HG: Homegrown Column April 18, 2009
What to do with ash borers
A salesman from a lawn care company left a brochure on my front door with a message that said, “Ash borer in front tree — very serious — see holes.”
Can you please explain what this is and what I can do to fix the problem before it gets worse?
Ash borer (properly called lilac-ash borer) has always been present in the Grand Valley, but it seems to have become more prevalent the past several years.
Ash has been an extremely popular shade tree choice for the past 25 years, and there’s so many around that the critters that feed on them started to increase as well.
This is precisely why we like planting a variety of different trees. The diversity tends to discourage problems such as this from showing up. But that’s a topic for another discussion.
Anyway, a very important thing to understand about most borers is that they are almost always a stress-related problem.
Borers are attracted to plants that are weak or under stress. In many cases, the stressed tree emits chemical signals called pheromones that actually attract the borers to them. “Hey all you bugs out there, I’m hurt. Come eat me!”
So the first line of defense against borers is to maintain your plants in a healthy and vigorous state. A healthy tree has natural defenses that usually repel borer attack.
Once borers have started in on a tree, or if the tree becomes stressed, we’ll fall back to our second line of defense: chemical sprays.
Controlling borers this way can be tricky because when they’re under the bark they’re pretty well protected from our sprays.
Systemic insecticides rarely work because the insecticide doesn’t concentrate well enough to affect most borers or doesn’t get into the plant tissues the borer feeds on.
What we have to do is try to take advantage of the borer’s life cycle.
The destructive phase of the insect is usually the larval stage, when they’re chewing through the plant. Eventually, they’ll pupate and emerge as adults to find mates and for the females to lay eggs on the tree. This is when they’re vulnerable.
Our goal is to apply a protective coating of insecticide on the tree to either kill the females when they are laying the eggs or to kill the young larva when they hatch and try to bore into the tree.
So it’s just a matter of spraying at the right time of the year.
What complicates this is that different trees have different borers and those different borers have different life cycles. Each requires their own particular spray schedule.
The time to spray for ash borer is mid- to late April.
You want to spray the tree with a solution of 38 percent permethrin. There are other permethrin products out there that aren’t as strong (they’re usually about 2.5 percent), and they just won’t give the control that the 38 percent stuff does.
You want to spray the bark of the trunk of the tree and all of the branches that are 1 1/2 inches in diameter or larger.
Essentially, you end up spraying the entire tree.
I don’t know how big your tree is but a standard hose-end sprayer will usually reach 20 or 25 feet high.
If your tree is substantially bigger than that, you might consider having a spray company do the job for you.
The good news is that you only have to spray once to protect the tree for an entire year.