Drought Stress Problem for Oaks Two-lined Chestnut Borers

By: Dominick Bierdek, Certified Arborist WI-0635A

Winter 2006 Newsletter in Adobe PDF format The Two-Lined Chestnut Borer population is on the rise due to the many drought stressed oaks in southeast Wisconsin.

Two-lined chestnut borer is not a scarylooking bug, but oaks certainly need to be scared of them. Normally, we see two-lined chestnut borers in oaks under various stresses, like those caused by construction or defoliation by gypsy moths. However, the continued drought has put many trees under stress. Two-lined chestnut borer numbers have been increasing, and even if we do get adequate moisture, they could still reach epidemic levels for the next few years, as they did after the drought of 1988.

Drought stress is present in many oak trees after our prolonged four-year drought. If a tree is not in good health, every effort should be made to strengthen it. Stressed or weakened trees emit volatile gases which attract the adult female beetles of the two-lined chestnut borer. The females are able to key in on these trees where their offsprings’ chances for survival are best. If attacked, only a strong, quick defense will save oaks from the two-lined chestnut borer. Keeping trees healthy is the best defense.

Females lay eggs which hatch in 10 to 14 days in bark crevices. The new larvae burrow through the bark and feed between the bark and wood in the phloem and cambium. This disrupts the tree’s ability to take up water and nutrients. Larvae usually take one year to mature. Adult two-lined chestnut borers emerge through D-shaped exit holes during themonths of June through August. The beetles are bluish-black in color and have two whitish lines running down their backs.

Attacks usually begin in the upper crown and progress downward as the tree weakens. Heavily infested healthy trees can die within three years. Very weak trees can die in one season. Deadwood pruning in winter and good sanitation can reduce the number of two-lined chestnut borers in a given tree or area.

The best defense is good tree health because healthy trees are less likely to be attacked. If a healthy tree is attacked it is by fewer borers. Healthy trees with good energy reserves are able to resist borer attack. Urban trees can be kept healthy by the things arborists encourage, such as adequate watering, proper mulch beds, proper pruning and sanitation, soil conditioning, fertilization, and pest control.

Wachtel offers all of these services, and more.

If your tree has branch dieback or its leaves are small, yellowing, or dropping off, it is time to investigate. Early diagnosis by a Certified Arborist from Wachtel means simpler treatment options may be used and increases chances for success. Treatment options depend on the stage of attack, location, and health of the tree. As with any treatment, proper application and timing increase the chances for success. The Certified Arborists at Wachtel are your partners for healthy trees. We can assist you in making an informed choice if trees are candidates for retention and treatment.

© Copyright 2006 – Wachtel Tree Science & Service, Inc.

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