insects that eat books, clothes
蛀 centers on destructive insects that bore into and consume materials, especially books and clothing. The core idea is insect damage to stored items.
蛀 combines 虫 (insect) with 主, which likely provides the sound. The character specifically refers to insects that damage books and clothing.
An insect 虫 is the master 主 of destruction, eating through your books and clothes. Picture a bookworm proudly standing as the ruler of a ruined library.
For シュ, imagine a shoe infested with book-eating insects: shoe -> シュ, and the bugs have chewed holes right through the leather.