It resembles a sequel to a B-movie science fiction thriller: Alien Creature Attacks Forest Ecosystem.

The list of alien creatures includes Dutch elm disease, chestnut blight, beech bark disease and, most recently, the emerald ash borer. Each of these exotic forest pests has changed and continues to change the appearance and ecology of our forests.

And now, the new exotic forest pest is the hemlock woolly adelgid, which currently is devastating eastern hemlock, Tsuga canadensis, in 11 northeastern states.

Why, we wonder, can't someone do something to stop the introduction of these new pests before all the trees are gone?

"Although many insect and disease problems occur in cycles, it certainly seems that the global economy and problems created by a highly mobile society like the movement of firewood are taking its toll on the forests of North America," said Dr. Robert Heyd, forest health management program leader for the Department of Natural Resources in the Upper Peninsula.

The hemlock woolly adelgid (HWA) is a small aphid-like insect that feeds on hemlock twigs. It is native to Asia and the Pacific Northwest of North America.

"The insect sucks sap from the young twigs causing needles to discolor and drop. The loss of new shoots and needles often results in tree death a few years," Heyd said.

Since being introduced to the U.S. in the mid-1920s, the HWA now infests about half the native range of hemlock in the eastern United States. Hemlock is the second most abundant conifer species in this part of the country. In Connecticut, New Jersey, Virginia and portions of Pennsylvania extensive tree mortality and decline are common.

In Michigan, eastern hemlock is a universally accepted symbol of our old growth forests, and is a component of 2.3 million forested acres. The most recent survey estimates 102 million trees of all sizes. Originally, the Lake States forest was hemlock dominated, with hemlock in either mixed hardwood or pure stands.

"A hemlock often takes 250 to 300 years to reach maturity and may live for 800 years or more," Heyd said. "Spectacular hemlocks are valued features of many our northern Michigan parks, backyards, travel routes and forested landscapes."

In 2000, the Michigan Department of Agriculture established a HWA quarantine prohibiting the movement of hemlock seedlings and nursery stock, logs, lumber with bark, uncomposted chips with bark and uncomposted bark from infested areas.

Despite the quarantine, HWA was found on landscape hemlock trees in Harbor Springs in August 2006. This was the third time HWA had been detected in Michigan, but the first time it was found outside of nurseries on native hemlock.

The infested trees were used in landscaping and came from a nursery in West Virginia. The MDA continues to look at other sources of hemlock in the area to determine whether additional infested nursery stock have been out-planted in Michigan.

The HWA is easily recognized during most of the year, appearing as a dry, white woolly substance on the young twigs.

"It looks like a bit of cotton at the base of hemlock needles, and it is readily dispersed in the spring and early summer by wind, birds, deer and other mammals," said Heyd. "Humans also can disperse the insect by moving infested plants."

Monitoring efforts by the DNR and MDA are Michigan's first line of defense in protecting our hemlock from HWA.

"We have surveyed hemlock areas near forest recreation sites and sites adjacent to nurseries for the last several years," said Roger Mech, a DNR Forest Health Monitoring Program leader based in Lansing. "These sites are at highest risk for successful HWA introductions."

These annual surveys are part of a USDA Forest Service funded Forest Health Evaluation Monitoring project.

Mech said early detection of HWA is critical.

"If found soon after being introduced, there is a much better chance of removing it before it disperses and establishes permanently," he said.

These steps include the rapid removal and destruction of landscape and forest hemlock trees found to be infested with HWA and treating uninfested hemlock in close proximity to infested trees with pesticides to ensure complete removal of HWA.

Of course, preventing HWA and other exotic pest introductions is best. But, unlike the movies, the arrival and spread of these harmful species may have no happy ending.

"The more people become aware of the problems associated with moving plants and plant products like firewood, the better chance we have of stopping the growing threat of exotic insects, diseases and invasive plants," said Mech.

For more information, including links to other agencies and universities that work with the DNR to ensure that our forests remain healthy and productive, visit the forest health page on the DNR Web site at www.michigan.gov/dnr.