Mar. 9—BENZONIA — Hemlock woolly adelgids continued their spread north, this time forcing the closure of a trail in Sleeping Bear Dunes National Lakeshore.

Old Indian Trail and the area surrounding it will be closed to visitors for several months as park employees and others work to identify the extent of the invasive insects’ spread, park Superintendent Scott Tucker said. Keeping out the public for now aims to help stop the spread — hikers or their pets could inadvertently take an adelgid home, or spread them from trail to trail.

“We won’t have control over transfer of these things from wind and animals, but we can get ahead of it by at least treating the trees now and keeping the public out to take that one variable of transmission off the table,” he said.

August is the earliest that park administrators hope to reopen the closed section, said Julie Christian, the park’s natural resources division manager. That’ll give survey crews time to search an 800-foot circle around each infested hemlock and tag each additional hemlock they find. Treatments will involve injecting a pesticide into the trunks of hemlocks big enough to get the shot, allowing the chemicals to spread through the tree sap — smaller trees get a different pesticide sprayed around their trunks.

It takes time for the pesticides to spread throughout the trees, but once they do they can provide at least four years of protection from the tiny, sap-eating insects, with some studies showing that can last up to 10, Christian said.

It’s the latest hemlock woolly adelgid (say: “a-DELL-jid”) infestation to hit the park since the insect was found on a single tree in Platte River Campground a few years back. This time, it’s on property adjacent to golf course Crystal Downs, where surveys found an infestation in 2023.

“So that infestation was found last year, and we did a lot of heavy surveys on park property and we assisted in the Crystal Downs area too, but we did a lot of surveys of these exact areas last year and didn’t see any evidence of them,” she said.

The fact that the insects were as far blown in as they were prompted the NPS to close off a larger area, and now Christian and others are weighing whether to check every hemlock in a 1,000-foot radius from impacted trees.

It may seem like a lot of effort for such a tiny insect, but these bugs steal nutrients from hemlocks by feeding at the needle base, according to the NPS.

By literally sapping a tree of its health, adelgids can kill it in as little as four years if the insects are numerous enough.

Hemlocks serve important roles where they grow, giving food, shelter and shade to birds and mammals. That’s particularly welcome in early spring before other plants or tree leaves regrow.

These evergreens grow along stream banks, locking down soil with their roots and cooling water temperatures with their shade — more perks for fish and invertebrates that depend on clear, cooler waters.

Christian said surveys in January and February had crews searching for the insect’s namesake white, waxy coating that looks like specks of cotton, which they form over winter. In summer months, the adeglids will become more active.

The mild winter and recent warmups might accelerate that life cycle change, Christian said, adding park administrators put the closure in place as soon as they could.

“We’ll be out looking through the sites to monitor the efficacy of the treatments, and so we’ll be looking for the changes in the life stages as well,” she said.

Northwest Michigan Invasive Species Network will help the NPS find out the extent of the infestation, while working to find and contain others, said Zach Seguin. He’s an invasive species specialist with the network, which covers one of several Cooperative Invasive Species Management Areas across the state.

Surveys so far have shown the infestation has spread beyond Crystal Downs and Sleeping Bear Dunes National Lakeshore, Seguin said. That includes some areas on the southwest end of Crystal Lake, and along the lake’s north and south shores.

While Seguin said he wasn’t sure if the mild winter impacted the insects’ life cycle, survey crews with the invasive species network are making certain they’re decontaminating themselves just in case.

“This time of year right now, there should be very minimal, if any, risk of spreading it … with the warmer weather, we just want to make sure we’re being extra cautious,” he said.

Hemlock woolly adelgids showing up in Sleeping Bear Dunes National Lakeshore won’t have any impact on a quarantine the state Department of Agriculture and Rural Development expanded in August 2023, said Rob Miller, the department’s invasive species prevention and response specialist. That regulates the movement of hemlock and tiger-tail spruce trees, forest products and stock from the quarantine area.

State and federal agencies, in coordination with local partners, have been dealing with hemlock woolly adelgids in Michigan for a number of years, Miller said. That includes the ongoing surveys and treatment programs in Benzie County.

“We’ve been seeing a slow progression of hemlock woolly adelgid moving north,” he said. “Unfortunately this is part of that progression.”