Week of November 8, 2010

Invasive Species Threaten Future of N.Y. Forests

The spread of Asian insects kills key American species in Northeast

By A. Drew Muscente

Published on The Cornell Daily Sun (http://www.cornellsun.com)

It is illegal to transport some firewood across the state of New York — doing so may lead to consequences.

On March 29 of last year, the N.Y. Department of Environmental Conservation (DEC) instituted a series of regulations, strictly limiting the movement of firewood. Its import is prohibited unless the firewood is first treated with insecticides; local wood may not be transported more than 50 miles from its natural source; and all firewood must be accompanied by a receipt.

These regulations represent the response to invasive insect species, particularly the emerald ash borer, the hemlock woolly adelgid and the eastern longhorned beetle. One anonymous source characterized these species as “the holy trinity of invasive species” — a powerful trifecta of Asian insects capable of drastically altering the natural American landscape.

The emerald ash borer, Agrilus planipennis, originally immigrated from Asia to southeastern Michigan in 2002, probably by hitchhiking in wood packing material bound for Detroit’s industrial district. The species uses ash trees for reproduction, laying larvae in the inner-bark of the tree. Each larva feeds on the wood, creating intricate tunnels as it eats. Located below the bark, each tunnel obstructs the movement of water and nutrients, eventually killing the ash tree.

Infested ash trees can be identified by distinctive D-shaped spots — holes through which the mature insects exited the inner bark. In addition, these trees are frequently covered with patches of leaves, dead branches and white flecks, caused by woodpeckers, digging into the wood for the larvae.

Consequently, the invasion resulted in the destruction of ash tree populations throughout Michigan state.

Ash trees are not a significant component of Michigan forests, which are already fragmented due to midwestern agriculture. In contrast, local forests are contiguous across the state and have relatively more ash trees per unit area — this creates a superhighway for the spread of the insect.

“The issue in all of Pennsylvania and New York is that they are more heavily forested,” said Prof. John Vandenberg, entomology.

Vandenberg studies the emerald ash borer in Michigan. While returning to Ithaca last summer, Vandenberg discovered the symptoms on an ash tree at a rest stop in New York state. He explained that a traveler with firewood likely carried the ash borer to that tree.

Since then, the emerald ash borer has appeared in separate regions throughout the entire state.

According to Mark C. Whitmore, an extension associate in the department of natural resources, ash trees constitute only 10 percent of local forests. However, in conjunction with other insects, the emerald ash borer poses a serious threat to the natural diversity of forests.

The hemlock woolly adelgid, Adelges tsugae, first entered the U.S. from East Asia in 1924, and it is now found throughout the eastern states. It is responsible for the widespread mortality of hemlock, a group of pine tree species. This insect sucks the nutrient-rich sap from the hemlock wood, reducing the overall transport of necessary materials in the plant.

The eastern longhorned beetle, Anoplophora glabripennis, is native to China, but surfaced in New York City during the 1990s; in recent years, the beetle has invaded regions of Massachusetts, forcing the state to implement multiple quarantine strategies. Thus far, these efforts have successfully contained the insect.

Like the emerald ash borer, the Asian longhorned beetle reproduces in the wood of northeastern trees. However, this insect also kills poplar, willow, elm, chestnut, birch and maple trees.

“It’s hard to think about the northeast without sugar maple,” Mark Whitmore said. After all, the combined potential of the three insects is extensive — ash, hemlock, birch and maple trees represent significant portions of local forests. This leaves only oak, hickory, and beech trees, as well as some less dominate groups of plants, unthreatened by invasive species.

The trifecta makes forests increasingly vulnerable to further insect invasions. In addition, as they reduce the number of ash, hemlock and other trees, natural processes that emerged over thousands of years may also change.

For instance, hemlock trees are confined to moist, cool regions, typically on higher slopes and along bodies of water — this includes the northeastern side of Beebe Lake. Consequently, hemlock branches frequently shade lakes, ponds and streams, cooling the water.

The destruction of Hemlock stands by the adelgid could drastically alter the ecology of waterways, particularly the behavior of trout fish.

Similar, the ash borer and the longhorned beetle could potentially impact local economics. Ash trees are common sources of wood for high-quality electric guitars, drum and baseball bats. Ash trees are also common street trees.

“When those trees die, it’s going to be a significant health hazard,” explained Mark Whitmore. “Hopefully by planning, I think communities will be able to minimize the economic impact that will occur.”

Cornell extension services is currently working to reduce the spread of the insects, particularly the emerald ash borer. These efforts include mapping the distribution of ash trees throughout the state, educating the public about the risks of moving firewood, and working with scientists to introduce resistance genes from Asian trees into populations of the local Ash species.

Insecticides are an effective solution to save pet trees in backyards, but because insecticides must be used repeatedly in large quantities, it is not a practical solution to end the statewide infestation. Insecticides also pose significant health risks and will inevitably harm other plants.

“I think the only hope we have is to slow it down,” Whitmore said. “Communities need to start acting now; they have no time to wait.”

“Sometimes it’s the scientist who finds the species, but usually, it’s Joe Public who finds something in his backyard,” related Holly Menninger, coordinator of the N.Y. Invasive Species Research Institute at CALS. “We need as many eyes and ears on the ground to find these things.”

Communities can reduce the impact of the insect by taking inventory of their local ash trees, reducing the spread of contaminated firewood and communicating with state agencies. Scientists hope that they may eventually produce resistant ash trees, capable of dealing with the insect. This work, however, is still only in its infancy.

“People like to compare it to the loss of chestnut and elms … They just disappeared real quickly,” Mark Whitmore explained.

Dutch elm disease and chestnut blight — two diseases caused by fungi of Asian origin — obliterated populations of elm and chestnut trees during the 20th century. Elm trees actually adorned University property until they were killed by the disease. Both species survived.

Immature chestnut trees can be found in the wild. Because their roots are fairly resistant to the fungus, they survive, but produce only limited growth of shoots.

The difference is this: chestnut and elm tree genes remain in the stunted offspring of both species. When ash trees, hemlock and other species die due to the activities of this invasive trifecta, they will likely disappear forever — their genes will just simply be gone.

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