[IceHorses] help with bug bites

[Bia]

Hi all, I have a problem with one of my horses being very sensitive to the 
bites from different insects here... I'm in North carolina. Does anyone have 
this problem? My mare is very sensitive and gets welts from just about 
anything. I have been giving her a shot every 2 months which does work but I 
am wondering if anyone has success with "summer sheets" ? Do they actually 
keep biting bugs away or are they just to protect from the sun?...