For the beetles in particular, they're only natural inasmuch as logging is natural. The beetles are able to survive because of human activity. Logging is just as natural, because it's done by humans, which are a natural species.
We have to have some common sense boundaries on what is or is not natural for the term to have any usefulness in environmental discussions. In this case, the beetles are unnatural and invasive. Moreover, they're harmful in that they kill trees more quickly than the forest is equipped to handle.