag-protected patio didn’t compare to the potential feast down the street where people were less rigorous about wrapping up and discarding their trash).
Party at dropzone’s!!!
Sorry, that was during the Big Fly Outbreak of 2003. If you wanted a fly party I needed to be warned a month ago to prepare for it. Not even the yellow-jackets are anything like they were in 2009. 
ETA: I’m allergic to their venom so sitting on the car port spraying them as they came home for the night had a special edge.
ETAA: I won.
Holy shit, the bags actually work!
And you can’t get the straight dope anywhere except the Straight Dope.
(I searched for this; I was sure it was in here somewhere. Thanks!)
Yes they do work. I don’t really know why but it does help. We had a bad fly problem on our side patio. To the point you couldn’t stay out there a couple of mintes without being swarmed with flies. I had seen the water bags used at a local cafe/bar and was skeptical about it but thought it was worth a try. I hung 3 1-gallon ziplock bags of water from the rafters along the outside edge of the patio and by the next day there were very few flies. Now out in the yard (where they were also bad) they are still there, but within 10 foot or so of the patio, there are hardly any.
I have noticed in the late afternoon (patio is on west side of house) when there is a slight breeze causing the bags to twist and sway, the sun hitting them makes it look like we have a slow moving disco ball going. Maybe its the water acting like a prism, causing moving spots and flashes that scares the flies away. Maybe a wind chime made of mirrors would do the same trick?
Also, I have never heard or seen the thing of putting pennies in the bag. Not sure what that would do, other than, I did put a marble sized rock in the corner of the bag to have something to wrap and tie the string around…maybe the first person to do this used pennies and got something started??
All I know is that if you bring a bag of water to the airport, it will probably work as an anti-fly there. 
I am going to try this on my back porch. I get a vortex of flies there, and since the four dogs do an awful lot of coming and going I often get flies in the house. Two things I’ve been doing that half-assed help: spritz the back door and screen door with vinegar, and keep a fan going in the kitchen - seems flies don’t like any sort of breeze.
One-gallon ziplock bags, check. Do you hang them with clothesline pegs, or what?
I’m guessing the point of ther pennies is to add to the reflective value of the bags.
FYI, I’ve used the following bags from Home Depot in my backyard for years. It’s fundamentally identical to hanging a ziploc bag, albeit a bit more refined / easier to clean up.
They sure as heck work-- it’s a housefly holocaust in a bag!-- although I too was ignorant of how.
The ones in your link are fly traps, the flies go into the bag and die, filling up the bag. The kind I use and I think the OP was talking about are sealed bags filled with water. They don’t trap the flies, they just chase them away. The bags I have up have been hanging for 2 years now, even went through freezing solid in the winter.