The Latest Invaders

By Arthur Melville Pearson

The emerald ash borer has arrived. What does it mean for our wild woodlands? And what about the next invader down the line?

Anyone remember Space Invaders? In the blockbuster video arcade game of the 1980s, it was a blast destroying wave upon wave of alien invaders as they dropped from the sky. Decidedly less fun has been dealing with a relentless succession of real life invaders from different quarters of planet earth. Several years ago it was the gypsy moth, followed by the Asian long-horned beetle, and now it’s the emerald ash borer, which threatens as much destruction as chestnut blight and Dutch elm disease. There is a lot we don’t yet know about the emerald ash borer. But learning from past invasive species campaigns gives us the best chance of finding the best solution for this and future threats to our urban and natural areas.

The New Borer on the Block

If you had to design an alien-looking invasive, the emerald ash borer, Agrilus planipennis, would be a good model in spite of its tiny size. Measuring in length about half the diameter of a penny, it is notable for its inscrutable, shiny-black eyes and its namesake metallic emerald coloring that lends its sleek, hard exoskeleton an air of invincibility. However formidable it may appear, the male beetle lives an average of only 13 days and the female about 10 days longer. During its short adult life span, between mid-May and late July pairs will mate, leave behind several dozen eggs in the crevices of ash tree bark, nibble on a few ash tree leaves, and then die. No big deal.

It’s what happens after the adults expire that causes all the trouble. Emerald ash borer eggs hatch in seven to nine days and the newly emerged larvae — flat, 10-segmented and milky white in color — bore their way into a tree’s cambial layer where they feed voraciously throughout the summer and fall. Lying between the bark and heartwood, the cambial layer comprises a tree’s circulatory system. As larvae feed, they carve ever-widening S-shaped tunnels, or “galleries,” throughout the cambial layer, eventually cutting off the circulation of water, minerals and nutrients. Its death sentence thus pronounced, a “girdled” tree snugly harbors the agent of its destruction throughout the winter. Beginning in late April, larvae pupate and soon thereafter emerge as adults to begin the cycle all over again.

Any of the 15,000 species of wood-boring beetles found worldwide can be a pest. Of the 657 borer species native to the United States, most are considered “secondary threats” to tree health. For instance, the bronze birch borer, indigenous to the Chicago Wilderness region, generally attacks only those birch trees already weakened by drought, root injury, old age, or some other stressing condition. Healthy birch trees rarely succumb to birch borer attacks, as they have the ability to produce a callous tissue around birch borer larvae galleries that inhibits larvae from reaching the adult stage of development.

Similar to the bronze birch borer, the emerald ash borer in its native habitat is little more than a nuisance to the trees there. In fact, so inconsequential is its impact in China, Mongolia, Korea, Japan, Taiwan, and the Russian Far East that relatively little is known about it even among the East Asian scientific community.

In North America, what we have learned thus far is that none of our 16 species of native ash trees (nor any of the horticultural cultivars) has the least natural defense mechanism against the emerald ash borer. We also know that unlike most other borer species, the emerald ash borer attacks healthy and weakened trees alike, often killing them within two years. The natural dispersal rate of these winged invaders ranges between one-half and two miles per year but is greatly aided by human transport of infested firewood, nursery stock, and solid wood packing materials. Since its detection in the Detroit area in 2002, the emerald ash borer has killed more than 20 million ash trees in Michigan, Ohio, Indiana, Maryland, and Ontario, Canada. Anticipating that the borer could wipe out entire populations of the ash tree, agencies plan to collect ash seed and store it at the National Center for Genetic Resources Preservation for use in re-establishing the ash tree if necessary.

Intercepting the Invader

Due to the severity of the threat, in 2003 Edith Makra of The Morton Arboretum convened representatives from nearly 40 stakeholder agencies, universities, conservation organizations, and green industries to develop the Illinois Emerald Ash Borer Readiness Plan (PDF). A critical component of the plan involved conducting a public outreach and education effort similar to the one that proved instrumental in the successful control of the Asian long-horned beetle. The emerald ash borer is more difficult to detect than the Asian long-horned beetle due to its comparatively small size and less prominent identifying features. Nonetheless, in June 2006, an alert homeowner in the Windings of Ferson Creek subdivision, located two miles west of St. Charles in Kane County, found the first confirmed emerald ash borer in Illinois. Another homeowner discovered adult beetles in Wilmette. Follow-up surveys by professional arborists found additional evidence of the borer in Evanston and Winnetka.

The Illinois Department of Agriculture subsequently announced quarantine areas around the Ferson Creek subdivision and in northern Cook County. Effective December 1, the U.S. Department of Agriculture imposed a quarantine upon the entire state. The purpose of the quarantines is to limit the spread of the borer by restricting the movement of potentially infested materials, namely firewood and nursery stock, which may harbor adult beetles or larvae.

To eradicate the borer, the Animal and Plant Health Inspection Service of the U.S. Department of Agriculture recommends cutting down all ash trees within a half-mile of known infestations. Certain chemical treatments may prolong the life of an individual tree that bears no evidence of infestation, but such treatments are expensive, necessary to repeat periodically, and potentially in vain if the treated tree lies within the boundary of a state-ordered control cut. Cutting down trees is a drastic and costly strategy, but it can be effective. After eight years, $72 million dollars and 1,500 trees removed from neighborhoods throughout northeastern Illinois, Governor Blagojevich declared July 12, 2006 as Asian Long-horned Beetle Deregulation Day, marking the lifting of the final ALB quarantine zone.

