Two lined chestnut borer

Two-lined chestnut borer creates a maze of trails in the trunk of an oak tree.

In mid-August, two lined chestnut borer (TLCB) damage began to show up in Itasca County. By late August, dieback, topkill and whole tree mortality were widespread in northern and southern Minnesota. Some oaks that did not appear to have been attacked by TLCB in 2001 were entirely brown and looked dead by the end of August 2002. Borer galleries could be found in the trunks of these trees down to the soil line.

In the northern counties, a few pockets of TLCB were seen in the fall of 2001 but the amount of topkill and mortality seen in 2002 was unexpected. A very dry April, May and June just as the trees were leafing out coupled with two or more years of forest tent caterpillar defoliation were likely the stress factors contributing to the success of the borers.

Aerial survey was flown in late August over about 84 townships in parts of Clearwater, Beltrami, Cass, Itasca, Mille Lacs, Aitkin and Crow Wing Counties. Approximately 10, 000 acres with oak mortality and top kill were mapped. Scattered damage occurred throughout northern Minnesota but the worst damage was in Itasca County. Approximately 7,000 acres of stands with mortality were mapped within a 10 mile radius of Grand Rapids. Additional damage continued to become evident through September. In some stands over 75% of the oaks suffered top kill or mortality. Severe damage occurred in stands that had been recently thinned or in areas where road construction or building construction had recently occurred. However, damage was not restricted to these types of stands.

Next spring in April, May and June, adult beetles of the TLCB will emerge from infested trees as well as from cut and split firewood from infested trees and fly to nearby stressed oak trees to lay eggs. Two lined chestnut borer management guidelines can be obtained from DNR Division of forestry Offices, or by contacting a Forest health Specialist listed on the last page of this newsletter. Management guidelines can also be found on the internet at Minnesota DNR: Two-lined chestnut borer.