Rodent

Damage to the bark of an oak caused by rodent chewing. Cayenne Pepper shown at left, is a deterrent. Photo by Matt Chroust

Rodents, which include squirrels, rats and mice, and opossum can be problems around bonsai. They can chew the bark of trees, dig in the soil and disturb the root system. They can be attracted by the irrigation of trees and the use of organic fertilizer, particularly the smell of fish emulsion and the decomposing fertilizer cakes.

A light dusting of cayenne pepper spread on the soil surface and around the trunk can be irritating to the mucous membranes and sensitive noses of some rodents.
Deodorized fish emulsion can be used to help avoid attracting pests.
A layer of chicken wire mesh or hardware cloth can be spread across the surface of the soil (cut a hole for the trunk, and a slot to install it, then twist or wire it back together)
Mousetraps can also work
a larger carnivore, like a cat or dog might be discouraging to these pests

Many rodents are nocturnal, so you might have to get up early to catch them at work!