What is killing the hop hornbeam trees? I have lost mature trees and there are several more that are dying. I have also seen sick trees along 7 Mile in Northville and at Ford Field in Northville. Can this be related to the emerald ash borer?
Hop hornbeams are in the birch tree family. The pattern of destruction is very similar to the emerald ash borer’s (EAB) attack on ash trees. Unfortunately it appears the bronze birch borer has infested and is decimating the hornbeams. Not only are birch borers very similar in size and shape, but their life cycle pattern mimics the notorious EAB. However, birch borers are native to North America, occurring throughout Canada and the U.S. to as far south as Maryland to Utah. They are not an alien introduction.
The first symptom of infestation is the dieback in the upper crown. This progresses to twig and branch dieback. Trees may take several years before dying, although they may die during a single year if conditions are optimum. The tree’s decline may be reversed in the early stages but not likely if more than 50 percent of the crown is damaged. The birch borer also mines extensive tunnels through the inner tree tissue like the EAB. When the weather cools, as is typical in a Michigan autumn, the larvae stop feeding and overwinter in the tree. Because of prolonged adult emergence and the long egg-laying period of this insect, all larval stages can be present during winter, from newly hatched to fully grown.
To reduce the number of adult borers, recently killed or heavily infested trees should be cut down and destroyed. This limits the adults emerging in spring. Prune out dead and dying branches slightly below the point of dead wood. Insecticides can be applied to the surface of the tree bark to kill larvae emerging from eggs. Sometimes as many as three applications are required from mid-May to mid-July to handle the repeated hatchings from the staggered egg laying. Vigilance, control, and integrated pest management are the only real tools against this destructive insect.