An ash tree infested with emerald ash borer from Michigan's Upper Peninsula is giving Michigan State University researchers insights into the pest's habits. The tree, harvested from Brimley State Park, had been girdled and used as a detection tree in 2004. It was taken down in September 2005 and brought to MSU.
"This is a white ash tree that showed no outward signs of EAB infestation, but when we peeled off the bark, we found 99 EAB larval galleries," said Deborah McCullough, lead EAB researcher and forest entomologist at MSU. "Any larvae that were found were saved and will be used in genetic studies to determine how closely related these larvae are to each other or to EAB from other locations found in Michigan, Indiana, and Ohio."
The tree was one of 60 ash trees that MSU researchers sampled at Brimley State Park this fall in cooperation with Michigan Technological University and the Michigan Departments of Agriculture and Natural Resources in an effort to understand more about EAB. Of the 60 trees sampled, only three had the D-shaped EAB emergence holes.
"Using dendrochronology, or the study of tree-ring patterns that can be used as the basis for dating or assessing a tree's environment, we can determine that trees in the park were first infested in 2003," said MSU entomologist Nate Siegert. "Those EAB required two years to complete their life cycle, and the exit holes were a result of adult beetles emerging in 2005. We find that most EAB develop in one year on stressed ash trees but many larvae take two years to develop on healthy trees."
The dendrochronology study also helped pinpoint when EAB came to Brimley State Park. Siegert said that EAB-infested ash firewood brought into the park sometime between the fall of 2002 and the summer of 2003 was probably the culprit.
After emerging, beetles did not have to travel far to lay eggs, McCullough said. The other infested ash trees in the park were less than one-quarter mile from the ash trees with EAB exit holes.
"This is the youngest EAB outlier infestation that we've dated," she said. "The earlier we catch an infested site, the better. Early on, larval densities are low, and there is a good chance that the population can be eradicated."
Of the larvae, 75 percent were found in the branch crotches. Researchers believe that EAB like to lay their eggs on rough bark. The branch crotches are where the bark gets rough and furrowed, especially on white ash trees, which often have smoother bark than green ash trees.
"What was so important about this early EAB find is that the trees looked perfectly healthy," said McCullough. "Trying to detect a recent EAB infestation by visual survey only is very difficult. The adult beetles seem to spend most of their time in the canopy of ash trees, where they feed on the leaves. Because most females deposit eggs near the leaves where they feed, the emergence holes, often the only obvious symptom, are more common in the tops of trees. This makes it difficult to find EAB emergence holes on taller trees.
"We are getting better at detection," she added. "We are getting better at finding EAB infestations when there are fewer larvae in the trees."
Populations of EAB do spread on their own, especially when populations are high. But McCullough emphasized that human movement of infested ash wood or trees, not natural beetle movement, is responsible for spreading the pest to many areas of lower Michigan, as well as areas of Ohio and Indiana.
"Most of the outlier infestations we are finding now are more than two or three years old," she said. "Many are as a result of ash nursery trees, firewood, or logs that were infested and moved to the area before we knew about EAB. Transport of ash nursery trees and logs can be regulated and monitored. At this point, firewood is probably the biggest concern. We know infested ash firewood can spread EAB as well as other pests and tree diseases. As more people learn about EAB and the dangers of moving firewood, we hope the number of new EAB discoveries will drop."
Working cooperatively, the MDA and MDNR removed a total of 940 ash trees to eradicate the U.P. infestation. Detection trees will be used as part of an intensive monitoring strategy around the area for the next three years.
For more information on EAB, go to www.emeraldashborer.info.
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