Forest INVADERS.

Balsam wooly adelgid. Brown spruce longhorn beetle. Hemlock wooly adelgid. Red-haired pine bark beetle. Smaller Japanese cedar longhorn beetle. This roster of renegades may not be as familiar to us as the European gypsy moth (or even the Asian longhorn beetle), but they threaten our forests just as surely.

Perhaps foremost among these "exotic aliens," invasive species from other parts of the world, is the hemlock wooly adelgid. These tiny aphid-like insects, native to East Asia, are stripping and killing vast acreages of hemlock forest throughout the East.

"The impact is catastrophic," says Sheila Andrus, a U.S. Forest Service entomologist and national program leader in forest entomology. "Hemlock forests provide over 4 million cubic feet of timber each year and millions of dollars worth of nursery stock. But it's their part in wildlife ecology, habitat health, and especially watershed protection and water quality that's simply irreplaceable."

Hemlock trees love cool, running brooks and rivers; there's hardly a ravine anywhere in the East that isn't clothed with hemlocks. No other tree, coniferous or deciduous, casts a shade so sheltering, summer or winter. Nor does any grow so well in such deep shade. Hemlocks may endure a century or more in the guise of small saplings until there's enough of a break in the canopy to shoot for the sun.

Several neotropical songbirds--black-throated green warblers, blackburnian warblers, Canada warblers, and blue-throated (solitary) vireos--use hemlock stands largely or exclusively. Barred and great-homed owls prefer them as nesting sites. Chickadees, which face 50 percent mortality each winter, find the best shelter from storms among the gracious hemlocks, as do many other animals.

The tree's low, sweeping branches moderate ground-level temperatures in winter, helping keep streams ice-free. That's as important for the ever-moving rainbow trout, brook trout, and Atlantic salmon in their high-elevation nurseries as is the hemlocks' cooling summer shade. Other conifers are no substitute for hemlocks, for they often drop their lower branches in their quest for sun.

Factor in the hemlock's delicate lacey-needled beauty, its Methuselah-like longevity (up to 900 years or more), and its size (the national champion in the Tennessee Smokies stands a stout 202 inches around and tops out at 165 feet), and it's clear the tree is an extraordinary species that we would be loathe to lose.

For four years Richard Evans, National Park Service ecologist at the 70,000-acre Delaware Water Gap National Recreation Area, has been matching up hemlock and hardwood-forested watersheds by terrain, soil type, and the size of the streams therein.

"We've found that hemlock-dominated watersheds support much greater species diversity--up to 37 percent more--than mixed hardwood forests," says Evans. "We've also found that hardwood-forested streams are much more likely to dry up during drought. We can't be sure if this is solely due to the moderating effect of the hemlocks or if it is simply a response to groundwater conditions. Still, it's safe to say that hemlocks act as hydrologic buffers, reducing the extremes of droughts or floods."

A dying forest, though, not only reverses the effect, it exacerbates the problem. Dead trees topple easily in high winds, forming debris dams across streams. During floods, a debris dam that breaks free becomes a debris flow, with dramatic consequences for bridges and roads, fields and homes downstream.

When hemlocks die, invasive weedy aliens may move in. Evans sees tree of heaven and Japanese barberry, garlic mustard and stilt grass invading the heart of the forest. Such invasives create vastly different habitats that dry up more quickly and support even fewer kinds of wildlife.

You can see hemlock wooly adelgids at work from central North Carolina and Virginia to Maine. They're in striking distance of the Great Smokies. Satellite colonies in western New York and Vermont have been caught and quelled--for now.

Taken one at a time, adelgids don't look formidable. These wingless, pinprick-sized creatures can't crawl far or fast; in fact, for most of their lives they don't even move. Soon after hatching they plug themselves into the base of a hemlock needle via mouth-parts formed into slender feeding tubes. To protect themselves, adelgids spin out long, waxy white filaments that coalesce around their bodies in a tangled, wooly-looking bali, confounding potential predators.

As infestations build, the balls pile up like bumps on a log. Beneath them sit the adelgids, sipping away through one molt after another and all through adulthood, even while laying their eggs. Their numbers quickly build to truly spectacular proportions: millions of bugs on a single tree.

While the woolies have no reason to leave as long as their hemlock host is still alive, when pickings grow slim they're inclined to depart. Newly hatched crawlers, the only stage that has functional legs, glom onto the feet and feathers of birds or the hair of passing animals, deer, or squirrels--or humans.

