Jane’s Gardening Journal: Hope for our ailing oaks

This little guy can do a lot of damage. (Purdue University photo)

by Jane Solomon

A tiny beetle has been quietly wreaking havoc on oak trees in the DC area. The twolined chestnut borer is a native beetle named after its traditional host, the American chestnut tree. Since the chestnut was driven to near extinction by the chestnut blight (which was a fungus) in the early 20th century, the chestnut borer turned its attention primarily to oaks.

Left unchecked, they’ll kill the tree in two to three years. However, if caught early, there is an effective insecticide.

We’re fortunate to have three magnificent white oak trees on our property which have always been in robust health. So it surprised me when last summer we noticed significant dieback in some of the upper branches of one of them. Shawn Siefers, my arborist from The Care of Trees, examined it and treated it for a range of possible culprits, but the tree offered no obvious clues to the cause.

My concern turned to panic this spring when about a third of its branches failed to leaf out. I feared the tree would have to be removed. While worried, Shawn’s advice was continued monitoring and we planned for a major pruning of the dead wood this fall.

Dieback in my white oak.

Upon returning from our summer in Vermont, a neighbor told me that her arborist said something is attacking oaks. It was with a small flicker of hope that I called Shawn to ask if this was true and if it was treatable. The answer to both was yes.

There is camaraderie among local arborists. They speak and share information fairly regularly. This summer, they had identified the chestnut borer as the cause of death in a significant number of trees they’ve removed – most of them oaks along with some beech. The borer attacks trees that are stressed by cultural conditions like heat and drought, or weakened from fighting some disease or other pest. Because it’s a native insect, it’s always present at some level in the population. However, there are periodic spikes when conditions are right, just as outbreaks of human diseases wax and wane.

Shawn Siefers is an arborist with The Care of Trees.

The symptoms of a chestnut borer infestation is early browning, leaf loss and dieback at the branch tips. It’s difficult to detect because for at least the first year, the insects are high in the treetops. The damage is caused by the larvae, which feed on the inner bark, cambium layer, and sapwood, cutting off the tree’s ability to carry food down from the crown, and water and minerals up from the roots.

The insecticide is applied as a root drench, so its success is dependent on the tree’s ability to carry it up. In the second and third year as the chestnut borers work their way into the larger branches and the trunk, the tree’s circulation becomes increasingly compromised, ultimately killing it.

Fallen branches provide the opportunity to diagnose the problem without getting up into the canopy. The chestnut borer leaves a characteristic D-shaped exit hole in the bark that is easy to identify.

The chestnut borer leaves a D-shaped exit hole.

I was able to find some of these holes in bits of fallen wood from our tree. That was corroborated once the arborists were up in the bucket truck to prune it.

After learning about the chestnut borer, I began to notice all the oaks in Forest Hills that are showing similar symptoms. Our trees are certainly stressed. These last two summers have been the hottest on record and this summer we had serious drought.

What can you do? Share this information with others. If you have an oak tree, examine fallen branches for holes. Next spring, look in the treetops for branches that don’t leaf out and consult an arborist. Routine preventive arbor care and necessary treatment does come at a cost, but it pales in comparison with the cost of removing a dead canopy tree, not to mention the loss of its benefits and enjoyment.

Meanwhile, I have my fingers crossed that we’ve treated our oak in time.

The tree after its fall pruning.