Ask the Arborist




What a pest

There’s a new critter in town attacking our majestic oaks—and this pest could have a devastating impact.

Recently I was driving around the Conejo Valley with my good friend John Innes, a landscape architect and consultant for the City of Thousand Oaks. He mentioned he’d seen several trees attacked by the California oakworm so I coerced him into showing me a few.

He took me to see a grove of at least 16 coast live oaks in Newbury Park that were in various stages of defoliation caused by the pest.

Oakworm infestations are nothing new to California, but this was the first time I’d seen an occurrence in Thousand Oaks. The destruction in this one grove gives me serious cause for concern.

Bear in mind, I tend to go to extremes when predicting potential problems, but that’s because I believe that being over-prepared is better than wishing you had done more after it’s too late.

What’s most disconcerting to me is the fact that there are so many drought-stressed trees in the Conejo Valley, and something like the loss of most of their leaves—caused by the oakworm— could be the straw that breaks the camel’s back.

The oakworm metamorphosis consists of the following: Adult moths lay eggs that hatch into caterpillars, which eat the leaves, then turn into pupae that mature into moths once again. In our area I think we can expect three generations of this process in a single year, which, if unchecked, means millions of caterpillars.

That is the worst-case scenario.

When the caterpillars first hatch, they’re very small and the damage they do is minimal. But the larger they are allowed to get, the more damage they can cause.

When I first looked at the trees with John it appeared as though the infestation was out of control and that it was not being managed. Now, however, there are little caterpillar corpses scattered around the trees. It appears someone decided to spray the trees to kill the caterpillars, which is good as long as the control method was limited to the affected area.

I did, however, still see many cocoons and live moths, which tells me that the problem is not totally controlled in that location. I have no idea if they have spread, but I am monitoring other oaks in the general area.

Valley oaks very close to the defoliated coast live oaks I saw show no damage and no caterpillar activity. It appears that these critters have a taste for only one kind of tree leaf.

This is not to say they won’t eat leaves on other plants: They may do so if their preferred food source is defoliated.

The bad news is—as I wrote in my last column—the drought is playing havoc with the health of our local trees of all species.

If a live oak is in decline already and just hanging on, the loss of so many necessary leaves could prove fatal. I just hope the location I saw in Newbury Park does not become known as ground zero in the Conejo for this pest.

So what can we do to avoid such a devastating outcome?

First, look for live oaks that appear to be losing their leaves. Look at the tree trunk and leaves for caterpillars, and look for small moths flying around the tree.

If you see any of these critters, please email me with the location. If we are all vigilant and deal with this problem on a tree-by-tree basis, it may not be too bad.

But just like floating embers from a brush fire, if these guys cause citywide defoliated areas that go unreported, our oak trees could have a big problem.

David D. Mortimer is a certifi ed arborist and a certified tree risk assessor with more than 35 years of experience in the tree care industry. Email questions to dmortimer@theacorn.com



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