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Riverway Board hears Emerald Ash Borer report
DNR Michael Hillstrong
DNR FORESTRY HEALTH SPECIALIST Michael Hill-strom made a presentation about Em-erald Ash Borer (EAB) to the board of the Lower Wisconsin State Riverway last Thursday in Muscoda. Hillstrom de-tailed the increase in prevalence of the pest, the threat to the state’s forestlands, DNR research about how to fight the pest, and strate-gies for landowners to manage their woods for EAB.

MUSCODA - The board of the Lower Wisconsin State Riverway (LWSRB), Executive Director Mark Cupp, and eight other citizens gathered at the group’s meeting on Thursday, Dec. 12 in Muscoda. On the agenda was an Emerald Ash Borer update, provided by DNR Forestry Health Specialist Michael Hillstrom. Also present from the DNR was Mike Finlay, Division of Forestry Team Leader for Grant, Crawford, Vernon, Richland, Iowa and Sauk counties.

“Emerald Ash Borer (EAB) is expanding rapidly in Southwest Wisconsin,” Hillstrom told the room. “At this point, the infestation is taking two years instead of five years to kill a stand, and the problem is that in its aftermath it leaves lots of dead hazard trees on the landscape.”

Hillstrom showed a map indicating the year that EAB was first identified in Wisconsin counties. Locally, it showed up first in Crawford and Vernon counties in 2009, in LaCrosse County in 2011, in Grant and Monroe counties in 2014, and in Richland County in 2015. 

The earliest identification of the pest was in 2008 in Ozaukee County, followed by Kenosha, Racine and Washington counties in eastern Wisconsin.

“Of course this map is becoming out of date because funding for trapping has been cut in recent years,” Hillstrom said. “We plan to conduct another aerial survey in 2020, which will give us some more updated information.”

“EAB is always emerging two-to-four years ahead of where the DNR first becomes aware of it,” Hillstrom said. “One easy way to identify an ash tree that is infested will be to notice a tree that appears more blond.”

An estimated 898 million ash trees are in Wisconsin's forestlands as part of northern hardwood, oak-hickory and bottomland hardwood forests. Ash species represent 7.8 percent of all trees in Wisconsin’s forests (counting all live trees one inch in diameter or larger). Black ash is the most common ash species and represents four percent of total tree density. White and green ash represent two and one-point-eight percent of total tree density, respectively. 

EAB attacks all native species of ash in Wisconsin, including white, green, black and blue ash. While other woody plants, such as mountain ash and prickly ash, have “ash” in their names, they are not true ash species and therefore are not susceptible to attack by EAB.

What is EAB?

According to the Arbor Day Foundation website, EAB is originally from Asia, and was first discovered in the Detroit area in 2002. It is believed to have entered the country on wooden packing materials from China. The bright metallic-green beetle may be smaller than a dime, but it is capable of taking down ash trees thousands of times its size. Adults are typically one-half-inch long and one-eighth-inch wide. Eggs are extremely small—approximately one-twenty-fith-of-an-inch, and are reddish-brown in color. Larvae are white, flat-headed, borers with distinct ‘bell-shaped’ segmentation.

Adults usually emerge in mid- to late-May from infestations to the trees during the previous year (earlier if the weather is warm), with females laying their eggs shortly after. The larvae bore into the ash tree and feed under the bark, leaving ‘S-shaped’ tracks in ‘galleries’ visible underneath. 

The feeding disrupts the tree’s ability to transport water and nutrients, resulting in dieback and bark splitting.

What kills ash trees?

According to Hillstrom, there are multiple things that can kill ash trees in addition to EAB. He said that often when a dead stand of ash trees are found, multiple different types of predators will be present.

“We once found a tree that was infested with EAB on one side of the tree, and bark beetles on the other side,” Hillstrom remembered. “Once a tree is weakened by an infestation of one species, that can make it more vulnerable to additional infestations.”

Woodpeckers are predators of EAB, and studies have shown that when EAB populations increase, woodpecker populations increase as well. However, according to Hillstrom, woodpecker predation alone is not enough to knock EAB back to a sustainable level.

Hillstrom also addressed another prevalent myth that cold temperatures will kill EAB. He said it is true that temperatures colder than negative 20 degrees can have some impact on EAB, but it is much less than has been reported in some media outlets in recent years.

