Emerald ash borers may be coming to a tree near you.
Since the discovery of an emerald ash borer infestation near Detroit, Michigan, in 2002, the assault on ash forests has become an epidemic. Michigan alone has lost more than 30 million ash trees, and 35 states and five Canadian provinces have been affected. As the blight mounts, the financial, aesthetic and ecological consequences will be enormous.
But an article in a December edition of the St. Paul Pioneer Press reported a curious phenomenon: the rate of ash loss seems to be slowing. While the 30-million-tree loss in Michigan took place in ten years, a similar ten-year period in Minnesota (beginning in 2009) resulted in the loss of less than a third as many trees.
The emerald ash borer (EAB) is a metallic green insect with an adult life of only a few weeks. It originated in Asia and probably traveled here in shipments of wood. The beetles chew the ash leaves but do little other damage. It’s their larvae that wreak havoc on the host trees. After the larvae hatch from their eggs in mid-summer, they chew through the tree’s outer bark to reach an inner layer of tissue called phloem, which functions as a bloodstream of sorts to transport nutrients from the leaves to the roots. As they grow, the larvae interrupt the flow of water from the roots to the sapwood. Infected ash have a 100% mortality rate.
Why should the march of death by EAB slow, and why now? Local scientists think they have an explanation.
The first infestations in Michigan came as a surprise (although later testing suggests the problem began in the mid-1990s). By the time of its discovery, the EAB had already infected a large rural area, where detection is difficult. Scientists and arborists were scrambling to create treatments and identify practices to slow the problem. Minnesota’s advantage was time and experience. When EAB crossed the Mississippi River, scientists knew a lot more about how the infestation worked and could plan accordingly. Plus, it’s just colder here, and no insects like cold.
Treating the trees is fairly simple: systemic insecticides are applied around the base of the tree (trial and error have eliminated some treatments that harmed nearby flowering pollinator plants). But identifying stricken trees in the disease’s early, treatable stages is difficult. Rob Venette, an entomologist and director of the Minnesota Invasive Terrestrial Plants and Pests Center, said that sighting woodpeckers helps, because the birds find the EAB larvae to be a delicacy. He noted, “Woodpeckers are much better detectors than humans.”
So are the EABs in retreat? Venette said he didn’t want to be pessimistic about the future. “We have so many beautiful mature ash trees on the landscape, and those mature trees provide a lot of value to the community. They provide habitat for wildlife, cooling and shade and are really important for the elimination of wastewater. And currently, a lot of trees are being treated, and an unknown number, through sheer dumb luck, never get infected.”
Venette noted, “Particularly in the southern half of the state, the ash trees are likely doomed. How quickly that happens is anyone’s guess. I feel confident in saying that eventually, in the Twin Cities, without anybody doing anything, the EAB will kill all of our ash. In the northern part of the state, that’s more complicated. The naturally occurring ash in areas that get down to minus 20 degrees are more resistant to the beetle. What makes projections difficult are factors like climate change and rare events, like the polar vortex. We’re really good at predicting what happens in average temperatures, but we’re not as good with the extremes.”
Jeff Hahn, an entomologist with the University Extension Service, agrees. He notes that in 2019, the EAB has spread to five Minnesota counties that had no previous cases. “We had a time advantage over the Eastern states and the benefits of more research. The rate of spread is not as high as it was. But it’s not slowing down. An infected but untreated ash tree may live two to four years, but ultimately most ash varieties will be gone.”
On the home front
Meanwhile, the battle continues. Columbia Heights received a $100,000 grant from the Minnesota Department of Natural Resources in February. The city’s urban forestry specialist, Liam Genter, said the grant will allow them to continue the 2014 EAB management plan which calls for the removal of 300 publicly owned trees in the next three years, with trees being replaced one for one. He added, “Genetic diversity in the urban forest is the only real defense against tree pests … and diversity of species will be a primary goal for future planting. The city will focus on planting native trees such as Ginkgo, Northern Catalpa, Swamp White Oak, Blue Beech and others, and well-adapted nonnative species that are reliably noninvasive.”
