Even if you have squirrels, my money is on the mice. They love insulation, and I've found them in houses, dead in the walls, burned to a crisp because they were eating through live wires. The mothball idea is good, and I've seen it work.
On my last bike restoration - 1998 Yamaha Virago - the bike had been kept in a garage and not started for about 6 years. The owners manual was about 1/4 eaten, and I found about 500 sunflower seed shells in the air box. Both were the work of some busy (and industrious) mice.........