Tiny woolly adelgids threaten state's mighty hemlocks

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Tiny woolly adelgids threaten state's mighty hemlocks

Monday, August 07, 2000

By Don Hopey, Post-Gazette Staff Writer

It announces its presence quietly by producing what look like tiny wisps of cotton candy in the short green needles of eastern hemlocks but there's nothing soft or sweet about what the woolly adelgid is doing to Pennsylvania's state tree.

Adelgid egg masses on a hemlock branch.

Throughout the Appalachian Mountains from Connecticut to North Carolina, and including the eastern third of Pennsylvania, the tiny, aphid-like bug has been sucking the life from hemlocks in wood lots, forests and residential areas at a rate that threatens the long-term survival of one of the region's most ecologically important trees.

"It's a real possibility that we could eventually lose them all," said Richard Evans, an ecologist at the Delaware Water Gap National Recreation Area, on the eastern border of Pennsylvania. He has been studying the effects of the woolly adelgid on hemlocks there for eight years.

"The adelgid has already killed thousands of trees, maybe 15 percent of the hemlocks in the recreation area, and it's expanded into every stand over the last five years.

If the infestation stopped now, it wouldn't be a major concern, but it's clear to me that this is just the beginning."

Gypsy moths, other bugs also eating well
The woolly adelgid is not the only insect bugging Pennsylvania's 17 million acres of forested land this summer.
Creepy-crawlies including the gypsy moth, locust leaf miner and fall webworm are displaying good appetites as they munch the greenery of Penn's Woods this summer.
The gypsy moth has been a particular problem in the central part of the state, where it has defoliated more than 550,000 acres of trees, according to the state Bureau of Forestry.
"It's real bad in the middle of the state, from the southern border all the way up through the northern tier counties," said Barry Towers, chief of the state's Forest Pest Management Division.
Although all of the state's regions haven't reported defoliated acreage yet, no one expects defoliation to top the 4.5 million acres reported in 1990 when gypsy moth, elm spanworm and forest tent caterpillar populations were all big and hungry. A tree is considered defoliated when 30 percent of its leaves have been eaten.
This year, Towers said, the other regular diners in the forest canopy, like the elm spanworm, forest tent caterpillar and even the fall webworm are eating light, or not showing up at all.
"Populations of these insects are cyclical; they tend to build for several years, suddenly collapse, and then gradually build up again," Towers said.
Jim Unger, pest management specialist for the 10-county southwest quarter of the state, said minimal gypsy moth defoliation has occurred in Allegheny and Washington counties. But the bugs have taken bigger bites out of Bedford County (20,919 acres) and Chestnut Ridge in southern Fayette County (10,593 acres), where they were joined by fall cankerworms. Throughout the southwest region, 75,641 acres were defoliated this summer, almost all by the gypsy moth, more than 15 times the acreage defoliated last year.
Most of the trees defoliated this spring and summer will survive. Studies have found that many trees that were 60 to 70 percent defoliated 15 years ago have recovered completely, especially when rainfall in subsequent years was good.
The state sprays insecticide in state forests to control gypsy moths and other forest insects, and the U.S. Forest Service does similar spraying in the Allegheny National Forest.
-- Don Hopey

Evans said if trends continue, all the major hemlock stands in the 70,000-acre federal recreation area could be lost in 10 to 15 years.

"Hemlocks are not the state tree for nothing. Pennsylvania used to have the world's largest leather tanning industry because of the hemlock trees," Evans said. "I think this is a very major ecological change that's occurring."

Hemlocks are also stressed by other bugs -- mites and loopers -- and by drought. But the tiny adelgid, which feeds on the sap in young hemlock branches causing premature needle drop and branch dieback, can be the fatal blow.

"The adelgid is the number one enemy of forest health in the East right now," said Brad Onken, an entomologist in the U.S. Department of Agriculture's Forest Health Protection Division in Morgantown, W.Va. "It has the potential to be worse than Dutch elm disease, but maybe not as bad as the chestnut blight."

Single-sex populations

The soft-bodied insects, only 1 millimeter long, overwinter in the trees as wingless adult females, which lay eggs in April. Newly hatched nymphs, or "crawlers," migrate to the young branches and settle at the base of the needles to feed. In June, new adult females secrete the white, waxy covering that looks like tiny tufts of cotton or wool in which they lay more eggs, and for which the insect was named.