“You have to remember that there is no wind chill inside of a tree,” Hillstrom said. “An early cold snap, before the larvae have become accustomed to colder temperatures actually has as much potential to kill as extreme cold in winter.”

Hillstrom said that the timing of cold weather is crucial to its impact on EAB, and that microclimates matter. He said that larvae at the bottom of a tree may literally be more vulnerable than larvae at the top, and the same could be true of larvae on trees higher up on a hill versus at the bottom of a valley.

Silviculture guidelines

Hillstrom told the group that the DNR has just updated its silviculture guidelines for EAB. While the updated guidelines contain many points, he said that the main point is the first one on the list – “Anywhere in Wisconsin, manage forests for EAB as soon as practical.”

On the DNR’s website at, there are many resources for landowners concerned about the pest and its impacts on their forested land. Hillstrom said to be careful if your property is enrolled in the Managed Forest Lands (MFL) or Conservation Reserve Program (CRP) programs because any change in the productivity of your woodlot, should it become infested with EAB, could affect a landowners tax credits.

The website also contains a stand management decision-making model about how to address an EAB infestation, resources for dealing with wet stands, and a lowland reforestation species guide to help decide how best to replace ash stands with other species that can survive in the environment.

Reversing the trend

There has been a federal quarantine against moving wood from ash trees, but given the wide spread of the infestation, Hillstrom anticipates that quarantine will go away in the next few years. The State of Wisconsin has had a quarantine in place statewide since 2018, and it is legal to move the wood anywhere in the state at this point.

As far as factors that can help to prevent the spread of EAB, Hillstrom says that no one tactic will work. He said that it will need to be a multi-pronged approach involving identification of resistant trees, insecticides, and biocontrols.

“There’s been a lot of debate about genetically modified trees now that the GMO Chestnut Tree has been released on the east coast,” Hillstrom said. “I don’t want to get into that debate, but selecting trees that have survived an EAB infestation to be used in a breeding program is one tool in our toolbox.”

Insecticides, Hillstrom said, can be part of a strategy, but are not an option for broadscale application. He said that if someone has a 30-year-old ash tree in their back yard shading their deck, then it might make sense to use an insecticide. But for a forest situation, it is not a realistic option.

Perhaps the most promising strategy is that of biocontrols. The department has experimented with the release of three species of tiny wasps for the last six years, and this approach has shown some promise.

The species are:

• Tetrastichus planipennsi (released in bolts)

• Oobius agrili (released in oobinators)

• Spathius galinae (released in cups)

Some of the earliest releases of these species of wasps occurred in Vernon and Crawford counties between 2011 and 2018. Hillstrom explained that a lot of factors go into selecting a release site – he said they look for a location where at least 20 percent of the tree population is ash, where there is a river corridor or other open area for the wasp to spread, and for a stand which will survive for at least five years so there is time to see what the impacts of releasing the wasps are on EAB.

Ash tree harvest

Hillstrom seemed especially frustrated with the variety of factors that may cause landowners to delay management of infested stands of EAB.

“The tariff scares have depressed wood prices recently and the weather last winter was terrible for logging,” Hillstrom explained. “Even if the prices aren’t currently optimal, a landowner is better off participating in a staggered harvest rather than waiting for prices to go up and then entering a flooded market.”

Ron Leys said that he had driven up the Lake Michigan coast from Milwaukee to Sheboygan, and the devastation of the forests along the shore was “terrible to see.” He asked Hillstrom if Southwest Wisconsin just has less ash than in the eastern part of the state.

“Some areas in Southwest Wisconsin have a lot of ash, especially in the lowlands,” Hillstrom said. “In the uplands, there tends to be more diversity of tree species.”

LWSRB Executive Director Mark Cupp further  elaborated that “there are some pockets of ash in the bottom lands of the Riverway, but it only accounts for about 15-20 percent of the species present. DNR Forester Mike Finlay agreed with Cupp, stating that “there is quite a bit of diversity in the Riverway bottomlands.”

Hillstrom said that he is always willing to come out and take a look at a landowner’s stand. Hillstrom can be reached at 608-513-7690 or by e-mail at