"We have to brush ourselves down after we've been in the field," says Carol Cheah, entomologist with the Connecticut Agriculture Experiment Station (CEAS). "Even the egg masses get caught in our hair."

Nor is this "passive dispersal" tactic the only one in the woolies' repertoire. A brisk breeze can do the trick. "As branches start moving in the wind, the crawlers simply release their hold and drop," says Brad Onken, Forest Service entomologist at the Forest Health Protection Laboratory in Morgantown, West Virginia. "They weigh barely more than a speck of dust, so they just drift away on the wind."

Like many invasives, the hemlock adelgid came to Narth America on infested nursery stock. At first it seemed that it was just a sedentary pest in residential plantings, where it could be controlled with dormant oil or insecticidal soap sprays. Identifying the woolies is easy once you've got an infestation: Just turn over a twig and look for a lumpy strand of wooly balls. The trick is catching the infestation early on; a few miscellaneous balls tucked here and there in a forest full of hemlocks are easy to overlook.

Indeed, the adelgid's presence in hemlock forests went almost unnoticed for several decades. But by the late 1980s it had gained enough critical mass to be scary. Only recently has there been hope of stopping it--an almost equally tiny predator from Japan, a type of ladybug no bigger than a pappy seed. Cheah and another entomologist with CEAS, Mark McClure, found that the ladybug, as larva and adult, is little more than an adelgid-eating machine. But rearing the ladybugs in sufficient numbers to make a dent in the numbers of woolies is still a dream.

"We're basically playing catch-up," says McClure, who has been releasing hatches of laboratory-reared ladybugs since 1995. "Though we've seen that the ladybugs are surviving, increasing, and spreading, it's still a numbers game. We'd have to release the ladybugs very early on to keep on top of the adelgids, yet we're at the mercy of stumbling across infestations while they're still small. And with adelgids, even a few is billions."

Public awareness is a major factor in detecting adelgids, according to Onken. "Each time we find an infestation and there's a local media blitz, people look around and find them other places."

And that's where the hope lies. The rampant destruction caused by these pests and parasites makes clear how fragile our forests are and how closely the health of ecosystems rides on the wellbeing of trees. It's equally clear that vigilance is now needed--by people who stop and look and touch the world, the trees, the hemlocks--partly to marvel at such beauty, but also as advocates and sentinels.

The next time you're in the woods, examine the underside of a hemlock twig. If you find so much as a couple of waxy, wooly balls, be an advocate: report it to your local Cooperative Extension office.

Mary M. Woodsen of Willseyville, New York covers natural history and environmental issues.

Backwoods Bullies Take Root

Japanese stilt grass (Microstegium vimineum) is well established in forests from Tennessee to Pennsylvania and looking to extend its range. At a distance, this sprawling 3-footer with short green leaves looks appealing--if you didn't know what grew there before. From river bluffs to wetlands, mature forests to your own backyard, Japanese stilt grass is a ready invader of moist, neutral, or acidic soils.

It usually crops up in disturbed shaded areas along streams that flood annually, or in soil disturbed by mowing, tilling, and digging. Although an annual, stilt grass roots from running nodes and each new plantlet produces up to 1,000 seeds.

Garlic mustard (Alliaria petiolata) is a native of Europe that has swept across eastern North America, recently turning up in scattered sites around the Northwest. This weed thrives in all but the most acidic soils. A cool-season biennial, garlic mustard stays green all winter; a rapid colonizer, it can grow during every maw. As soon as the second-year plants set seed and die, that seed germinates into pretty new rosettes that quickly monopolize light, moisture, nutrients, soil, and space. Many of our loveliest spring wildflowers--trillium, wild ginger, Dutchman's breeches, and hepatica among them--simply can't compete.

Herb Robert (Geranium robertianum) is common in many eastern woodlands, but in the Pacific Northwest it's a rampant weed with an ecological amplitude ranging from dense hemlock understory in mountain passes to sea-level rocky outcrops. How could such a weak-stemmed, weak-rooted annual be so competitive? Research indicates that one year's decaying leaves may inhibit other herbaceous species' seeds from germinating the next year.

