I have used these jugs before and yes they will fill up with flies, but from what I have seen, the flies it fills up with mostly are the ones with the iridescent bodies, and those aren't the ones that pester the deer. The ones that pester the deer are smaller and bite the deer, and us as well.
Now I'm not saying they aren't worth using or that they don't catch at least some of those biters as well. And I'm not suggesting the iridescent flys are ''ok'' to have around. I'm saying those aren't the ones that blacken our deer's legs and make our deer miserable through their relentless biting...biting to the point of drawing blood.
I've used Golden Malrin around the areas where grain has spilled and gotten wet, or have put Golden Malrin on what was left of a fish I just filleted, and my driveway flatout gets covered with dead flys of all kinds, and lots of gnats fall prey to it as well. Yes one must be VERY careful where to place the Golden Malrin as it will kill non-target critters quickly.
Now...to disclaim...I'm NOT saying here my Golden Maldrin is the way to go over the jugs. I'm saying it is something else to do for fly and gnat control, and maybe using both could help our poor deer deal with them biters that make life miserable for our deer in the summer. It makes my life miserable just watching them!
I will say this - A completely closed in shed with no windows eliminates the fly problem. I've watched my deer being literally chased by a cloud of flies, run to my shed, and as they enter the doorway the flies act like there is a barrier there and do not enter with them. So, the deer go in lay down in complete comfort, and don't have to race around all day trying to escape the relentlessness of those biters.
Why do the flys not enter? I don't really know except I believe it has something to do with the lack of light...it almost has to. Like I said...a completely closed in shed with no windows, yet with some top ventilation.