The hemlock woolly adelgid is almost impossible to see with the naked eye. But the tiny insect causes lots of damage to hemlock trees and their surroundings. That’s likely to get even more serious as a warming climate allows it to move further north. Caitlin Whyte went to Ithaca, New York to learn about plans to fight back.


On a rainy day, City Forester Jeanne Grace takes me on a tour of the City Cemetery where tall, evergreen trees hang over many of the graves. Hemlock trees.

The cemetery has the peace and quiet of any cemetery, but if you take a closer look at the hemlocks,  you’ll spot the hemlock woolly adelgid.

“They’re very small,” Grace says. “I don’t know if you’d be able to see it even… you need like a microscope or a hand lens to see one.”

This hemlock tree, like many in the Great Lakes region, is coated with this white, fluffy material, right where the needles meet the branch. It looks like a light snow has fallen.

LISTEN: “Scientists Try to Fight Back Against Tree-Killing Bug”

Once the tiny bug makes its way to a hemlock tree, it sticks its mouthparts into the branch, hides in this coating and essentially sucks the life out of the tree.

“So when you have millions of these on a single tree, that’s making millions of little wounds in the tree that the tree then tries to wall off,” Grace says. “And then it just kind of plugs up the whole vascular system of the tree.”

Think of it like little scabs preventing nutrients from freely flowing through the tree.

“To come up with a simple response especially to a complex interaction like a tree and an insect is just a fool’s game. But, that being said, I’ll admit to being a fool.”

The hemlocks are easy to protect in a contained space like the cemetery, and the city uses an insecticide. But it’s harder in areas like gorges, where many hemlocks grow and are essential to the surrounding ecosystem.

“They can survive without a whole lot of light so they can live deep down in the gorges where they may not get full sunlight all the time,” Grace says, moving toward a nearby gorge. “And the kind of geological situation we’ve got here is all this really loose shale stone that slowly erodes as the gorges get bigger and bigger.

“And what the trees do is they kind of create a network that holds all of that soil together. And kind of holds the gorge and prevents it from eroding as fast as it would without the trees there.”

So, lose the hemlocks and you eventually lose the gorge.

But scientists are looking for ways to fight back, as the hemlock woolly adelgid spreads across the Great Lakes region.

Mark Whitmore, an entomologist at Cornell University in Ithaca, suspects that warmer winters are contributing to the insect’s spread.

But establishing cause-and-effect relationships is complicated, he says. “To come up with a simple response especially to a complex interaction like a tree and an insect is just a fool’s game. But, that being said, I’ll admit to being a fool.”

He says hemlock woolly adelgids are sensitive to cold. But last winter, one of the warmest on record, populations went through the ceiling.

“I was amazed that they could go through actually three years of very high mortality and still bounce back,” Whitmore says. “And I expect them now, with the mild winter we had and perhaps another mild winter, the populations to begin spreading much more rapidly.”

The laricobius nigrinus beetle. Photo: Caitline Whyte

Whitmore’s new 1.2 million dollar lab is dedicated to protecting hemlocks.

This is where the laricobius nigrinus comes in. It’s a beetle just slightly bigger than a poppy seed.

In Japan, where the hemlock woolly adelgid originates, these beetles are a natural predator, and keep the adelgids in check.

Under a microscope in Whitmore’s lab, a branch covered in hemlock woolly adelgid fuzz sits in a dish. The laricobius nigrinus is tearing apart the adelgid fluff, like a kid unwrapping presents on Christmas morning. When it finds and eats the adelgid inside, it moves on to the next one.

Still, Whitmore says we will never get rid of the hemlock woolly adelgid.

They travel by wind or perhaps on birds and deer. And they’re asexual, so all it takes is one to start a new population.


Great Lakes Today is a collaboration led by WBFO Buffalo, ideastream Cleveland and WXXI Rochester.