A tiny, boring bug is the cause
A reader sent me a picture this week wanting to know what was wrong with her tree. Numerous branch tips had died, as signaled by brown, drying leaves. What caused this? Was her tree a goner? Fortunately, she asked the Grump.
The culprit is a tiny beetle called a black twig borer. It’s about the size of a grain of rice. Females bore a pinhole into a thin branch about a foot from the end of the branch. Then they hollow out a cavity in the middle and lay eggs. The eggs hatch into grubs that feed on fungus brought in by their doting mothers. By late summer and fall, their activity blocks water and nutrients from passing into the branch tip and it dies. Females spent the winter inside the dead branch tips and then emerge in spring to find another branch.
Black twig borers attack lots of different trees and shrubs. From my observations, ‘Little Gem’ Southern magnolia (show above) is a favorite. Borers also nailed my beautyberry, pawpaw, winter hazel, and sweet shrub. Dogwood, redbud, pecan, and red maple are just a few of their other common targets.
Insecticides are useless once the borers get inside. Fortunately, the damage they cause is cosmetic and doesn’t pose a serious threat to the target plant. The best control is pruning off the dead branch, cutting back to living wood, about an inch from the pinhole. If you can’t find the pinhole, bend the branch. It will snap off right at the pinhole. Bag the dead branch tips and throw them out with the trash. Because the females are still inside the dead tips, you’ll be trashing them too.