Do you mean just mosquitoes and flies? Or ticks? If it’s flies and mosquitoes then you just have to let them go away, but I would try using a fly spray to keep them off. If it’s ticks or another bug that attaches itself then a) take them all off and b) ask your vet if they have a medicine or repellent for that specific bug. If your horse is getting hives from the flies (some horses at my barn do) then I suggest using a fly sheet and fly spray. Hope this helps!