Pest eating giant holes in PA trees

One of the treasured aspects of living in an old, established neighborhood is the mature trees that shade the sidewalks.  Their branches toss in a lively breeze, in the spring they break through the grey of winter with attention-getting flowers and in the fall, the street flames as their vivid leaves change to shades of yellow, orange, and purple.

As I walked home not long ago I was alarmed to see evidence that some large pest had worked its way through the neighborhood chewing holes in some of the largest trees.  I wonder what impact this has on the overall health of the tree, its longevity, and its ability to withstand storms (and I wonder this as Hurricane Irene blows her way up the east coast).

I wonder, do the pests in those trucks know what they are doing?  Are they certified arborists, or just employees who come to work one morning and are handed a pole with a blade on it, to be sent out to wreak havoc on neighborhood and countryside character?