It's an odd bug from Asia that doesn't need males to reproduce and spends almost all of its life in one spot, eating and eating, as it encloses itself in a waxy, wool-like cocoon for protection.

And it poses a major threat to Michigan's 170 million hemlock trees, with implications far beyond forest health — also threatening the health of rivers and streams and ultimately the state's recreational fishing industry.

The hemlock woolly adelgid has been a problem in the eastern U.S. for decades, but the invasive species had been held largely in check in Michigan. Now, that seems to be turning for the worse.

On June 21, the Michigan Department of Agriculture and Rural Development announced a quarantine prohibiting the movement of hemlock trees out of four west Michigan counties — Allegan, Muskegon, Oceana and Ottawa — after larger-scale hemlock woolly adelgid infestations were discovered in hemlock tree stands.

Seeing the devastation happening in eastern states, the Michigan Department of Agriculture and Rural Development installed a pre-emptive import quarantine on hemlock trees coming in from affected states in 2002. But the adelgid still made its way here, first detected in Harbor Springs in Emmet County in 2006 and popping up in other counties ever since.

Previous infestations were limited to individual trees or small groups and were turned back through the use of pesticides, said Mike Bryan, a plant industry specialist with the state agriculture department.

"These recent detections are more widespread," he said. "Once you get beyond a single property, it becomes difficult to control."

The tiny, aphid-like adelgid — oval-shaped, dark gray females are only about 1 millimeter long — is native to Japan, and was inadvertently transported to the U.S. in the 1950s. It's hard-wired to want to feed on hemlock trees, turning down even other evergreen species.

An adult hemlock woolly adelgid, removed from its host tree and cleaned for magnified imaging. (Photo: Kelly Oten, North Carolina Forest Service,

"This pest has a very specific taste," said Jennifer Holton, spokeswoman for the state agriculture department.

While the adelgid attacks western hemlock trees, it's the eastern hemlock that's particularly harmed by them.

"They have sucking mouth parts, and they attach to the tree permanently; they don't really move around," Bryan said.

The bug's target? The base of hemlock needles.

"Think of its mouth like a long crazy straw," said Deborah McCullough, a professor of forest entomology at Michigan State University. "They move it through the needle tissue down to the shoot, and they suck nutrients out."

Eventually, starved needles and branches die. And for eastern hemlock trees, adelgids stimulate a hyper-sensitivity response, like an allergic reaction, McCullough said.

"Trees start dropping needles; buds die," she said. "Eventually, the whole tree can die."

A band of dead hemlock trees, killed by the invasive hemlock woolly adelgid, is visible in the forests of the Great Smoky Mountains in North Carolina. (Photo: Ignazio Graziosi, University of Kentucky,

The adelgids have two life cycles per year, in winter and spring. Reproduction is asexual; no males involved.

"The female feeds, she matures and she produces eggs, and the eggs are all genetically identical to her; they're like sisters," McCullough said. "That means all it takes is one adelgid and you start a whole new colony. They don't need to find a mate."

As the adelgids feed, they secrete fine, white strands of wax that eventually encase the insect. The bug then lays its eggs within the protective wax case, and upon hatching, the new crawlers can move around and find their own place on the tree to settle and start the process again — or be blown to other locations by the wind, or carried by birds or animals.

Once they settle in a spot, the adelgids spend the entire summer in a dormant state, until beginning to feed again in the fall.

A hemlock woolly adelgid in its immature crawler phase. After it moves and find a suitable spot on a hemlock tree, the bug attaches at the base of a needle with sucking mouth parts, never to move from the spot again. (Photo: Kelly Oten, North Carolina Forest Service,

McCullough has a theory as to why the adelgid boom is spreading from the Lower Peninsula's far west.

"If you remember the 2013-2014 winter, when it was so bitterly cold ... My theory is a lot of adelgids were killed during that winter, and being very close to Lake Michigan may have protected some of them," she said. "Because you get the lake-effect snow and the temperatures don't get quite as cold there."

A couple of mild winters in a row since has the bug on the move, she said.

"As an infestation builds, you have a better chance that birds, animals and even the wind will spread the adelgids when they are in their crawler phase," she said.

The hemlock tree's nemesis has ripple effects throughout the ecology.

"Hemlocks are kind of picky about where they grow," McCullough said. "They tend to grow in certain kinds of soils. They are often near waterways, streams and rivers, and they tend to be in areas where there is a high water table."

As hemlock trees die near streams, erosion increases. The water becomes less clear. Hemlock foliage is more acidic, meaning dead needles falling into a stream can affect the water's pH levels. And with more sunlight reaching the stream through the forest canopy, water temperatures can increase.

All of this impacts the tiniest aquatic organisms in a stream — and that affects the fish that feed upon them, on up to the prized trout that help fuel Michigan's multibillion-dollar fishing tourism industry.

"A trout stream has trout in it because it's very pure water, very clear water, and has organisms in it that trout feed on," Bryan said. "If you kill off the hemlock trees along the banks of the stream, it's got a lot of effects on forest undercover and stream quality."

A stand of hemlock trees shows the damage from a hemlock woolly adelgid infestation. (Photo: Jason Van Driesche,

While researchers are studying possible biological controls on the adelgids, the best defense method now is early detection and preventing the spread of the invaders.

"That's kind of the idea, to see if we can hold the fort to some degree," McCullough said. "Most of the state's hemlock trees are in the northern Lower and Upper Peninsula — that doesn't mean the hemlock trees in the southern part of Michigan are not important; but they are not as widespread.

"So we look after the northernmost populations and try to push them back."

Michigan residents who notice white, waxy material at the base of the needles on hemlock trees, should not move potentially infested branches from the site, but should instead take photos, note the location of the affected trees and report it to the Michigan Department of Agriculture and Rural Development via e-mail at, or call the MDARD Customer Service Center at 800-292-3939. Potential adelgid-spotters can also use the Midwest Invasive Species Information Network (MISIN) online reporting tool at

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