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.........

2007 1800T, Memphis Shades 21", Kury Engine Bar & Pegs, HondaLine Light Bar, Ultimate Seats, Cobra Slash-Cuts, Progressive 430 Shocks & Springs, HondaLine Saddlebags, Iron Horse Tail Bag, Fender Rail, Color-Matched Radiator Housing, Show Chrome Fringe, Kury Iso-Grips, Custom Breather Cover.
GO TEAM DREW!!!