Solution
Holes in leaves, made by iridescent green beetles and their and fat, black grubs.
Organic
Mint is a prolific plant, so there's no need to control the beetles. If you're worried, you could try growing pennyroyal, Mentha pulegium as a sacrificial crop, or simply remove beetles and larvae by hand and squish them.
Chemical
Again, there's no real need to control the beetle, but you can kill them by spraying with a bug spray that contains imidacloprid or thiacloprid. Both imidacloprid and thiacloprid are systemic pesticides, meaning they are absorbed into the plant and can be present in concentrated forms in the nectar and pollen of flowers. If you do use these chemicals, remove any flowers before they open, as they will also kill any bees that visit them.
