Locust borer can be a real problem for shade trees

When we moved in five years ago, a purple robe locust tree sat in the corner of our fenced-in area and seemed to thrive. This area can sometimes be on the dry side but I have always made sure to add water if the grass under the tree looked dry. This spring, the hanging flowers seemed smaller than usual and after the tree bloomed, it seemed as if the crown of the tree lacked the usual lushness and the leaves looked sparse, with a few even turning yellow here and there. There are no spots on the leaves, but on close inspection, I found a little bit of “sawdust” in the main crotch of the tree, and I think some borers may be the problem.  What pesticide would help and when should it be used, if at all?

— Barbara

It sounds like you have locust borer. This insect pest has become a bigger problem the past three or four years.

It’s almost gotten to the point where I’m recommending people take preventative measures and spray whether they see evidence of this guy or not.

Before I get into specifics, there’s some basic information you need to understand about borers in general. There are a number of different species of borers that trouble our shade trees. Most of them damage trees by tunneling around under the bark, disrupting and killing the vascular tissues of the tree, causing dieback and death of the tree. Some can structurally weaken the tree, making it more prone to wind breakage (this is a big problem with locust borer).

The most important thing to keep in mind is that these types of borers are almost always a stress-related problem. A tree that is not as strong and robust as it could be is more likely to develop borer problems.

The long term solution to borer problems is to try to identify anything that may be putting the tree under stress and to correct it to restore the tree to a healthy, robust condition.

Doing this won’t cure your tree, but it will help tip the balance more in favor of the tree. You’ll still have to do some spraying to stop this little monster.

The problem with spraying for borers is that when they’re in their larval stage and chewing under the bark, they’re pretty well protected from anything we can throw at them. Unfortunately, for most borers in most situations, systemic insecticides won’t work well either. They’re not effective because they don’t works on some kinds of borer or that the borer spends most of its time in the heartwood of the tree where the systemic doesn’t penetrate.

What we try to do is to take advantage of their life cycle. Eventually, the larva will pupate and emerge from the tree as an adult. It will then find a mate, and the female will lay eggs on the tree that hatch out and start the life cycle over again.

While the adult is out of the tree it is vulnerable to insecticide sprays. Unfortunately, different species of borers do this at different times of the year so it’s important to know exactly which borer you’re dealing with.

With locust borer, you’ll want to spray the bark of the trunk and lower main branches in late July and again in late August. The best spray to use is 38 percent permethrin.

I have irrigation water and this last year provided me with an enormous amount of silt that is rich with BIG worms throughout. Is this dirt safe to mix up with my vegetable garden?

— Tammy

Actually, I’m usually pretty careful about using that silt from the ditches since it sometimes can be pretty salty. In your case, however, with those worms in it — they wouldn’t be there if it was too salty — I doubt this is an issue.

That silt isn’t bad soil to begin with. The texture is usually pretty good and there’s a good amount of organic matter in it. Having the earthworms is really a bonus as they help aerate and enrich the soil. Use it to your heart’s content.

Dennis Hill is the nursery manager at Bookcliff Gardens, Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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