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 if 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 tn 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 {hat 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 long term, best way to control the pests HWA is spraying.