Insect damage, including damage from caterpillars, can cause some damage to cannabis plants but not enough damage to worry about. Plants can suffer a little damage and have no major problems.
Powdery mildew and bud rot are a different story. Bud rot can be cut out and removed from the garden. Powdery mildew gets into the system of the plant and can spread from plant to plant quickly. Jorge discusses different tactics to control these fungi.