Lilac tree borer in Sept

Asked September 13, 2020, 1:50 PM EDT

My grandmother's lilac tree which is over 20 ft tall has bark shredding off of it. I noticed holes in the lower area and what I thought was a wasp but now no is a borer. I read you cannot treat what's already in the bark. What can I do now to try to keep them out and prevent them from eventually killing her tree. It is September and they are still going in the tree.

Greenbrier County West Virginia

1 Response

The insect pictured is a baldfaced hornet. They can sometimes nest in hollow trees and if they are going into the tree at regular intervals, I would worry their nest is inside. However, it is more likely that they are simply harvesting dead wood to build their nest elsewhere. They typically live in large papery nests found attached to limbs of trees and shrubs.

I would, however, highly recommend verifying that there is not a nest in the stems before disturbing the shrub. Baldfaced hornets, while beneficial, can be extremely aggressive if their nest is disturbed and their stings are quite painful.

The lilac seems also to have sapsucker damage (the horizontal rows of holes) as well as other decay in the older stems. But, I don't see anything of significant worry. Decline in older stems is natural for lilac and some regenerative pruning may be beneficial.

Due to the age of this plant, I recommend utilizing the following method proposed by Richard Jauron at Iowa State University:

"...cut back the overgrown shrubs over a three-year period. Begin the procedure by removing one-third of the large, old stems at ground level in late winter. The following year (again in late winter), prune out one-half of the remaining old stems. Also, thin out some of the new growth. Retain several well-spaced, vigorous stems and remove all the others. Finally, remove all of the remaining old wood in late winter of the third year. Additional thinning of the new shoots should also be done. Since lilac wood needs to be 3 or more years of age before it blooms, this pruning method should allow you to enjoy flowers every spring.

When properly pruned, an old, overgrown lilac can be transformed into a vigorous attractive shrub within a few years. Once rejuvenated, pruning should be a regular part of the maintenance program for lilacs. The shrub can be kept healthy and vigorous by removing a few of the oldest branches every 3 to 5 years."

The rest of the article can be found here: https://hortnews.extension.iastate.edu/1993/2-10-1993/lilac.html