State entomologist: Better to focus on infected ash trees

Emerald ash borer infestation takes years to show up in affected trees, but residents can use a couple of tactics to control the pest, a state expert told the Rochester City Council on Monday.

The pest was found in Olmsted County in August , and thousands of ash trees in the county are expected to be affected. The emerald ash borer has spread to Ramsey, Hennepin, Winona and Houston counties as well, and all five counties are under a quarantine designed to contain movement of the tree-killing beetle.

In Olmsted County, the affected trees are near the interchange of Interstate 90 and U.S. Highway 63 north of Stewartville, about 45 miles from a similarly infected area in Winona County.

One of the options for fighting the insects is to remove affected trees and destroy them, said Mark Abrahamson, an entomologist with the Minnesota Department of Agriculture.

"That's actually a pretty viable management tactic," Abrahamson said.

Borer larvae cause the biggest problem as they tunnel into a tree's bark and beyond, he said. Emerald ash borer can be tough to manage because symptoms aren't usually visible until the pest has been inside the tree for years, Abrahamson said. It takes one to two years to turn the problem around in an infected tree.

"This is a process that took years to unfold," he said.

Another option for treating ash trees is injecting them with an insecticide, which can range in cost from tens to hundreds of dollars per tree, he said. Those insecticides will affect all insects inside the tree, not just the emerald ash borer.

"We could consider them pests as well for feeding on the tree," Abrahamson said.

Additionally, if any other plants are near the tree's root system, and the insecticide is soaked up via the tree's roots, those plants could be affected, which could affect bees who visit those plants, he said.

"Anytime you use an insecticide, there's going to be considerations for other impacts," he said.

It is within a city's interest to replace ash trees that aren't infected yet with other trees as a forestry management tactic, but it won't help in stopping the emerald ash borer, Abrahamson said, adding that concentrating on infected trees is the best offensive action.

"If you're removing and destroying infected trees, you're having a direct impact on the emerald ash borer population," he said.

So far, the City of Rochester has removed and replaced about 2,000 ash trees since 2009, but only those that were infected or that interfered with infrastructure, said City Forester Jeff Haberman.

"We have not removed any good ash trees to date," Haberman said.

People with ash trees on their properties can watch for early signs of infestation, such as bark starting to split open vertically, more woodpecker activity and a characteristic S-shaped tunnel made by the larvae, Abrahamson said. To find that tunnel, people will need to pull back the tree's bark, he said.

"Just cutting away the bark isn't going to hurt the tree any more" than the emerald ash borer is, Abrahamson said.