He’s speaking of the hemlock woolly adelgid (HWA), an invasive insect that has devastated hemlock populations on the east coast since it arrived in 1951 from eastern Asia. The eggs are carried by the wind and the many creatures who call the trees home. Once they’ve landed, they hatch, crawl, feed, and reproduce prodigiously: One adelgid may lay more than 300 eggs twice per year.

The hemlock woolly adegid’s tell-tale egg masses. To slow the spread of these destructive invasive insects, the U-M will spend an estimated $50,000 treating the Arb’s hemlocks individually with an insecticide. | Photo: Mark Bialek

“They just reproduce without sexual reproduction,” says Michael Palmer, research coordinator and pest management specialist at Matthaei Botanical Gardens and Nichols Arboretum. “It’s like one of those horror movies.”

Left untreated, infestation is a death sentence for a hemlock—and by May of this year, HWA had made its way to the Arb. It was discovered not by an employee or a scientist, but by a visitor who recognized the distinctive woolly masses—they look like tiny cotton balls—that gather at the base of each stem in cold weather. This citizen scientist reported the infestation to the Michigan Department of Agriculture and Rural Development (MDARD). As a result, the trees will be treated and will survive.

Normally, Palmer explains, the Arboretum doesn’t treat insects. But they made an exception for HWA.

“The state of Michigan is trying to keep it at bay, to slow down the spread, because it could ruin hundreds and thousands of acres up north of here,” he says. “The eastern hemlock is a really important tree species in those ecosystems.”

MDARD estimates that there are 170 million eastern hemlocks in Michigan, most of them concentrated in their native habitat to the north, where they prevent erosion in dune ecosystems along the Great Lakes and keep inland waters cool enough for trout to mate. As the region’s only shade-tolerant evergreen, they also provide cover and habitat for animals year-round. 

Infested hemlocks are treated with insecticides that kill the HWA without harming the tree, while also providing years of future protection against the bug. 

“We apply them in a very responsible manner,” says Robert Miller, with the MDARD invasive species program. “It’s not like we’re broadcasting it across the landscape. You apply it to just one individual tree at a time.”

The insecticide is applied either to the bark or injected directly into the tree; because hemlocks are wind pollinated, pollinators like bees are unharmed. Although this is effective, Miller says, it’s not a long-term solution. These are still powerful poisons, and using them is costly, in terms of time, effort, and money—according to Palmer, the estimate to treat the hemlocks in the Arb tops $50,000. The fact that it’s applied to individual trees means it’s hard to scale up over large swaths of forest. Ultimately, Miller says, they’re buying time for researchers to design better solutions.

“It’s our hope that we can build better pesticides that would be more selective [in the insects they target], and allow more broad applications,” Cernak says. “The main body of research that we’re involved in right now is trying to be able to identify which trees are most susceptible.”

Cernak’s lab is comparing the chemical signatures of Asian hemlocks with ones here in the U.S. to see which, if any, of our trees might be more resistant. Other labs across the country are researching potential HWA predators that could be safely introduced to affected ecosystems. Predator insects from Asia are a no-go, but there is a beetle—“Little Larry, or Laricobius nigrinus,” says Cernak—that eats adelgids that feed on two western U.S. species of hemlock.

But there’s a snag: The white pine, Michigan’s state tree, also has an adelgid and a Little Larry beetle, Laricobius rubidus, that feeds on them. And in New York, the two Little Larry beetles are beginning to hybridize.

“So one worry is that this hybrid won’t want to feed on either,” Cernak says. “Or if we stop the Laricobius rubidus from feeding on pine adelgids, then in principle the eastern white pine could be in a bad way.” 

While Cernak and his team work toward a solution, Palmer advises Ann Arborites to avoid buying hemlocks imported from out of state—or at all. If you do buy a hemlock, he adds, check that the trees have been cleared for HWA through the Michigan Department of Agriculture. The other thing people can do, all three experts emphasize, is keep an eye out for HWA and report it to MDARD—at
michigan.gov/hwa—if they find it.

“Basically, you’re flipping over the underside of that branch, and you’re looking for those white little cotton balls,” Miller says. “November and December would be good times to look for it, all the way through the winter.

“We don’t want people doing that in the summertime, even from spring into late summer, because there’s a potential that they might accidentally spread the insects doing that.”