Defending against the ash borer

I have an ash in our front yard that looks diseased. I am wondering if it needs to be taken out, or if the sickly branches could be pruned? It has a lot of bare, dead branches on it and it looks like more are going to die, too. Is there anything I can do to save it?

— Maeve

I’m pretty sure you have lilac-ash borer in your ash tree. 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 just under the bark. They pupate inside the tree over the winter and then the adults emerge in the spring. Left untreated, they will kill or severely damage an ash tree. We’ve always had this little monster around here, but it seems like there’s a lot more of them the past several years.

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.

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. What we try to do is 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 to find a mate and for the female to lay eggs on the tree. 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. What complicates this is that different trees have different borers and those different borers have their own individual life cycle. They’re all different from the others and require their own particular spray schedule. The time to spray for lilac-ash borer is in spring. We recommend that you spray the tree twice. The first spray should be in mid- to late-April and the second one a month later. The spray to use is 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.

People often want to treat borers with a systemic insecticide. The problem is that most systemic insecticides don’t work well because the insecticide doesn’t concentrate in the plant tissues where the borers are, or because the insecticide doesn’t work well on that particular type of insect.

That’s especially true with lilac-ash borer. The common systemic in the homeowner market today is Imidacloprid and doesn’t do a good job controlling moth and butterfly larva. There is a systemic called Dinotefuran that should do a good job on them, but it’s hard to find. If you can get your hands on some, you’ll want to apply it to your tree the end of March to the first part of April next spring.

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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