Early indications are that the fight against the emerald ash borer could be significantly more expensive and severe. If the Illinois Department of Agriculture determines that the infestation at Ferson Creek remains small and localized, the likely course of action would be the removal of more than 2,800 ash trees in and around just the one subdivision. Warren Goetsch, bureau chief of environmental programs for the Illinois Department of Agriculture, is quick to add that if the extent of infestation has spread significantly, the response might shift from eradication to containment. The objective of a containment strategy, such as the one employed in response to the gypsy moth, is to limit and slow the spread of the invasive species, effectively buying time until science comes up with an effective solution. However, unlike the gypsy moth, which generally does not kill its host trees, the emerald ash borer kills its hosts quickly. One way or the other, time for ash trees in infested areas grows short.

When the Borer Finds the Woods

While homeowners brace themselves for what the emerald ash borer might do to their tree-lined streetscape, many people are equally concerned about the potential impact on our region’s natural wooded areas. The two most common species of ash native to our region are white and green. (Far less common are blue, black, and red, also known as pumpkin.) Both range widely throughout our various wooded community types, with white ash tending toward upland areas and green ash tending toward lowlands. To date, a trap tree program, in which more than 3,000 ash trees throughout natural areas in Chicago Wilderness have been manually girdled to attract adult borers, has revealed no infestations in any of the region’s publicly owned woodlands. Nonetheless, land managers are preparing for what seems inevitable.

Al Zaleya, forestry crew chief for the Lake County Forest Preserves, acknowledges that while any infestation on public land would be addressed on a case-by-case basis, the recommended course of action is the same as in urban areas: remove all ash trees within one-half mile of the infestation. In 2005, the Michigan Department of Agriculture cut and burned 880 ash trees on 130 acres of Brimley State Park and adjacent privately owned land in order to eradicate the only confirmed infestation in the Upper Peninsula. The low density rate of targeted ash trees allowed the MDA to require contractors to hand cut trees, and to remove them via approved skid steers and swamp machines. Follow-up surveys thus far have revealed no significant ecological impacts on groundcover and wetland areas within the control cut area.

In other infested natural areas, where the density rate of ash trees is much higher, the terrain prohibitive, or the extent of infestation simply too great, it may not be possible to remove all targeted trees. For these reasons, combined with the difficulty of monitoring for the pest, Zaleya and many other land managers believe that eradication of the emerald ash borer is highly unlikely. Whether by borer or arborist or a combination of the two, therefore, our region is likely to lose a significant number of ash trees in the coming years. In places like Ferson Creek, where ash comprises only five percent of all trees within the subdivision, the loss of even 2,800 trees might not be as visually arresting as in those urban areas where ash were planted in the aftermath of Dutch elm disease and make up more than 40 percent of the total number of trees.

Within our forested lands, in which ash comprises on average about six percent of the total number of trees statewide, the visual impact of any significant ash loss is likely to be minimal. As Drew Ullberg, director of natural resources for the Forest Preserve District of Kane County, points out, the loss of a thousand trees in an urban streetscape dominated by ash would be devastating. But, he says, lose the same number of trees across 150 acres of woodland and the average passerby might not notice any difference. Ecologically, the impact is likely to be more significant.

Consensus among most experts, including Mike Mason, chief of forestry for the Illinois Department of Natural Resources, is that the gaps left by dead or removed ash trees are likely to fill up with aggressive nonnative species including European buckthorn and Japanese honeysuckle, thereby further degrading the health of our woodlands. Others, such as Wayne Lampa, retired natural resources manager for the Forest Preserve District of DuPage County, foresee native sugar maples competing with nonnatives to fill the ash gaps, thereby exacerbating a long-term trend from oak-dominated to maple-dominated woodlands.

Further complicating the ecological picture is the fact that ash trees, in certain circumstances, are themselves considered invasive species. As revealed by the 2003–2004 Chicago Wilderness Woods Audit, the vast majority of our wooded lands are in fair to poor condition and degrading steadily due largely to the presence of invasive species, both nonnative and native. Although indigenous to the region, ash is aggressive without the natural controls such as fire that kept it a minor component of a healthy woodland. Lacking active management, ash (along with red oak) is steadily replacing bur and white oaks. Karen Glennemeier, science coordinator for Audubon–Chicago Region, suspects that unchecked stands of invasive ash trees actually might facilitate the spread of the emerald ash borer by providing ample and easily accessible nurseries for the larvae.

In a perfect world, the emerald ash borer would help conservationists by thinning the green ash overtaking our oak woodlands. Even if such selective thinning were possible, taking advantage of it would still require much human intervention to re-establish a healthy plant community. As Zaleya, Ullberg, Lampa, Mason, Glennemeier and others concur, the key to protecting all of our natural area community types from both current and future threats lies in restoring their native diversity through active habitat restoration. Healthy, diverse habitats, by their very nature, are best able to withstand periodic disruptions.

If there is a silver lining to the specter of a large-scale emerald ash borer infestation, it may be that we are coming to grips with the need to take a systemic approach to the ecologically and economically costly issue of nonnative species. In 2005, the USDA began requiring all international wood packing materials to be heat-treated or fumigated before being shipped to the United States. The early preparation of the Illinois Emerald Ash Borer Readiness Plan greatly increased our chances for an optimal response to this latest invader and also cultivated a comprehensive network of stakeholders better able to meet the challenges of future outbreaks.

The arrival of the emerald ash borer, coupled with the inevitability of yet more nonnative invasive species descending upon us, is enough to make it feel as if we were trapped within a malevolent Space Invaders game. But as those of us living within Chicago Wilderness have proved time and again, when it comes to protecting our natural areas, we are definitely gamers. We are concerned citizens keeping a watchful eye out for invasive species. We are agencies implementing coordinated control strategies. We are researchers developing improved monitoring protocols. We are nonprofit organizations coordinating information and advancing the regional dialogue. We are a formidable team. Game on.

If you suspect emerald ash borer in your area, call (312) 742-3385.
For more information, visit emeraldashborer.info.