European Hornets (Followup)

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.

European hornets

European hornet

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.

European hornets

European hornetsThese are European hornets.
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.
European hornets
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)

Bagworms and Bagworm Predators

This morning we did a pest monitoring visit for a good client, a 250unit townhouse complex.  Historically the worst pest here (invertebrate that is) has been the bagworm, because of the preponderance of arborvitae and juniper in the landscape.

A big problem with bagworms is that to the untrained eye they are not easily seen, so they’re often not noticed until late summer when they are no longer easy, or possible, to control, and the tree has been killed or severely damaged.

But early instar (young) larvae are EASILY controlled, you just need to know when and how to spot them.

juniper bagworm

cocoon of bagworm larva

Cocoon of early instar larva. Can you see it? The caterpillar has camouflaged itself by attaching juniper needles to the cocoon!

 We examined all the important host plants today, and only found one very small bagworm outbreak (thus the lousy photo-not a lot of subjects from which to choose.)

But what we DID find was lots of assassin bugs!  That’s a good thing!  They’re a predatory insect – one of the few predators of bagworm.

assassin bugs

assassin bug

assassin bug

Assassin bugs

The bagworm larva’s cocoon protects it from most predators, but the assassin bug can attack it successfully because it has a long, pointy mouth part (rostrum) that it can insert right into the cocoon.


See the rostrum? It’s the reddish brown spike curving down and rearward from the head

Anyway, the point of this story is that this clients landscape has very few pest problems.  And it is because they DON’T use regularly scheduled pesticide cover sprays.  When we encounter a pest problem that reaches a threshold requiring an intervention we just target the actual pest population, we don’t blast the whole landscape with pesticides.  And we use a control measure that can do the job with the least  impact on non-target species.  Bagworm can be easily controlled with Bt if caught in time.  Bt only kills Lepidoptera, no other insects.

So natural predators control almost all the pests for this client.  The bagworms rarely get out of hand anymore.  There are never any mite problems on the spruces or arborvitae or junipers.  This job is really easy if you know what you’re doing.

More insect eaters we saw today:

Damsel fly

Damsel fly

Predator mite (eating an earwig)

Predator mite (eating an earwig)

 Unfortunately a lot of companies still manage pests with regular sprays, whether needed or not.  This is stupid.  It’s like bombing the hell out of an entire country just to try to get one terrorist bad guy when you don’t even know if he’s there or not!  It’s a huge waste of money and ammunition, there’s loads of unnecessary collateral damage, and a lot of the casualties turn out to have been your allies!

The lesson : diagnose before you treat.  (treatment without diagnosis is malpractice)  Monitoring plants is the first and most important step.  It is the key element in an IPM (integrated pest management) or PHC (plant health care) program.