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I was in Nicaragua recently and noted many restaurants and convenience stores had small sandwich sized bags filled with water hanging at their entrances. When I asked why, they said it was to repel mosquitoes.

This seemed to be a widely held belief even extending to other countries in Central America.

I have not found a reliable source claiming that bags filled with water can repel mosquitoes, but I hope that these sites can help to corroborate what I personally noted.

  1. Plastic bags of water as mosquito repellent - JREF Forum
  2. Banish mosquitoes naturally on the cheap - DIY Life
  3. That hanging plastic bags filled with water will repel Mosquitoes

I am aware that the more common claim seems to be about flies and that this was busted on an episode of MythBusters. It is the claim that bags of water can repel mosquitoes which interests me however, given how common it was in some parts of Central America.

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mosquitos lay their eggs into water. there are only two reasons why hanging a plastic bag full of water helps to repel them. either they are totaly pissed, that they cant reach the water through the plastic and leave the area or they cannot imagine how water is able to fly and look for better places where the laws of physics apply. on a more serious note I either think this might have something to do with reflection of light... or is total BS –  Baarn Jan 22 '12 at 19:48
    
@WalterMaier-Murdnelch Is any of this substantiated or only your opinion? Please do not post answers in this comment section. Comments are reserved for inquiries/suggestions to help improve the post. Thanks. –  Robert Cartaino Jan 23 '12 at 14:39
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Assuming the Straight Dope can be considered a valid resource, here's an article about it, though they say it's for deterring flies, not mosquitoes. straightdope.com/columns/read/1956/… Note, the article itself doesn't quotes sources, that's why I posted this as a comment, not an answer. –  Dave Lancea Jan 23 '12 at 17:41
    
@RobertCartaino Please do not post nags in this comment section. Comments are reserved for inquiries/suggestions to help improve the post. Thanks. –  isJustMe Jan 24 '12 at 18:51
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@Rafael.IT What you call "nags" is providing guidance so folks can learn how to properly use this site. That's pretty much my job. –  Robert Cartaino Jan 24 '12 at 20:49

2 Answers 2

The sources that I found for this were in regards to this method being used to repel flies, and not specifically mosquitos, but at least one source mentioned that most claims of the theoretical effectiveness were based off of the interaction of the light refraction with insects with compound eyes. As mosquitos also have compound eyes, it seems feasible that the method would be just as effective (or ineffective) on mosquitos as the houseflies and biting flies that are more commonly referenced in conjunction with this method.

Most proponents of this theory agree that light refraction is the key:

In theory, refraction can be just as confusing for some species of insect, especially the housefly. It boasts a highly sensitive array of eyes which allow it to see in multiple directions at once.

The insect's head mostly consists of a pair of large complex eyes, each of which is composed of 3,000 to 6,000 simple eyes. These eyes can't move or focus on objects like human eyes, but they provide the fly with a mosaic view of the world around them. Each simple eye provides one small piece of the puzzle, much like the way a screen's pixel delivers one detail of the larger picture.

A housefly bases its sense of direction on the direction sunlight comes from. Some entomologists believe that when these complex, sensitive eyes experience refracted light, the insect becomes confused and flies away.

Does this actually work, though?

Perhaps not.

According to several sources (the howstuffworks.com article, this gardensalive.com article, and snopes.com), in 2007 a researcher named Mike Stringham ran a 13 week study on the effectiveness of this method at one or two egg farms (the accounts differ on the details of the study, which causes me some concern as to how accurately the study's results are reported).

Fly populations at each location were measured by placing cards that allowed the capture and measure of fly droppings. Some cards were placed in control rooms, others in rooms with bags of water.

The results purportedly showed that there was an actual increase in fly population in the rooms with the water bags.

However, snopes.com labeled this claim as "undetermined", due to the fact thtat Stringham's experiment was conducted in an environment that was "indoors with flourescent and incandescent lighting, a factor which may limit its applicability: it's possible the effects of direct sunlight on the bags may produce different results".

The popular show MythBusters also tested this:

Bags of water hung from the ceiling can repel flies.

BUSTED

This myth is based in the theory that refracted light in water confused flies’ compound eyes.

The Build Team made a rig consisting of three chambers separated by trap doors. The first chamber would hold the flies, the second would hold some rotten meat, and the third would hold both rotten meat and a bag of water. They then released over 5,000 flies from the first chamber and waited to see how many flies would go into each of the other two. After the chambers were sealed off, they let all the flies die and collected the corpses to weigh for comparison. The chambers with and without the water contained 35 and 20 grams of flies, respectively, busting the myth.

I haven't seen the MythBusters episode, but this preview does show a quick scene where Kari Byron appears to be lifting a bag of rotting meat out of the fly enclosure, which is outdoors.

So it seems unlikely that this is effective. However, many of the versions of this claim include mention of a few pennies in the water, so, as this article points out, it doesn't appear to be fully tested.

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Here's an article from the Straight Dope (assuming they can be considered a valid resource), They say it's for deterring flies, not mosquitoes. Here's their explanation on how it works:

The water bag acts a bit like a lens--try it some time--in which the movements of people in the area are reflected. Even if the fly is too far from the action to see it directly, it can see a shifting of light and dark in the water bag, which it interprets as nearby movement, and it will fly away from the bag.

(Note, the article itself doesn't quote sources.)

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I originally posted this as a comment, but since no one else answered it I reposted as an answer. –  Dave Lancea Feb 6 '12 at 19:24
    
If true, then that is a cool explanation. –  Jivlain Feb 7 '12 at 2:10

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