When foresters saw the flagging leaves on old oaks in Richfield's Augsburg Park last year, they thought they were seeing a disease called oak wilt. It was only when they sent a sample to a lab at the University of Minnesota that they discovered something else was killing the trees:
The two-lined chestnut borer, an insect that is native to Minnesota.
Normally an irritant that's shrugged off by healthy trees, it can be as deadly to stressed oaks as emerald ash borer is to ash trees. After several dry summers, there are a lot of stressed oaks in the metro area, said U of M entomologist Jeff Hahn.
"Talking with colleagues in the landscape industry, I know incidents of two-lined chestnut borer have gone up," he said. "We're getting more reports of oak trees in trouble and more oak trees dying.
"I pin it on the dry weather we've had the last few years. Oaks are more vulnerable."
Unless it rains more frequently, two-lined chestnut borer will probably kill more oaks in Minnesota, Hahn said.
In sheer numbers, the effect would not match that of emerald ash borer. Black ash is the fifth-most common tree in Minnesota, according to the Department of Natural Resources (DNR), while oak isn't even in the top ten.
But mature oaks often dominate residential landscapes with a majesty and impact that even big ash trees can't match, so the effect in the Twin Cities area could be significant.
Two-lined chestnut borer also attacks chestnut and ironwood trees, but in Minnesota the pest's main target is oaks. The adults, which are skinny black beetles one-fifth to one-half inch long with two gold stripes on their backs, are attracted to stressed trees and lay eggs on them in May and June. Larvae feed under the bark in July and August, disrupting the flow of nutrients and water to trees and essentially starving them.
Fighting a two-borer war
The pest's emergence creates a dilemma for cities like Richfield, which are trying to save money when budgets are tight and have already invested in the battle against emerald ash borer.
This spring Richfield committed about $21,000 to buy chemicals and equipment to try to save more than 300 oaks in Augsburg Park, treating the trees with pesticides twice over a four-year period. The 21-acre park, home to the city's community center and senior center, includes some oaks that are more than 100 years old.
"Those trees define that park," said Mike Eastling, Richfield's director of public works.
Eastling said he believed this is the first confirmed two-lined chestnut borer infestation in the city. Borer damage was spotted last summer after people working in the park noticed dying branches in a cluster of 17 trees with thinning crowns.
"Having a tree or two die is not unusual," Eastling said. "Having 17 die is. The forester knew something was going on."
City workers and staff from a tree service working in the park assumed the trees had oak wilt, a fungal disease whose main symptom is branch-by-branch wilting that begins in the top of the tree. But when the city sent a tree sample to the U, they found out the villain was borers. Seventeen trees that were most affected were cut down.
How to detect the pest
According to the DNR, signs of infestation become visible after mid-July. The top of an infected tree may have dead and leafless branches, with wilted red-brown leaves in the middle of the crown and lower branches looking healthy and green. The DNR calls the characteristic pattern "dead, red and green."
Diagnosing a borer infestation in oaks can be difficult. Symptoms can be confused with oak wilt, and proof of infestation is high in the tree, where the borer's D-shaped exit holes will be visible and borer tunnels show if bark is peeled back. The more obvious symptoms of dying branches may not be obvious until the second year of infestation, Hahn said.
"It depends on borer numbers and the size of the tree," he said.
Water is best prevention
The best way to combat borers is through prevention, he said. That means keeping trees as well-watered as possible, which can be hard for homeowners with big trees and impossible in areas like parks. Chemical treatments may not be enough to save the Richfield trees if summers continue to be dry, Hahn said.
"If we get normal rainfall they will survive, so the insecticides will help stave off the borers until then," he said. "But if we have drought year after year, that's not going to work."
Richfield is on the watch for two-lined chestnut borer both on public and private land. If trees on private land are affected, homeowners will be notified and it will be up to them to decide what to do, said Chris Link, operations manager for streets and forestry.
Mark Hall, the city's park and fleet operations manager, said it's worth trying to fight the borer in Augsburg Park. Chemicals will probably be injected into trees at the end of May.
"If all those trees died off, we would have a prairie there," he said. "Everywhere you look, it's oak trees. It's just beautiful. If those trees were gone, it would be devastating."
Mary Jane Smetanka • 612-673-7380