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 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.

　　1. The passage gives a general description of an invasive insect, HWA.