] We had the same problem with our older mounts. We contacted our taxidermist and he had the solution that worked GREAT!
Get a can or Raid House & Garden. Spray the mounts lightly (like you would hairspray). Then take a damp cloth and rub the fur lightly in the same direction the hair grows. The area around the eyes and mouth are the most vulnerable so spray some of the House & Garden directly on the cloth and dab it on those areas.
After it has dried (several hours) lightly brush the fur to soften it, if it looks any different than it did before you sprayed.
An insect bomb isn't a bad idea either but the mounts will still need to be wiped with a damp cloth since the bombs leave a residue.
If that does not solve the problem (and it did on our 30 plus mounts), send me a PM and I can send you to a link that is a professional that "injects" the mounts.
I do know that the insects get on birds first and then migrate to the mammals.
Good luck and I hope this helps!