A winged form of the insect develops from the first generation, and migrates to other hemlock or even spruce. That, along with wind and birds, plus the biological quirk that all adelgids are female and therefore egg-layers, accounts for the insect's rapid spread.

In Virginia, hundreds of thousands of hemlocks are dying along the ridges and in the eastern foothills of the Appalachian Mountains. All of the hemlock stands in Shenandoah National Park are infested, and 90 percent of the trees dead or dying. New Jersey has lost half of the 26,000 acres of scattered hemlocks in its forests.

The bug came to the United States as an uninvited Japanese import in the 1920s and was first reported in southeastern Pennsylvania in 1969. It is now found in 32 counties, where it is affecting hemlock stands in state forests, gamelands, parks and even residential areas. In southern Bedford County, woolly adelgids are quickly killing off old growth hemlock in the state's Sweet Root Natural Area.

The first known adelgid-infected hemlock in Allegheny County was reported earlier this year, the result of a residential planting imported from a Philadelphia-area nursery.

That tree's sap-sucking forest pest has since been destroyed, but the concern of state forestry officials has not.

Kevin Carlin, a state Bureau of Forestry entomologist, said he doesn't think the adelgid will wipe out all hemlock in the state, but it could affect its distribution, especially where the tree has extended westward out of its natural range and is more susceptible.

"It's really been a bad problem on the state gamelands, where hemlock is one of the major wildlife cover trees," Carlin said. "It is having a major impact that can't be denied."

State Game Commission lands in the southeastern end of the state have been particularly hard hit. Don Little, the commission's regional forester for 193,000 acres of gamelands in 10 southwestern Pennsylvania counties, said game officers are on the lookout.

"We haven't seen them yet out here," Little said, "but we have been put on alert."

Risks to nature, commerce

In addition to the hemlock's value for wildlife -- it provides winter thermal cover and nesting spots for songbirds, food for a number of animals and cover for deer -- it is an important commercial timber tree in New York and New England, where more than 1 billion board feet are cut annually. That could be lost if stands are decimated.

And tree nurseries in North Carolina are sitting on $34 million worth of ornamental hemlocks -- the most utilized tree in urban landscaping --that they can't get rid of because of possible infestation.

But most important, hemlocks are a unique ecological component of the eastern hardwood forest, where they make up five to 10 percent of the trees.

"Hemlocks are disproportionately important to the ecology of the area. That's one of the main reasons we need to pay attention to this," Evans said. "If you spend any time outdoors you realize that hemlock forests have distinct micro-climates that provide habitats for all kinds of wildlife."

In winter, deer will seek out hemlock stands where the thick branches of the trees allow the animals to avoid deep snow and browse for food. The stands also act like a quilt to trap radiating heat from the ground, giving deer a place to sleep.

In summer, Evans' research has shown that streams running through hemlock groves are three times as likely to have native brook trout as streams flowing through other hardwood forests.

"Hemlock forests tend to be less disturbed and that is habitat trout like," he said. "People like them too because they are cool and shady and tend to grow along streams with waterfalls. Most state parks include large hemlock stands and those have great appeal."

Attempts to control the adelgid have had limited success. Silvaculture control methods, such as timbering some trees to reduce the density of hemlock stands, don't work.

And although pesticides, horticultural soap and oil applications can be effective on ornamental plantings in residential settings, they are too expensive and ecologically damaging to other plant and animal species for use in a widespread forest spraying program.

This problem is particularly tough for forest pest managers because unlike other pests, adelgid populations don't seem to build and then crash on a natural cycle. Even if they succumb to extremely cold winter temperatures, the population rebuilds rapidly because they are all females.

Adelgids have few natural enemies in the United States, but several species of beetles in Asia feed on them and have been imported and released with some success at test sites in the Delaware Water Gap recreation area and state forests in Franklin, Adams and Cumberland counties.

"The beetles have been effective in the lab and on individual trees, but we don't know yet how well they'll do in a forest-wide setting," Onkin said. "We may have to employ more than one predator insect and there are others to test. That's the best hope for the forest.

"We have to come up with some kind of biologic controls that work, or we're looking at losing the hemlock resource."