We do have evidence of mountain pine beetle activity that goes back over 8,000 years in western North America. That evidence is primarily based upon fossilized bits of insects collected from lakes. All that can really tell us is that the beetles were there, and we can only assume they were doing then what they are doing today.
The other reference I was making was to studies that have looked at fossilized evidence of other types of insect disturbance from as much as 56 million years ago, during a time when we had a climate change event that was similar in rate and severity as what we're expecting today. In that particular case, there was a significant increase in forest-like disturbances back in those times as well.