By KEVIN WALKER
Michigan Correspondent
 
LANSING, Mich. — The Michigan Department of Agriculture (MDA) has recently confirmed small infestations of Hemlock woolly adelgid (HWA) in Emmett, Macomb and Ottawa counties.

In Macomb County just north of Detroit, a customer at a Wiegand Nursery brought suspicious looking tree stems into the store where she had bought a hemlock tree and showed them to an employee. This was last November.
“We want people to be aware of it because we want those eyes out there,” said Jennifer Holton, a spokeswoman for the MDA. “HWA is not easy to spot. That alert homeowner reported it to the nursery, the nursery reported it to MSU (Michigan State University) and MSU reported it to the MDA.”

The USDA’s Animal and Plant Health Inspection Service (APHIS) describes the adelgid as an insect pest destructive to ornamental and forest hemlock trees in the eastern United States. People who purchase or already own hemlock should be on the outlook for white, cottony masses at the base of the needles and report any suspected infestations to the MDA.

The adelgid uses its long, siphoning mouth parts to extract sap from hemlock trees. Originally from Asia, this variety of adelgid was first discovered in Virginia in 1951. Since then it’s spread over an area from Georgia to Maine, decimating hemlock trees throughout the region.

An APHIS report on HWA from 2009 paints a bleak picture of what the insect has done so far and what it can be expected to do to what’s left of the uninfested stock of Eastern hemlock.

“Heavy infestations have killed trees in as little as four years,” the report reads. “HWA now infests 18 states and it is estimated that approximately 40 percent of the T. canadensis (hemlock trees) range and 100 percent of the T. caroliniana range within the United States is infested … (HWA) should continue to spread in eastern North America until it occupies the entire range of T. canadensis.”

Mike Philip, an entomologist and pest survey specialist with the MDA, took issue with the report’s analysis. First, he said the conversion between Celsius and Fahrenheit contained in the report is inaccurate.

“I think it’s a little too bold a statement to make, considering what we do and don’t know at this point,” Philip said.

A local television report on the infestation compared the recent findings of adelgid to the invasion of emerald ash borer (EAB), a wood-boring beetle that has infested millions of ash trees. Philip said such a simplistic comparison is misleading.

Although it’s true there are similarities between EAB and the adelgid, there are also important differences. “Hemlock woolly adelgid does not thrive in extremely cold temperatures,” he said. “The insect is not good at dispersing on its own.”
In addition, there are important differences among the trees they infest. There are 15 different species of ash and the EAB kills all of them, Philip said. With hemlock, there are two species in the East, and the adelgid infests both of those. In the western part of the country there are two other species, both of which are highly resistant to the adelgid.

Also, ash is much more useful to humans as firewood, lumber and as a landscape tree. Hemlock, on the other hand, takes a long time to grow and while it is used as a landscape shrub, it’s never used as a street tree. Because of this, ash is transported by people much more so than hemlock, which has contributed greatly to the spread of EAB.

He added that in order for the adelgid to infest trees in Michigan through natural rather than human dispersal, the insect would have to go through Canada, which would be a problem for it, given its sensitivity to cold temperatures.

As it stands now, the adelgid has infested a total of 40 trees in Michigan over about four years. These include a small infestation in the Petoskey area in Emmett County in 2006 and again last November, in a tree that was being monitored from the original infested area.

Trees in Grand Haven and Holland, in Ottawa County, were found to be positive for adelgid. Those are in addition to the trees in Macomb County. MDA officials said they believe the trees were all brought in from areas where the adelgid is now endemic. There are an estimated 100 million hemlock trees in Michigan.
“To date, we have been pretty effective at managing the pest,” Philip said.