Renee wanted me to help her get rid of the hornets. So I had to decide how to go about it. The most effective way to control European hornets would be to destroy their nest and colony. But we have no idea where the nest is – probably not even on her property. If we went to spray the hornets we see on the trees with an ordinary contact insecticide we’d just kill those individuals present at the time.
So I opted to coat the bark of the trees with a material called dinotefuran. It’s a neonicotinoid, like Merit, with which you may be familiar. But it’s much more soluble, so when applied to the bark of a tree it can be absorbed, and then conducted through the tree’s phloem. And the phloem of the birch branches is where these hornets are feeding.
I made a visit to Renee’s today to check the results. There were no longer any hornets on the tree. But there were MANY on the ground, all either dead or dying.
So I think we made a good choice. Though we did have to intervene with a pesticide, the one we used – dinotefuran – is unlikely to cause harm other than to the target pest. It is extremely low in toxicity to humans. And with the trunk application method, the material all goes where we want it – in the tree. The only exposure is to whatever eats the tree. And, being so highly soluble, it dies not last very long (like Merit does). So it won’t be affecting beneficial insects like bees next year, after its job is done.
Post Script: I found a really good article since I had this experience. It’s by Frank Santamair, in the Journal of Arboriculture from 1984.
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)