文档介绍：大学四级模拟610
Part I Writing
Directions： For this part, you are allowed 30 minutes to write a composition on the topic: Ciblack-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 . 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 . 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 th