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Sep. 24, 2012

Tiny Invasive Insect Rewrites the Landscape

by Kara Rogers

Click to enlarge images
In a stream trickling along the western slopes of the Great Smoky Mountains, brook trout dart around rocks and fallen branches, snapping up insects and snails. The water is shaded and cool, and the rocks along its edges are covered with slippery green moss. Overhead, the canopy is dense, and red squirrels and martens lope along the branches of hemlocks. The scene is iconic in the eastern United States, but it is being slowly erased from the landscape by a tiny invasive insect known as the hemlock woolly adelgid.

This aphid-like pest, which is almost impossible to see without the aid of a magnifying glass, has caused massive damage to eastern hemlocks (Tsuga canadensis) and Carolina hemlocks (Tsuga caroliniana) -- the only two types of hemlock native to eastern North America. Since its discovery in eastern forests in the 1950s, its threat has intensified. Recently, it spread to more northern areas of the eastern hemlock range, which some scientists have suggested may be linked to climatic warming. The insect’s migration north could prove disastrous for the remaining 50 percent of eastern and Carolina hemlock populations, the majority of which had been buffered from infestation by cool temperatures that held the insect at bay.

The hemlock woolly adelgid is named for the white wool-like material that it secretes and that becomes conspicuous on hemlock branches when females lay their eggs (which they do twice each year). Newly hatched young feed on sap, and it is this loss of nutrients that ultimately kills the tree, though the process is gradual, taking anywhere from four to 15 years. New generations of adelgids may be spread to other trees by the wind or by birds and other animals. Humans also contribute, such as through the transport of parts of infested trees into previously unaffected areas, which may be how the insect got here in the first place.
 
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Hemlock forests are unique in that they are defined largely by shade. Because only low levels of light reach the forest floor, the forest stays cool and moist in the summer. In winter, the close canopy buffers the forest from wind and snow accumulation. Hemlock forests are further characterized by the slow decomposition of the trees’ acidic needles and wood, which produces a thick layer of humus, and by an open understory dotted by plants such as rhododendron and young, shade-tolerant hemlocks.

The loss of eastern and Carolina hemlocks poses serious threats to the many animals that make their home in the forest. Of special concern are the impacts of hemlock mortality on the cycling of water and nutrients through the forest ecosystem, and what those impacts mean for aquatic species, like brook trout and the insects and invertebrates they eat.

Hemlocks, in buffering the forest from temperature extremes, prevent streams from drying up in summer and from freezing solid in winter, which ensures a more or less constant flow of water and reliable habitat for aquatic species. Furthermore, since the soil of hemlock forests has only low levels of nutrients such as nitrogen and calcium, there is very little leaching of nutrients into the water. As a result, water quality is high and aquatic life is abundant in an intact hemlock ecosystem.

That the tiny hemlock woolly adelgid is killing off old-growth hemlock trees -- many of which measure at least three feet in diameter and stretch upwards of 100 feet in height -- seems improbable. But that we are defenseless against it and that it is slowly bringing about fundamental change in eastern forests is simply frightening to me. Defeating the adelgid may come down to natural resistance, with trees that survive infestation emerging as the last hope for the eastern and Carolina hemlock populations.
 
About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.

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