An invasive insect, the emerald ash borer (EAB), continues to kill ash trees across southern Wisconsin, including Dane County, and officials said tree mortality rates have risen.
The insect is metallic green and only attacks ash trees. While the adult beetles eat ash leaves, EAB larvae eat the trees’ inner bark, killing them from the inside out.
Michael Hillstrom, the invasive forest pest coordinator at the Wisconsin Department of Natural Resources, told The Daily Cardinal EAB is from east Asia and arrived in the United States in the early 2000s in packing crates.
Wisconsin has four types of ash trees: green, white, black and blue, according to PJ Liesch, entomologist for the University of Wisconsin-Madison Extension. Green and white are commonly found in urban and forested areas, while black is found in swampy areas in the north, and blue is exceedingly rare in Wisconsin, he said.
Hillstrom said that although EAB hasn’t reached the black ash yet, when it does it’ll be difficult to control.
“We’re going to have major issues, where you have large areas that there are no other trees in there other than black ash or very few other trees. So it’s really difficult to access those sites to do anything about it,” he said.
Wisconsin has a similar climate to EAB’s native range and is home to plenty of ash trees, making the state a perfect place for EAB to invade, according to Liesch. When EAB first arrived in Wisconsin in 2008, there were around 770 million ash trees in the state.
Liesch said this is in part due to the die-off of other species of trees, like elms. When Dutch Elm disease killed off elms, ash trees became the replacement tree in many urban centers such as Milwaukee, he said.
Why is EAB so threatening to ash trees?
EAB larvae make s-shaped galleries in the water and food-moving tissues of the tree, which blocks the trees from moving nutrients around, Hillstrom said. As more and more larvae eat away the inner bark, the tree starts to die.
“Each individual emerald ash borer larva might get up to about an inch long, but it’s death by a thousand paper cuts,” Liesch told the Cardinal.
EAB is also incredibly hard to detect in the early stages. Female beetles lay their eggs in the upper branches of the tree, making it hard to detect the presence of EAB until it’s too late, Liesch said.
Hillstrom said woodpeckers will eat EAB larvae, especially in the winter, by flicking off pieces of the bark in a process called “flecking.” However, woodpeckers also eat other insects, so officials have to check for other signs of EAB including D-shaped exit holes or S-shaped galleries formed by the larvae.
Currently, southern Wisconsin, including Dane County, has approximately a 99% mortality rate, according to Liesch. Mortality rates are highest where EAB has been present the longest, and northern Wisconsin still has lower rates for now, he said.
Hillstrom told the Cardinal that in the last few years, mortality rates have increased in Dane County, and the few ash trees left in urban areas are typically being treated for insecticide.
“The vast majority of ash [in Dane County] that are not being treated by humans to keep them alive are dead at this point,” he said.
How does EAB spread?
Both Hillstrom and Liesch agree the biggest factor contributing to the spread of EAB is human activity. The adult beetles can fly but only up to a few miles, according to Hillstrom. The main way EAB is spread is through firewood, which is why it’s important to heat treat, age and buy local wood, he said.
“If somebody from Madison’s ash tree dies and they take that wood and move it up to their cabin up north somewhere, the insects can still hatch out of that firewood and start a population in a new area,” Hillstrom said.
How can EAB be controlled?
Hillstrom said the most important thing to slow the spread of EAB is to create time. This allows cities and forests to respond to the dying trees as they slowly replace ash trees with other species.
High-value ash trees, such as ones in residents’ yards, can be treated with insecticide, according to Hillstrom. Additionally, approximately 10,000 ash trees in Madison are treated by the city multiple times a year.
Another approach to controlling EAB is biological control, a small silver lining, according to Liesch. Scientists found a small species of wasp that lives in the native range of EAB and only parasitizes EAB, he said.
Hillstrom said these wasps don’t sting, and they feed on the larvae of EAB. They are released into forested areas with a lot of ash trees to combat EAB invasions.
Researchers spent a lot of time and money to make sure these wasps wouldn’t harm native insects, Hillstrom said.
“Those [wasps] can kill 20 to 80% of the larvae in an area, so it’s not something that’s going to get rid of emerald ash borer, but it’s another piece of that puzzle of knocking the population back, so it doesn’t spread as fast, and those trees have a chance to get bigger and produce seed,” Hillstrom said.
Lindsay Pfeiffer is the science editor for The Daily Cardinal.
