I've been surprised and heartened at how tolerant Theoden is of insects when hacking out. I cover him in Lincoln's Fly Repellent and Fly Stuff around various susceptible regions of his body, help him get rid of any I can reach but otherwise he contains himself well and just becomes a little introspective, flopping his ears down like a dressage horse. Considering that this is a horse that you used to blame me if he even so much as tripped, this is progress!
I'm not surprised that some horses are anxious about being sprayed or wiped with insect repellant or washed down with the hose. At times like this, horses are even more sensory defensive than usual and suspect that everything that touches them is a creepy crawly. They react first and think later and sometimes it takes a little while to persuade them that something is friend not foe.