What will happen to the emerald ash borer when all the ash trees are gone?
Will it die of starvation or will it settle for the taste of black walnut?
That’s just one question contemplated by researchers at ground zero of the devastation caused by this invasive species from China.
Therese Poland, research entomologist with the United States Department of Agriculture Forest Service, works at the Northern Research Station on the campus of Michigan State University. She has been studying the borer since 2002 when it was discovered in North America after arriving in Detroit via a shipping crate in the 1990s. Since then, it has killed at least 15 million ash trees in Michigan alone and has spread to 13 states as far west as Missouri, as far south as Virginia and as far east as New York. At risk are an estimated billion trees nationwide, according to the USDA.
Pearson Park in Oregon was ground zero in Ohio in 2005. Governor Robert Taft visited Pearson to assess the impact and defend the state’s effort to cut down all the ash trees in the park, even the healthy ones, an estimated 17,000 trees. While that approach proved fruitless, the subsequent quarantine and public education may have slowed the borer’s advance.
Much has been done since 2002. Poland said researchers were originally concerned the borer would attack other species as it has in parts of Asia. Dr. Robert Haack, another USDA researcher, said this belief stemmed from a dispute within the scientific community about the classification of the borer. Some claim the borers that feed on black walnut trees in Japan and elm trees in Korea are the same species. Others believe the borers are similar but slightly genetically different.
This concern led scientists at the Northern Research Station to conduct tests in which they introduced our borer to black walnut, American elm, hackberry and lilac. When given a choice between these trees and ash trees, all borers chose to lay their eggs on ash trees. When given no choice, borers laid eggs on these non-ash trees, but the larvae could not feed and subsequently died.
“Of all the species we’ve tested them against, they can’t attack anything other than ash,” Poland said. However, there is a cautionary note. There is no guarantee, given our global economy, that genetic variations won’t hitch rides on containers bound for North America.
Then there is evolution. “Trees and insects co-evolve over millions of years,” Poland said. “Insects develop the ability to metabolize and detoxify the defense compounds in the tree, while the tree tries to develop different toxins and defense compounds against the insects. So, it’s kind of a battle that evolves over time.”
Poland said a new treatment has proven effective for ash trees in backyards and parks, although it is not practical for forest applications. Called Tree-age, it is registered for use by professionals. The active ingredient is emamectin benzoate. Tree-age is 100 percent effective for two years, Poland said. It is injected into the trunk.
Two other treatments have shown promise in lab tests and could help control the borer in forests and other areas where trunk injection is not feasible. Leah Bauer, another researcher, has led studies that have shown limited success with three parasitic wasps from China. These wasps, less than one millimeter in length, deposit their eggs in the borer. The parasite grows inside the borer, feeds on it and emerges from the dying borer. The wasps were released in 2007 in three areas in Michigan. Monitoring will continue until 2012, according to the USDA.
Poland also said lab tests have shown that a natural soil-dwelling bacterium that infects insects but not people has proven successful. It is now being tested in the field. Called Bt (bacillus thuringiensis), it can be dispersed in an aerial spray, similar to treatment used in forested areas for the gypsy moth. This is an organic spray, not a pesticide, said Poland. Adult borers feed on leaves covered with Bt and eventually die.
So, the battle continues and we are learning to live with yet another invasive species, like we are learning to live with the estimated 185 foreign species which have invaded the Great Lakes from the ballast discharge of ocean going ships.