Forests Under Siege:
Nonnative Insects Are Threatening Our Trees

by Pam Owen, Fall 2007

	Above, the hemlock wooly adelgid, which gets its name from the wool-like filaments it produces to protect itself and its eggs from natural predators and from drying out. Its young like to eat the starch in the twigs at the base of hemlock needles, robbing the hemlock of nutrients and eventually killing it. Photo courtesy SNP.

In the heat of summer, many human visitors to Shenandoah National Park have found relief in the cool, damp stands of eastern hemlock along the Limberlost trail, in the park's central district. Spared from timbering before the park was created, the stand has represented a scarce old-growth ecosystem that also is home to resident and migrating wildlife.

Today, the hemlocks in the Limberlost are no more than brown skeletons, thanks to the hemlock wooly adelgid (Adelges tsugae), an aphidlike insect from Asia. First spotted in the eastern United States in 1951 near Richmond, by 2005 the adelgid was established in parts of sixteen states and has been infesting the stands of hemlock in Shenandoah since 1988.

This pest's young like to munch on the starch in the twigs at the base of hemlock needles. Starved of this nutrient on which they depend for their growth and long-term survival, the hemlocks decline and die within three to ten years.

"The long-term prognosis for eastern hemlock survival in Shenandoah is uncertain," says park biologist and Forest Pest Manager, Rolf Gubler. "At this point, there are no large contiguous blocks of hemlocks left in the park." With more than 274 cultivars of eastern hemlock in use in landscaping, the adelgid is affecting more than the park's hemlocks.

	Above left, the Limberlost Trail in Shenandoah National Park in 2003, shaded by old-growth eastern hemlocks. On the right, the Limberlost Trail today, after the hemlocks have been destroyed by the wooly adelgid. The hemlocks are dead, but the loss of the forest crown has allowed black ash seedlings and saplings to take advantage of the increased sunlight now reaching the ground. Photos by Rolf Gubler.

Hemlocks are not the only trees in eastern forests that are under attack by foreign invaders. The European gypsy moth (Lymantria dispar L.), which has been plaguing eastern forests since it first arrived in 1869, likes to eat the leaves of a variety of trees but is particularly fond of oaks, especially white oaks. White oaks are an important part of oak-hickory forests—which have the largest range of any of the Eastern Deciduous Forest communities.

On its way to our area from the Midwest is yet another insect pest: the emerald ash borer (Agrilus planipennis Fairmaire). An Asian cousin of our native ash borer, this small, bright-green beetle targets ash trees exclusively. Its larva feed under the bark, disrupting the tree's nutrient and water transport system. Infestations of this pest have destroyed 20 million ash trees so far. Gubler projects the infestation will probably arrive in our area within five to ten years.

A website (www.StopTheBeetle.info) set up by several states fighting this pest warns, "If it is not contained and eradicated, the impact of the emerald ash borer on the ash resource in North America will be similar to that of chestnut blight and Dutch elm disease, which devastated woodland and urban forests in the 20th century."

Forest pests thrive when tree health is already compromised by other stressors—from other biological assaults to air pollution, acid rain, and increasing cycles of drought. Drought can be a particular problem, since many of the species that help control pests are more susceptible to drought than the pests are.

The loss of any species of tree can mean the loss of other species that depend on it, which eventually can lead to the collapse of the entire ecosystem. However, the future or trees species targeted by insect pests is not completely bleak. As with human disease, biological attacks rarely spell the end of the host species. The healthiest trees, and those with genetic characteristics that make them pest-resistant, are likely to survive and produce a better genetic strain of trees that, in turn, will thwart new infestations.

And sometimes the disappearance of one native species in an area can mean the emergence of another. With the passing of the hemlock in the Limberlost, for example, the forest canopy has opened, allowing sunlight to reach species that have been lying in wait on the forest floor. Black birch seedlings and saplings are now filling the area where the towering hemlocks once reigned.

Acknowledging that complete eradication of forest pests is unlikely, Shenandoah has changed its adelgid management strategy to trying to save enough healthy individual hemlocks to ensure regeneration after infestations of the pest have subsided. When it comes to the hemlocks, Gubler says, the park is "committed to preserving a lasting remnant…for future recovery."

Where possible, and particularly in ecologically sensitive areas, management of forest pests is also shifting away from heavy reliance on chemical controls, which can often do more harm to an ecosystem than good, toward a mixed strategy that employs more biological controls along with some targeted chemical application.  

Populations of gypsy moth throughout the park were greatly reduced in 1992 from a naturally occurring virus (nucleopolyhedrosis) and an introduced fungus (Entomophaga maimaiga), along with some targeted applications of other biological and chemical controls by park staff. However, with the recent drought, the fungus is not thriving and the moth population is back on the rise, particularly in the northern end of the park.

Methods for suppressing emerald ash borer infestations are still being explored, but the best hope seems to lie in wasps that parasitize the larvae. Fungi, parasitic nematodes and mites, predatory insects, and woodpeckers are also likely soldiers in this war.

Since native biological controls have proved ineffective in combating hemlock wooly adelgid, some insects that are known to feed exclusively on this insect have been introduced from Asia and are slowly becoming established throughout the infested region. In the meantime, the park is injecting a systemic pesticide in the soil surrounding individual trees, which has proven to be effective.

While some pest-management strategies are hard for an individual landowner to undertake, others are doable. In any case, it's important for those with native trees on their properties to learn the signs of infestation and alert their local Virginia Cooperative Extension agent or state forester.

For more information on managing forest pests, contact the Virginia Department of Forestry (www.dof.virginia.gov) or your local Extension agent.

For more information on the Biodiversity Task Force, contact Pam Owen at biodiversity@RLEP.org.