These are European hornets.
I never really thought of them as tree pests before. When I see them on trees, usually they’re feeding on sugar that’s been excreted by aphids or scale insects, or on the alcoholic wetwood flux oozing from a mulberry or a dying elm. But I may be changing my mind a little bit , after what I saw today. Renee, from Audubon, showed me these insects, which she had attempted to identify by searching on the internet. She had noticed them before, but could no longer tolerate them because her son was stung by one of them, and it was a pretty bad experience. They were congregating on two of her river birches. I waved my hand a few inches from a group of them and they didn’t react at all. They really aren’t very aggressive, normally.The branch of the birch tree was stained with sooty mold, indicating sugar, such as from an insect injury.
On closer inspection, I could see that the hornets were not feeding on the sugar; they were actually causing the injury that produced it. These hornets had chewed away the bark all the way around one branch, killing it!
I know they need cellulose to make the paper to construct their nests, but this is the first time I’ve seen this type of damage. You learn something new everyday!
One more interesting observation: while I was trying to get a photograph, I watched a baldfaced hornet approach a group of the European hornets. The Europeans reacted immediately and chased it away. After that, their behavior was completely changed- they were very aggressive toward ME and would no longer let me get close! (check back later to see what we did about it)