Wausau ramps up Emerald Ash Borer plan after beetle confirmed in city trees

By Shereen Siewert

Parks officials in Marathon County are ramping up their response to the Emerald Ash Borer, a beetle responsible for destroying tens of millions of trees worldwide, after the pest was confirmed in Wausau trees.

Staff in 2016 presented a plan to the Wausau City Council that called to reduce the ash population in boulevard areas by 30 percent within seven years and treat the remaining trees to prolong their life. So far, the ash population has been reduced from about 5,200 to about 4,518. Additionally, there are 95 ash trees being removed by a contractor and 444 marked and recorded for removal in the near future.

Many trees that have been removed have been replaced with alternative species.

The council approved $107,000 in the 2019 budget for treatment, removal and planting per the 2016 plan, which was approved again in the 2020 and 2021 budgets.

But that plan was in place when EAB had not yet been detected in the city.

“Now we have found it in three sites,” Parks Director Jamie Polley told the city’s parks committee.

Polley said about two-thirds of the city’s trees have been treated with Emamectin Benzoate, which is the industry standard and provides about two years of protection. Left alone, Polley said, all the city’s ash trees will die.

Staff is now recommending a more aggressive approach to removing and replacing ash trees to maintain public safety.

Parks officials now seek to remove all ash trees in Wausau within about 12 to 15 years in phases, replacing trees along the way to maintain canopy coverage. Treatment will also continue.

Unlike elms, oaks, and maples, ash trees use a thin ring of conducting tissue to supply water from the roots to the entire tree.  Emerald ash borer grubs damage these functional water pipes as they chew just beneath the bark inside trunks and branches. This causes the tree to dry quickly and the structural wood to become prone to cracking, according to researchers at Purdue University.

Internal breaks in the structural wood that bear the weight of the tree are often hidden from view by tree bark. As such, limbs can break and fall at any point along the branch at any time. It is not uncommon to have sizable limbs snap 30 feet off the ground on a calm day, researchers found.

The threat of falling limbs is not limited to just dead ash. A comparative study of ash trees conducted in Ohio shows that structural integrity of ash trees can begin to decline even when trees are mostly green and have two thirds of the canopy still intact.