Hi!
I know this is an older thread but it actually came up in a Google search when I put in “when bugs get under fly sheets”
I got my horse an expensive fly sheet, which seems like nice quality and I noticed it was protecting her against flies and smaller flies.
I was watching her graze and yikes! Some bee or stinging insect went under her sheet in the loose area. My poor horse kept kicking at it and swishing her tail. The bugger was caught in her fly sheet!!! An expensive fly sheet caught a stinging insect, yikes. Fortunately I was there so I lifted up the back part of the sheet and it flew out. I think she got stung or at least that bug was definitely aggravating her.
I’m re-thinking this.