Most species of bush honeysuckles (Lonicera maackii, L. morrowii, L. tatarica. and L. x bella) are Asian imports now hybridizing over much of North America and acclimating to everything from cool deserts to swamps. They invade open or disturbed woodlands, and even intact forests, crowding and shading native understory flora. Even before the aspen and willows leaf out in the spring you'll see honeysuckles' green haze in abandoned fields and across wooded hillsides. You can tell them from the native honeysuckle bushes they're displacing by their red or yellow fruits; natives are mostly blue- or black-fruited.

Few large ecosystems south of the tundra and north of the Sonoran desert remain free of multiflora rose (Rosa multiflora). The average multiflora sets roughly a million seeds a year, and encapsulation in tasty "hips" readily devoured by birds primes them for propagation. Plus, every arching cane, with its sharp snaggy thorns, can grow 12 feet and root at the tip.

A mild, damp climate makes English ivy (Hedera helix) the kudzu of the Pacific Northwest, where it turns urban parks into ivy jungles. This supremely shade-tolerant vine soaks up every last bit of light that tries to eke through the canopy and can overpower any tree not up to the challenge. Its weight massed in the crowns makes trees prone to toppling in a strong blow.

New to southern Florida's cypress domes is wetland nightshade, also known as aquatic soda apple (Solanum tampicense). Prickles adorn the veins of leaf tops and bottoms, while straggly 15-foot perennial stems, well armed with recurved thorns, drape themselves over shrubs and small trees to form impenetrable thickets in sun or shade. Give wetland nightshade a few months under mature trees, and it will take over. Slice through it with a machete, and its cut stems will root anew. Frost kills it but not the seeds in its handsome red berries.

Even so, an industry dedicated to eradicating invasives is also springing up. Some companies use goats to eat exotics; Steve Manning, of Invasive Plant Control in Nashville, follows an integrated-pest-management approach, which often includes herbicides, to combat everything from kudzu to stilt grass. --Mary M Woodsen

The 'Best' of the Rest

At high altitudes where hemlocks don't range, an adelgid cousin, the balsam wooly adelgid, has destroyed the cool, damp mantle of Fraser fir that once blanketed the high peaks of the Great Smokies in Tennessee and North Carolina. Now as the peaks warm and dry, some species of animals--ice-age relics of northern climes--are vanishing too. The balsam adelgid has infested and killed grand, silver, and especially sub-alpine firs in the Cascade and Blue Mountains of Oregon, Washington, Idaho, and British Columbia. Predator releases have had little effect, but there's hope that a Japanese ladybug could develop an appetite for the balsam adelgid as well.

* The brown spruce longhorn beetle has established residence in the port city of Halifax, Nova Scotia, during the past decade. In its home range, western Siberia and north central Europe. it attacks only dead and dying trees. But in Nova Scotia it is attacking live, seemingly healthy red spruce. The Canadian Forest Service has contained the beetle so far by cutting "fire breaks" that circle the city, but the thing flies. The beetle has also been found on live black spruce, white spruce, and Norway spruce. Because in its native haunts it feeds on dying fir, pine, and larch, those trees' North American counterparts are likewise at risk.

* In coastal North Carolina the smaller Japanese cedar longhorn beetle cropped up in 1997 on a dying tree in a residential area. The following year it was found in Milford, Connecticut. And though in its East Asian homelands this beetle feeds on dead or weakened trees, in Connecticut it has infested live, healthy-looking arborvitae trees in nurseries and homeowners' yards. Since then it's been found in southeastern New York and New Jersey. It seems likely the trees, which sustain some injury when balled and burlapped, give off "plant volatiles" that attract the beetles. Cedars and junipers may be at risk.

* Fresh on the scene is the red-haired pine bark beetle. A single specimen was trapped in 1994 and another in '95 in the Great Lakes port city of Rochester, New York. "Then it slipped from view and we kind of forgot about it," says Rick Hoebeke, entomologist with Cornell University Cooperative Extension. "But in the winter of 2000 we found it--quite by accident, because we were looking at some diseased stock--wintering under the thick bark of a stump in a Christmas tree plantation 20 miles outside the city. It was well established there." The beetle, native to southern Eurasia and North Africa attacks dead or dying trees. But before mating and laying eggs, the beetles feed briefly on the live roots of young pines (and occasionally larch, spruce, and fir). The danger? The beetle is a carrier for blackstain root disease and other destructive pathogens.

Mary M. Woodsen