The bot fly knives really do work. I think many folks don't realize they are ONE sided
If they are turned the wrong way against the horse, they aren't capable of getting a speck of dirt off.
The best way I have found is to soak only the area you are immediately working on in WARM water. Wait just a couple seconds, then use the bot knife and the eggs come right off.
The logic behind the warm water is that it tricks the eggs into thinking the horse is licking them, so they loosen up.
If you wait until the moisture cools down the eggs won't come off. If that was the case, those of us that live where there is a lot of rain would never have bot eggs on our horses