I would not have believed this water trick would repel houseflies if I didn’t see it happening (or not happening, depending on your point of view). Tested with transparent plastic bags, it may or may not work with transparent water bottles. Tested outdoors, it may or may not work indoors.
If you place them around the perimeter of your yard, your ordinary houseflies will avoid your yard. I don’t know what the science is behind it. Again, I wouldn’t have believed it would work until I saw what was happening with my own eyes.
Josie and one of her aunts saw this water trick being used at an outdoor restaurant when they were on vacation on some islands other than Luzon in the Philippines in 2017. She asked one of the guests about it, and he told her he saw it while on a Caribbean cruise and forgot about it. Josie kept it in mind for an upcoming gathering.
The large gathering took place in our backyard in April 2017. Josie had a bunch of people put the bags of water along the backyard fence. Two sides were completely covered and part of a third one was covered. The only parts that weren’t covered were the backside of the house and the opening where people could get into the backyard. The only place the houseflies came in was at that opening.
There’s a pig slaughterhouse near enough to our house that we can hear the pigs squealing at night when it’s otherwise quiet. It’s rarely quiet enough with all the dogs barking, cats fighting and roosters crowing in the neighborhood.
I’m sure that’s where most of the flies come from. The fence was a barrier for houseflies coming from that direction, at least with this water trick. A lot of people were in the backyard that day, eating and playing cards. The water trick works and you can take my word for it. When we had large gatherings like this before, we were constantly shooing away houseflies (and other insects). That day, there wasn’t enough of any pests to bother anyone.
Edited and updated. Originally published at one of my other websites in April 2017.