Tiny invasive insect attacking hemlock trees in Michigan

A tiny insect threatens more than 170 million trees in Michigan, and the cascading ecosystem effects could be catastrophic. MSU researchers are working to stop it.

A tiny insect — no larger than 1.5 millimeters in length — threatens more than 170 million trees in Michigan, and the cascading ecosystem effects could be catastrophic. Michigan has already experienced this with previous invasive pests, but this latest one — hemlock woolly adelgid (HWA) — threatens not only trees but the health of aquatic species and wildlife as well.

Deb McCullough, professor with the Michigan State University (MSU) Department of Entomology, knows the enormity of the challenge facing the state. She has been through it before with the emerald ash borer and the gypsy moth. “It’s a tree killer,” said McCullough, in reference to HWA.

It preys on hemlock trees, which are vitally important to the ecological health of aquatic species and wildlife throughout Michigan. Hemlocks provide dense shade during periods of extreme weather that helps to protect animals. They typically grow along the banks of lakes and rivers, where they help to provide nutrients to soils and waterways.

In the eastern United States, the death of hemlock caused by hemlock woolly adelgid has altered soil temperatures, nitrogen cycling and decomposition rates. It has also increased erosion, raised water temperatures and reduced water quality.

“People might assume that when you lose hemlock, those birds and animals will find somewhere else to go, or another tree will take its place,” McCullough said. “But hemlock is unique. Losing big, mature hemlock trees will affect the function and quality of these ecosystems.”

The HWA is native to Asia and was transported into the Pacific Northwest in the 1920s. Both Asian hemlock and western hemlock are highly resistant and are rarely killed by the insect. Eastern species of hemlock, however, have been devastated by the adelgid. The pest was first reported in the eastern United States in 1951 and has since spread to 19 states from Georgia to Maine, killing hundreds of thousands of trees.

Scientists suspect that the pest was brought into Michigan on hemlock trees imported from nurseries. The adelgid may have been in Michigan for about 15 years, but if so, the populations stayed low and went unnoticed. Small, localized infestations near the Lake Michigan shoreline were discovered in 2015, and additional adelgid hot spots have since been found in four western Michigan counties.

The insect does not fly, but its eggs can be blown by the wind or transported by birds or animals or on clothing. The insects feed at the bases of hemlock needles, sucking up moisture and nutrients from the woody shoots. Insects secrete strands of white wax as they feed. This wax or “wool” helps to protect the insects. Populations can increase rapidly because two generations of the pest occur each year.

The Michigan Department of Agriculture and Rural Development (MDARD) has implemented an interior quarantine in infested parts of Allegan, Muskegon, Oceana and Ottawa counties, limiting the movement of nursery stock and hemlock branches, trees or similar material in and out of the four-county area.

The challenge posed by the adelgid requires cooperation from several entities, and McCullough said the partnerships formed at the local, state and national levels have been invaluable.

She communicates with other forest entomologists throughout the eastern United States who have been confronted with the HWA firsthand, learning about identification, evaluation, insecticide treatments and biological controls.

McCullough is also collaborating with officials at the Michigan Department of Natural Resources, forest health specialists with MDARD and the U.S. Forest Service, and land managers in western Michigan to develop strategies and tactics to respond to the infestations. McCullough and other MSU researchers are working to develop a statewide hazard map that will identify where hemlock is abundant and the pest is most likely to thrive. Those areas can then be prioritized for HWA surveys and mitigation efforts to reduce impacts of the pest.

MSU climatologist Jeff Andresen is analyzing winter temperatures going back to 1980 to identify regions of the state where the winter climate is likely to cause HWA mortality in at least some years as well as areas where the moderating influence of Lake Michigan might allow HWA populations to thrive.

McCullough and her colleagues will also be monitoring temperatures at various tree heights and directions to see how temperatures vary during the winter depending on exposure to sunlight or snow depth.

“We want to use as much data as we can to get a better sense of where and how quickly the adelgid is going to impact hemlocks throughout the state,” she explained.

Another big part of McCullough’s work is outreach and education.

“We are working with several partners to get the word out about HWA. The West Michigan Cooperative Invasive Species Management Area will play a key role in carrying out HWA surveys and in organizing and informing property owners about the threat of the adelgid and the treatment options that can protect trees.”

She will also be working with partners in MSU Extension on outreach efforts to inform residents about the pest and to encourage the public to report sightings of HWA on the Midwest Invasive Species Information Network. That spirit of cooperation helps to drive McCullough.

“We have a long history of working together to solve problems in Michigan,” McCullough said. “Think about Tahquamenon Falls without all those beautiful hemlock trees. We have so many forests that have been affected by invasive pests such as the emerald ash borer, beech bark disease and oak wilt. Now we have another pest that could knock out hemlock. Where does it stop? There’s rarely a vacuum in nature. Something will grow there, but it will change those systems, and the forests become less diverse and less resilient.”

Despite the daunting task ahead, McCullough said she is committed to fighting as hard as possible to protect Michigan’s hemlock trees, no matter how long it takes.

“I learned a long time ago that, if you’re going to do anything related to forests, you better have a long-term perspective — rarely does anything happen fast.”

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