EMERALD ASH BORERS A SILENT INVADER

BY HOLLY DODGE

The emerald ash borer, Agrilus planipennis, is an invasive beetle native to Asia that has caused extensive damage to ash trees across North America since its discovery in Michigan in 2002. The adult beetles are metallic green and about half an inch long, but their larvae pose a significant threat. These larvae tunnel under the bark of ash trees, disrupting the tree’s ability to transport water and nutrients, eventually leading to the tree’s death. Infested trees often die within three to five years of the initial infestation.

Emerald ash borers spread primarily through the movement of infested wood products, such as firewood. This rapid spread has led to quarantines in many areas to prevent the further spread of the pest. Despite these measures, the beetle has continued to advance, killing tens of millions of ash trees across the United States.

The emerald ash borer’s impact is ecological, economic, and social. Ash trees are common in urban and forest areas, and their loss significantly alters ecosystems and landscapes. The cost of removing and replacing dead trees can be substantial for municipalities and homeowners alike.

Efforts to manage and mitigate the spread of emerald ash borers include using insecticides, biological control agents, and public awareness campaigns. However, the fight against the emerald ash borer is ongoing, and proactive measures are essential to protect remaining ash tree populations.

According to North Mankato Water & Parks Superintendent Duane Rader, the city first encountered the invasive emerald ash borer in 2022 when an infected ash tree was discovered on Center Street. Once the tree was removed, the telltale signs of the borer’s presence were unmistakable, revealing extensive damage caused by the insect’s boring activity. This discovery set off action aimed at managing the threat and mitigating its impact on the city’s ash tree population.

In 2023, North Mankato received a grant from the Department of Natural Resources (DNR), which stipulated that for every infected tree removed, another must be planted. This initiative led to the removal and replacement of 100 community ash trees. The city’s proactive approach continued into 2024, with another 99 trees removed and replaced. The DNR grant has facilitated these efforts, allowing the city to contract larger removal projects.

North Mankato has approximately 1,100 ash trees along boulevards and in parks. Homeowners are responsible for removing infected trees on their property and must act soon after signs of infestation. Unfortunately, by the time symptoms are visible, the borer has likely been present for three to five years, and the tree’s health has already been affected. As the infestation progresses, branches become brittle, entire trees can topple, and large limbs may fall, posing great risk to homes and vehicles. Residents are urged to act sooner rather than later.

While treatment options are available, such as trunk injections, these must be administered every two to three years and can become quite costly over time. Removing and replacing the tree may be the more practical and economical choice for many.

Starting in 2025, North Mankato will focus on removing additional boulevard trees. The city will no longer replace these trees, but homeowners can replace their trees at half the cost in November. The city offers a selection of five to six different tree species for replacement, which are planted in May and come with a one-year warranty. This initiative aims to encourage residents to take an active role in maintaining the urban trees.

The removal of large, mature trees has understandably upset some residents, as these trees provided significant canopy cover and shade. However, the city’s strategy aims to be as aggressive as possible given the resources available, ensuring safe and sustainable community trees for the long term.

The North Mankato city office is committed to addressing resident concerns and providing valuable information. The city employs knowledgeable staff, including a former forester and an experienced tree crew member, who can offer advice and assistance. Residents are encouraged to seek support and guidance on managing emerald ash borers and maintaining their trees.