To Save Trees, Fighting One Alien Insect With Others

Lori Valigra
for National Geographic News
October 4, 2005

Rusty Rhea sighs wistfully as he talks about the beauty and peace of standing amid a grove of deep green hemlocks in Appalachia, some of them up to 160 feet (50 meters) tall and more than 500 years old.

"This is a very special tree," said Rhea, an entomologist for the U.S. Forest Service's Forest Health Protection program in Asheville, North Carolina. "I was brought up here, and I don't want to see another species go by the wayside."

The evergreen trees, a hallmark of southern Appalachia's national parks, are under attack by an invasive insect barely visible to the eye but potent enough to fell the giants of the eastern United States' old-growth forests.

Already the tiny bug from Japan, known as the hemlock woolly adelgid (HWA), has killed upward of 95 percent of the hemlocks in Virginia's Shenandoah National Park. Now they are making their way through the half-million-plus-acre (200,000-plus-hectare) Great Smoky Mountains National Park in North Carolina and Tennessee.

The hemlocks shade streams, keeping water temperatures just right for brook trout and other fish. They also house birds such as the black-throated green warbler, solitary vireo, and northern goshawk, all three of which mainly shelter in stands of hemlock trees.

Because of the insect's broad impact on the entire ecosystem of southern Appalachia, HWA stands to cause wider damage than the American chestnut blight of the early 1900s. That fungus from Europe killed off the once dominant chestnut trees from the northeast United States to the southern Appalachian Mountains.

In addition, a species related to HWA, the balsam woolly adelgid, has already killed about 90 percent of the mature Fraser fir trees in the Smokies.

Acting Quickly

HWA arrived in the U.S. Pacific Northwest via nursery plants from Japan in 1924. By 1951 the tiny invader had been found in Virginia. Since then the insect has spread to more than 15 U.S. states.

The key to killing the HWA is to catch it early and act quickly. It's already well established in the Great Smoky Mountains, where Rhea and others are trying to stem the spread of the bugs.

HWA multiply quickly: All of the insects are females that reproduce asexually, laying several hundred eggs a year. When they get to the nymph, or crawler, stage, they are dormant from about June until October, after which they emerge and establish themselves on trees.

Winds and birds and other animals spread the crawlers through the forest.

Continued on Next Page >>




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