How to battle those holes, ash borer in Grand Valley

We recently transplanted a young ash tree from a friend’s house and noticed a hole in the trunk with ants coming in and out. The main branches still feel supple and have some green to the bark when broken but there are no leaves forming yet. Is this tree a goner? If not, how do we treat the ants?

— Carol

The ants are just taking advantage of a little hiding place. What I am concerned with is what caused the hole in the first place. The most common reason for this is a borer of some kind. Lilac-ash borer is the most common culprit here. This insect is actually a moth that looks like a wasp. The larva tunnel around under the bark, killing off the vitally important cambium tissue. They pupate inside the tree and then chew an oval exit hole that’s about an eighth of an inch across. Left untreated, they will kill or severely damage an ash tree.

Ash borer has always been present here 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 are so many around that the critters that feed on them start to increase as well, since there’s so much for them to choose from.

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. 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. In your case, the transplanting was certainly stressful to one extent or another.

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 protected pretty well 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 that 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 then emerge as an adult. This is when they’re vulnerable. Our goal is to apply a protective coating of the insecticide on the tree to either kill mama when she’s laying the eggs or to kill the young larva when they hatch from the egg and try to bore their way into the tree.

So, it’s just a matter of spraying at the right time of the year. The time to spray for ash borer is usually mid to late April. Now, you’ve obviously missed that date, but you could still spray to try to protect the tree. You may have missed some of the early ones, but I’d bet that the spray would still be helpful.

You want to spray the tree with a solution of 38% Permethrin. There are other Permethrin products out there that aren’t as strong and they just won’t give the control that the 38% stuff does.

You want to spray the bark of the trunk of the tree and all of the branches that are one and a half 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 up to 20 or 25 feet. If your tree is substantially taller than that, you might consider having a spray company do the job for you. You only have to spray once to protect the tree for an entire year.

I am concerned that your tree hasn’t pushed out any leaves yet. This may be due to the shock of transplanting or it could be a sign that the tree is a goner. Since you still have green, live tissue under the bark I’d hang in there and see what develops. You should know what direction the tree is going to take in a couple of weeks. Give the tree a good deep soaking when you do water but allow the soil to dry slightly before soaking it again. 

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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