grubs in wood, worms in books, clothes, etc, used figuratively of corrupt officials
螙 centers on destructive, hidden infestation: wood-boring grubs, bookworms, or clothes moths that eat away from within. The figurative sense of corrupt officials extends this idea to people who secretly undermine an organization.
螙 combines 木 (tree, wood) with 䖵 (a doubled insect radical), suggesting insects that infest wood. The exact historical development is uncertain, but the component combination clearly points to wood-boring pests.
A tree (木) is being eaten from the inside by a swarm of insects (䖵). Picture tiny grubs tunneling through the wood, leaving it hollow and weak—just like corrupt officials hollowing out an institution.
For ト, imagine a tiny insect going 'toe-to-toe' with a giant tree, boring into it one bite at a time: toe -> ト.