Are you referring to this project: https://www.hometalk.com/43424486/keep-mosquitoes-away-with-this-diy-spray?expand_all_questions=1?
Try posting on the project page so the poster can see it and respond.
the beer does not have to be stale
I believe the recipe is: 3 cups of Epson Salts, 3 cans of stale beer, and a large bottle of Blue mouthwash. (Any brand will work. It just has to be blue) The beer should be stale. It's something about the yeast content. Pour these three ingredients in an extra-large pitcher to mix until the Epson Salts dissolve. Then put it in spray bottles and spray everywhere in your yard. As long as it doesn't rain, it will continue to work quite awhile.