Re: Why people use plastic bags filled of water to frighten the insects

Dear Carolina,

Thank you for your question about flies. I have tried to locate scientific research into the use of water-filled bags as fly-repellants, but have been unsuccessful. I’ve found a lot of people who believe that it works, but no scientific evidence. I even found a website with a picture of a man hanging a bag of water to repel flies. It seems that using bags of water for discouraging flies is particular to Central and South America, as I have only been able to find people familiar with this idea from those places.

I spoke with two scientists born in South America that work with me in the department of Tropical Medicine here at Tulane University in New Orleans. I had never heard of using bags of water to scare flies, but they were very familiar with this practice. One of the women thought that the bags worked, while the other thought that they might not work. In the end, it seems that no one has yet done the experiments to give us the answer.

The experiment would be pretty easy to do. You would need some sticky flytraps, bags of water, and nails to hang them up. You should find at least six houses with many flies. Randomly, you should divide the houses into two groups. In one of the groups, place a bag of water in one corner of the doorframe, and in the other corner place a flytrap. In the second group of houses, only place a flytrap in one corner of the doorframe, and do not put anything in the other corner.

Wait one week, and then count the number of flies in each of the flytraps. Did you catch more flies in the houses without bags of water? Is the difference large? Scientists love to repeat experiments over and over again. If the results from the experiment are the same many times, then it becomes accepted at scientific evidence.

If bags of water do scare flies away, then there could be many reasons for it. The most common guess that I have read is that the flies might mistake the bag of water for a wasp nest. This point of view is stated by an unnamed Alabama pest control technician. Wasps hunt other insects for food, and so it makes sense that a fly would want to avoid a wasp nest. I have not seen any evidence that flies really do know to avoid things that might be wasp nests, but it could be true. In the end, this is only guessing because no one has done the experiments to make sure that that is the case.

Flies are very interesting insects for many reasons. One reason is that there are just so many kinds of flies, some of which are very beautiful. Look at the USDA’s fly website to see a few of the beautiful flies that live in this world. Make sure you see the picture of the Bat Fly. My best friend, who loves to go insect collecting with me, found a Bat Fly on my head when we were in the Virgin Islands. I still have no idea how it got there.

If you have any more questions, please send them to the MAD Scientist Network!


Sincerely yours,


John Carlson MAD Entomologist