The further into the bot egg season we get, the stickier the "glue" gets (that holds the eggs on the hair) and therefore the itchier, so the horse bites to make the nasty little creatures stop itching.
Laying a paper towel with WARM water on top of the eggs for only a few seconds will help make removal easier.
Warm water makes the eggs think the horse is licking or biting and they will release themselves. Rain is cold, that's why it never washes the eggs off.
I scraped what bot eggs I could see off the horses when I brought them in last night --- and that wasn't very many under the false lights. It took me nearly two hours to get every d*** egg off four horses this morning. Needless-to-say, nobody got brushed, just scraped half skinless.
Also, I am seeing a lot of throat bot eggs this year. Those are different bot flies that lay their eggs deep inside the hair under the jowels, just behind the chin.
Sometimes you have to pull the hair backward fairly tight to see them and they aren't easy to get with the bot knife.
These bot eggs migrate themselves into the mouth
If I can't get all of them removed, I put some kind of ointment (not cream but ointment) to smother them. Generally, at the next feeding I can grab onto them with a paper towel and wipe them off the hair.
Ointment can be anything from hemerhoid ointment to whatever antibiotic ointment is sitting around; it's thick enough the eggs can't move and won't harm the horse.
I knew when tick season was below normal, the bot flies would be hard at it