Dr. Dan what do I give a horse that is hypersensitive to insect bites, she has been on bug check for years, but still scratches herself raw!
Increase the Bug Check amount (double even since any amount is only healthy and all situations are different).
Also get her off all commercial feed. They are just so full off hydrogenated fats, and other "junk" likely to cause allergies. Bugs only trigger itching (allergies).
Try to follow the feeding advise at http://WhatToFeedYourHorse
In addition I would add the Aller Check supplement to the feeding advise. It will support the immune system to help overcome the allergies… of course there are other potential issues but you just wouldn’t believe how many of the above problems I see.