Emerald Ash Borer Expected to Kill Nearly all Ash Trees in Milwaukee

Milwaukee is a “mortality hotspot” for pest-induced tree death, particularly due to the emerald ash borer, an invasive beetle expected to kill nearly all ash trees and cost $13 million in Milwaukee by 2050, according to a new study from the Journal of Applied Ecology.

In response, the city and county are removing ash trees in public spaces and street trees en masse, and the city has dedicated millions in funds to fight the beetle, but this comes as the city has committed to an urban greening effort.

“It really just changes the whole looks and feel of the park,” Riverwest artist Jessica Laub said in response to tree removal in Kern Park. “Lots of people enjoy the space, I know I do, but I figured [the county] had their reasons for cutting them down.”

The study, done by a group of researchers from the US and Canada, looked into the impacts of 57 species of invasive insects on 48 tree genera in around 30,000 street tree populations across the US. 

They found that nationally 1.4 million street trees, or 2.5%, will be killed by invasive insects between 2020 and 2050 which will cost an average of $30 million per year nationwide.

However, this will not happen equally nationwide: 96% of all insect-induced street tree deaths will occur in a roughly 350,000 square mile area in the northeast and great lakes region. A quarter of urban centers will experience nearly all insect-induced tree mortality; Milwaukee, Chicago and New York will be the hardest hit. 

Furthermore, emerald ash borer (EAB) will be responsible for 90% of all tree deaths; it is expected to kill virtually all ash trees in over 6,000 communities, including Milwaukee, the study said.

The study estimates that the total cost associated with street tree mortality in the top 10 mortality hotspot cities to be $50 million between 2020 and 2050. $13 million of this will be in Milwaukee alone, despite the city being smaller in population and size than other hotspot cities such as Chicago, New York City or Seattle. 

Forestry Services, part of the Department of Public Works, in the city of Milwaukee has been dedicating special funds to respond. The 2022 budget allocates $860,000 to emerald ash borer readiness and response, less than in 2020 ($1.1 million) and 2021 ($950,000). This is considered a forestry project and all forestry projects are being financed through a transfer of Stormwater Management Fee revenue to the Capital budget, the budget says.

The Forestry Services budget has been decreasing. In 2022, the department’s budget was $12.1 million, down from $14.2 million in 2021 and $13.5 million in 2020, the city budget shows. 

Urban forests and street trees are becoming more important, the study notes; the urban population in the US is 82% but has not yet peaked. 

This comes as many cities, including Milwaukee, are committing to “urban greening,” or public landscaping and urban forestry projects. 

The City of Milwaukee Sustainability Plan for 2013-2023, also called the ReFresh Milwaukee plan, calls for doubling the city’s tree canopy. In 2013, Milwaukee’s tree canopy coverage was 22% and, in order to maximize environmental, social and economic benefits, it should be increased to 40%, the plan says.

Xiaohan Gu, a graduate student in the urban studies department at UW-Milwaukee who teaches a course on green cities, says that urban greening is also an environmental justice issue.

“It concerns urban and social equity, climate change, and the broader impacts on the city,” Gu said. “There is a lot of research on urban green spaces, such as how they promote and improve equity, help revitalize neighborhoods that are struggling with disinvestments, and how they contribute to protecting the environment.”

She wonders if the effects of the EAB will be felt equally across Milwaukee communities.

The study from the Journal of Applied Ecology says that urban greening can lower cooling costs, protect against flooding, improve air quality and boost citizens’ mental and physical health, which is why so many cities are doing it. Milwaukee’s plan also says that the increase in tree canopy will increase employment in tree care jobs.

Thousands of ash trees have been removed in nearly all 157 county parks, according to Ian Everett, marketing and communications manager with the Milwaukee County park system. The removed trees are being used for flooring, furniture, firewood and landscaping material, the website says.

The Milwaukee County Parks website says that trees infested or killed by the EAB might lose large limbs or fall over completely, making them a safety concern. The county parks forestry department has had to shift almost all its effort to removing ash trees.

“[Emerald Ash Borer] has been described as producing extensive tree damage while carrying the impact of a slow-moving hurricane across tree communities,” the website says.

A park may have more than 50 trees removed at one time, as a result, because it is more effective to remove all the infested trees at once, rather than removing a few at a time, according to the county parks website.

The county says it does not have the staff or budget to replant trees at the rate they are removed. However, it is not always recommended to replant immediately after removal as the ground may not be suitable for replanting and it is best to stagger planting to achieve age and species diversity.

Milwaukee County Parks removed 42 trees from Kern Park in Riverwest this past winter, 31 of which were ash trees infested with EAB, according to Everett.

Laub, who has lived in Riverwest since 1997, frequently visits Kern Park to exercise and used to bring her son there when he was young.

“I usually see families having picnics and using the playground and community groups have celebrations there and there’s always people playing basketball and baseball…,” Laub said.

She said that Kern Park is a fixture of the Riverwest community and, as a nature lover, losing so many trees changes the feel of the park.

“I feel like we should have a tree funeral for them,” Laub said in a Facebook post. “Thank you trees! Goodbye!”

Moving forward, the goal is to increase species diversity in Kern Park to protect against invasive species, Everett said; the objective is to replant within the next one to three years.

According to Milwaukee’s Forestry Services, EAB was confirmed in Milwaukee in 2012, after being in the midwest since 2002.

Ash trees are a genus of trees. A genus is a rank used in biological classification that falls above a species and below a family; there are between 45 and 65 different species of ash trees, according to the Britannica encyclopedia.

According to the department of entomology at UW-Madison, there are four native species of ash trees in Wisconsin: green, white, black, and blue ash. Ash trees usually are over 50 feet tall and are identified by two key characteristics. The first is an opposite branching pattern, which means two branches come off the main stem directly across from each other. Secondly, leaves on ash trees grow in odd numbers, usually in groups of five, seven or nine.

Ash trees are often used as street trees because they are tolerant of urban environments, according to the department of entomology, which is why there are an estimated 5 million in urban areas of Wisconsin. 

Milwaukee’s Forestry Services website has an interactive map showing where EAB is in the city, as well as resources for identifying, treating and removing infected ash trees. Likewise, the Wisconsin Department of Natural Resources has a website with EAB information and guidance.

The Journal of Applied Ecology is part of the British Ecological Society. The researchers in the study are Emma Hudgins and Brian Leung with McGill University in Montreal, Frank Koch with the US Department of Agriculture Forest Service and Mark Ambrose with North Carolina State University.

The study was funded by grants from the Natural Sciences and Research Council of Canada and from North Carolina State University.

An EAB-like insect that targets maple and oak trees could kill 6.1 million trees and cost $4.9 billion over a 30-year period if one were to emerge, the study concluded, which is why staying on the lookout for future invasive insects is important.

“These results can hopefully provide a cautionary tale against planting a single species of tree throughout entire cities,” the British Ecological Society said in a press release.