The City of Madison has begun its annual search for an insect predator that is responsible for killing ash trees across the nation.

According to Madison Parks spokesperson Laura Whitmore, the culprit of these tree killings is the emerald ash borer insect. The insects’ larvae kill trees by tunneling into the wood and feeding on the tree’s nutrients, ultimately depleting them.

University of Wisconsin entomology professor Richard Lindroth said the larvae of emerald ash borers further decimate the ash tree population by burrowing underneath the bark and disrupting the vascular systems in the tree. The beetles disrupt the flow of fluids from one part of the tree to another, which can be detrimental to tree growth and survival, he said.

Whitmore said although these beetles can only fly half a mile, they are able to travel quickly through the transportation of lumber and firewood.

There are around 20,000 ash trees planted between the street and the sidewalk across Madison, comprising 25 percent of the city’s tree population, Whitmore said. She added they are called “street trees” because they can withstand numerous stresses imposed on plants in urban areas, including pedestrian and vehicle traffic as well as salt.

“This tree holds up really well to those conditions, which is why it was chosen to be a good street tree at the time,” Whitmore said. “But now a beetle has come in and threatened the population of ash trees.”

Lindroth said the killing of ash trees is a serious concern because they are vital to the survival of Wisconsin’s wood products industry and forest ecosystems.

“Ash trees are a very important part of our forest communities, and ash trees, as far as we know, show no natural resistance to emerald ash borers,” Lindroth said. “So once they’re infested, they will die.”

According to Whitmore, emerald ash borers were recently discovered in the Milwaukee area, signaling a cause for concern for Dane County. She added early detection methods for the beetles have been enacted, such as branch sampling.

Branch sampling is a process that includes cutting off some branches of ash trees in Madison and peeling away the bark of those branches to look for emerald ash borer larvae, Whitmore said. The process can take several months.

According to a statement from Madison Parks, the forestry staff will sample five ash trees for beetle larvae in designated plots that are spaced apart at one-mile intervals.

Whitmore said branch sampling is a way for the city to be proactive about this problem rather than reactive. She stressed the importance of the measure because the beetles have killed tens of millions of trees, which equates to tens of millions of dollars lost.

The statement said Madison established a task force in 2008 to combat the beetle concern. The task force provided preventive recommendations to Mayor Paul Soglin and City Council last November.

Emerald ash borers are not the only pests the Madison Parks Department combats, Whitmore said. Problems with Japanese beetles and gypsy moths are also addressed every year. These insects defoliate mostly oak trees from April through the end of summer, she added.