填空题

To Save Trees, Fighting One Alien Insect with Others
Rusty Rhea sighs wistfully as he talks about the beauty and peace of standing amid a grove (小树林) 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(鲑鱼) 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(枯萎病) 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 (无性地), 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.
HWA crawlers feed on the new growth of hemlocks by piercing the twigs that hold the branches, sucking the sap, and injecting toxic saliva. The needles turn from a deep green to a grayish green and eventually die, depriving the tree of nutrition from photosynthesis.
An infected tree usually dies within five years of initial attack. Infection is signaled by either a white, cottonlike material that appears along a tree’s twigs or by the "baldness" of a tree’s upper branches.
Plans of Attack
In the Pacific Northwest the hemlocks seem to be tolerant of the creatures’ feeding, and in the cold northeast, winters seem to keep them at bay. But in the warm southeast, with weather approximating that of the insects’ native Asian homes, they thrive.
Chemical sprays--such as insecticidal soaps and horticultural oils as well as trunk or soil injections--have helped to kill some of the HWA infestations.
But spraying must be repeated every six months, and injections are expensive and last only two years at most. These methods can’t be used conveniently or safely in remote areas or near the streams where hemlocks grow thickly.
Long term, the best way to control the pests appears to be releasing other insects that feed exclusively on HWA. Scientists have studied HWA in Japan and China and identified three such species. One of them, the Sasajiscymnus tsugae (St) beetle, was released in areas of Great Smoky Mountains National Park in 2002.
Studying what controls a species in its native habitat--including climate, predators, and host resistance--provided clues about which insects to use against HWA, said Kristine Johnson. Based in Gatlinburg, Tennessee, Johnson is a supervisory forester for Great Smoky Mountains National Park.
"Biological control is the only long-term hope to save the trees in the backcountry(穷乡僻壤)," she said. "We have 800 square miles (2,100 square kilometers) of contiguous wilderness. We value the native forest, and it’s entirely worth defending."
Risky Business
Releasing one species of non-native bug to kill another could be risky business, potentially creating another type of infestation. But scientists first quarantined and studied the HWA-killer insects.
They believe the St beetles are the best answer to the HWA problem and that they won’t cause side damage. This tiny black female beetle, the size of a poppy seed, is already spreading in the Great Smoky Mountains.
But the beetle and other HWA-killer insects are seasonal, so it will take several different ones operating year-round to keep HWA in check, Rhea said. He doesn’t believe HWA will be completely eradicated (根除) but will instead be kept in balance by the predator insects. "We’re trying to insert a balance in a system that’s out of balance," he said.
Each St beetle can lay 200 to 300 eggs, said Ernest Bernard, professor of entomology and plant pathology at the University of Tennessee in Knoxville.
Bernard’s laboratory is one of several that are breeding the beetles.
"Each beetle eats hundreds of baby adelgids a year," he said. And about 120,000 of the beetles have been released in the past couple years in the Smokies, but it is still too early to measure their impact.
One good sign, Bernard said, is that some beetle larvae (幼虫) have been found in areas where they were not released, indicating that the HWA killers may be reproducing and spreading.
The key to killing the HWA is to catch it early and act quickly.