While treatment of trees was not part of the grant funding, the city will continue to treat and preserve approximately 300 of the highest-value ash trees with funds appropriated for that purpose in the annual budget. The trees are getting injections at three-year intervals, approximately 100 trees each year.
Kevin Hansen, head of the Columbia Heights Public Works department, said at the March 2 City Council work session the
city estimates as many as 3,000 to 4,000 ash trees are on private property.
Hansen said the city has chosen four contractors to help residents remove their trees. They’ll pay an amount similar to what the city pays, based on the diameter of the trees. Previously, the city chipped in 25% of the cost of removal. With so many ash trees on private property, Hansen said, the program became cost prohibitive.
Genter said, “The city has featured EAB stories in the two most recent newsletters to residents. The stories detail the threat EAB presents to ash trees in Columbia Heights and stress the importance of making a decision about ash tree management soon. Further outreach efforts will include additional mailers, city social media activity, future newsletter stories, and city website updates.”
The Minneapolis Park and Recreation Board is responsible for the planting, maintenance and removal of all trees on public property in Minneapolis. MPRB’s forestry director, Ralph Sievert, said there were 40,000 ash trees (one-fifth of the total public tree population) when the Ash Canopy Replacement Plan began in 2014. Each year, 5,000 ash trees are removed and replaced with new trees the following year. He added, “There has been a slight increase in infested trees, but it is expected to dramatically increase over the next five to 10 years. We remove and replace only public trees. The big increase will be on private properties.”
Approximately 1,000 boulevard trees are being chemically treated by property owners; the city does not treat trees. Asked about outreach to property owners, Sievert said EAB information is on the MPRB website and will be provided at the annual Arbor Day celebration on April 24.
He noted, “Since we are removing all the public ash trees, we are not looking for new infestations. However, we do record them when we find them. We consider any ash tree to be infested or likely infested but not showing symptoms yet. We are actively looking for infested private ash trees so that they may be condemned for removal. This helps slow down the spread of the insect. In 2019 we condemned 826 private ash trees, 121 of which were in Northeast Minneapolis.”
Jeremy Gumke, St. Anthony Village Public Works Superintendent, said his department will be working with Hennepin County in the near future to conduct a tree survey. After that, ash trees will be removed and the wood will be ground into chips, using city-owned equipment. New trees (disease-resistant elms and other shade species) will be planted in September and October.
Christopher Linde, board vice chair of the St. Anthony West Neighborhood Organization (STAWNO), provided some history of the earliest Northeast infestation: “It all started in March of 2014 when 22 mature boulevard ash trees on 2nd Street NE were found to have EABs and were taken down. It was shocking to have that many trees gone on one street overnight. A once-beautiful tree-lined street was transformed instantaneously. I didn’t want to see it happen to other parts of my neighborhood. We knew we were infected, we’d lose additional trees and the pest would spread.
“We came up with a plan to treat 20% of the trees in the neighborhood. The goal was to keep some of the mature trees around longer while the newly planted trees grow. The neighborhood boulevard trees were all ashes. The MPRB’s Forestry department provided a map of all the ash trees in our neighborhood. After we chose our provider (Rainbow Tree Service), we had them evaluate 125 trees to treat out of about 500 trees, and later that summer, they began the first round of treatments.”
When MPRB decided to take down all the public-property ash trees in the city, STAWNO moved forward. Linde said the organization hoped, with the treatment they provided, some of the mature trees would be around long enough for the newly-planted replacement trees to catch up. Money for the work came from NRP funds. He added that the cost of the chemical has come down and the savings has made another round of treatments possible. STAWNO hopes to eventually (within ten years) have the homeowners whose boulevard trees are saved to take over the cost of the treatments.
The stately trees that surround the St. Boniface church parking lot, home in good weather to the Northeast Farmers’ Market, are all ash. When Rainbow Tree Service was gearing up to treat the selected boulevard trees in St. Anthony East, they used the trees around the parking lot for practice. Every tree was treated.
Below: Forestry workers were active this past week in Minneapolis felling ash trees. (Photo by Mark Peterson) Graphic showing EAB spread in Minneapolis by neighborhood (courtesy of Minneapolis Parks)
