Why does some taxidermy get eaten by insects?
Insects and taxidermy have a very complicated relationship. In some ways, a piece of taxidermy can be compared to a fur coat or wool sweater, except instead of hanging in your closet, the piece is on display. Just as your natural fiber based garments are susceptible to moths and carpet beetles, the same can apply to taxidermy-especially for pieces that are of unverified provenance, or haven’t been consistently cared for (such as vintage or garage sale pieces). The use of arsenic-based solutions in commercial tanning processes has been banned for decades due to environmental and health concerns, but it also meant the only permanent (or at least super effective) bug proofer is no longer in wide use. Since the ban took place in stages (so commercial operations were able to transition out of the old toxic products and into the new without losing too much), it can be a bit difficult to determine whether or not a piece has been treated with arsenic or not (so you should always handle old pieces with care!)
Despite all this, a good professional taxidermist takes many important steps to protect their work from insect damage-including thorough fleshing, cleaning, degreasing, and proper tanning/preservation. There are absolutely no shortcuts or substitutes for these steps. Many choose to add a bug proofer to their mounts, but none of these bug proofers are permanent-so its is important a customer properly stores and maintains their pieces. Store in a cool, relatively dry place away from direct sunlight, and regularly clean/dust your pieces, and make sure to keep your home clean too, sealing up and cracks that insects may use to enter. Pieces can also be kept in a glass display dome (some even hide a moth ball in the habitat) or display cabinet. Pheromone baited moth traps are also helpful, as they can alert you to a moth problem and can eventually slow the reproduction of larva (as its the immature larva that eat the fur, not the flying adults).
Insect damaged pieces can be cleaned and treated, but since techniques and preparations have varied so much, each piece should be professionally evaluated on a case by case basis.