The use of mothballs to ward off rodents is highly discouraged as mothballs are potentially toxic to household pets and children. Mothballs often contain naphthalene or paradichlorobenzene which emit very noxious, irritating and potentially toxic fumes and are difficult to dispose of once they have been put down. These are chemicals you don’t want in your planting beds.
See IPC’s blog post “Moth repellents: Repulsive or not?” for more information.