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Chestnut borer killing northern oak trees

SPOONER Wis. -- A native insect known as the two-lined chestnut borer is causing some oak trees in northern Wisconsin to die back, according to Department of Natural Resources Forest Pest Specialists.

“Infested trees show browning of leaves and top die-back, which progresses downward and becomes more visible in mid-July through September,” said Kyoko Scanlon, a DNR forest pest specialist. Peeling bark of trees killed by the borer reveals the characteristic meandering narrow feeding tunnels under the bark, she said.

This is a different disease, Scanlon notes, than the oak wilt disease that is infecting and killing oaks in some areas of southern Wisconsin.

In the adult stage the chestnut borer is a slender black beetle with two golden lines on its back, but it is in the immature or larval stage that the insect causes damage. That is when borers feed on trees’ nutrient and water conducting tissues beneath the bark girdling and killing either a single branch or the entire tree.

While lightly infested trees may recover, those losing more than 50 percent of tree crown are not likely to survive, Scanlon said. Such trees usually will die within a year or two. The insect is a secondary pest usually attacking only weakened or stressed oaks. Stress from several years of heavy defoliation caused by the forest tent caterpillar together with hot and dry summers, unfavorable soil conditions, and perhaps human disturbances weakened many oaks this year.

The best preventive measure, Scanlon said, is to maintain healthy trees. For yards, if dry conditions persist, water trees during the growing season, she advised.

“Minimize activities that could damage the trees and their root system,” she said, “and remove dead and severely infested trees from the site before spring.” This, she added, may reduce the pest population for the next year.

People should mark dying trees before leaf drop and take them down during the dormant season to minimize disturbance. Homeowners who choose to keep infested trees for firewood should completely cover the woodpile with a plastic tarp from mid-May to July.

“This prevents emerging adult beetles from laying eggs on nearby trees,” Scanlon said. There is some good news, she said.

“Defoliation caused by the current outbreak of the forest tent caterpillar is ending,” she said, “so the problem with the chestnut borer should subside as trees in the north regain their health.”

FOR MORE INFORMATION CONTACT: Shane Weber 715-635-4156 (for Bayfield, Douglas, Sawyer, and Washburn Counties) Kyoko Scanlon 715-365-8934 (for Florence, Forest, Lincoln, Oneida, and Vilas Counties)




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