Ash and Emerald Ash Borer: How Do Trees Defend Themselves from a Deadly Beetle?
Editor’s note: This is the first installment in the “Behind the Science” series by Laurel Haavik that peeks into the lives of scientists. See other posts in the series.
By Laurel Haavik, Ph.D.
You have probably heard about a tiny green beetle from Asia that is blazing across North America, leaving millions of ash trees in its wake. The emerald ash borer (EAB) is one of a growing number of invasive species that are able to dominate ecosystems to which they have been introduced by proliferating uncontrollably. This reduces biodiversity and can disrupt normal ecosystem functions. Invasive species can achieve this feat because they lack a shared evolutionary history with existing species in the ecosystems that they invade. EAB is possibly the most destructive alien species to invade North American forests to date. EAB kills nearly every ash tree that it encounters, and it is spreading across the continent at an alarmingly rapid rate. The future for ash trees looks bleak.
But scientists are working hard to find a way to help ash trees survive EAB’s onslaught. Justin Whitehill began studying EAB soon after it was discovered in North America. His dissertation was a quest to find out exactly what allows ash species from EAB’s native home to survive beetle attack, and what defensive capabilities are lacking in North American ash species.
Justin was intimidated when he began work on this project. Though he completed a research project as an undergraduate at Otterbein College, his science education had mainly versed him in how to identify plants and insects, and how the two groups interact. For example, EAB eats ash trees, but only the immature stage is present throughout most of the year, and this stage is found inside the trunk. So, to collect an EAB, it would be a good idea to look underneath the bark of the nearest ash tree. In 2006, Justin became a new PhD student in Pierluigi (Enrico) Bonello’s Forest Pathology lab at The Ohio State University. In order to find out what chemical compounds and proteins existed in ash tree trunks, Justin had to learn about biochemistry and proteomics — two subjects that were completely new to him. Though it took him some time to ascend a steep learning curve, Justin was eventually able to develop some new methods that actually worked. Of this time in his development as a scientist, he says, “You learn what not to do, and how to fail in style. Every PhD student goes through a struggle, and then you beat it, you come through to the other side — then you have the confidence that you can do anything.”
Meanwhile, the EAB invasion was in full swing in Columbus, Ohio. A nursery had received ash saplings that were infested with the beetle. As these saplings were planted throughout the city, EAB spread to other ash trees street-side and in city parks. This put pressure on Justin to succeed, and at the same time placed great importance on his work, which would make it all the more rewarding if he could find some answers.
Justin was the first student in Enrico’s lab to study EAB and ash trees. This was an exciting, yet challenging, time and place! Nothing was known about the problem, leaving all avenues of inquiry wide open, but choosing which avenue to pursue could be difficult with no prior knowledge available. The prevailing idea was that there might be one (or several) chemical compound(s) — a silver bullet — that prevents EAB immatures inside trees from getting adequate nutrition or from properly digesting the wood they eat, preventing them from surviving for very long. If it existed, this compound would most likely be found in ash species native to Asia. Asian ash species, like Manchurian ash, probably possessed such an adaptation to combat EAB. Otherwise, the beetle would be able to kill large numbers of them with regularity.
Justin and others pursued this idea, and it looked promising at first. But further investigation revealed that it was much more complicated than just one single compound; there was likely something else involved. At this point, it seemed to Justin that a key piece of information was missing. He discovered a study by Eva Wallander that carefully defined the evolutionary relationships among different ash species. In other words, Eva’s study identified which ash species were most closely related to one another. Justin repeated an analysis of compounds in the trunks of Manchurian ash, white ash, and green ash, and added two other North American species that had not yet been tested: black and blue ash. This study confirmed his suspicions that the chemical relationships between ash species also matched their evolutionary relatedness!
This was a conflicted success for Justin. The results of his study had supported his hypothesis — indeed, very exciting — but the quest to identify the key to ash defense against EAB had suddenly become much more complicated. Manchurian and black ash are closely related and share a similar chemistry in the trunk. However, EAB has no trouble eating and killing black ash, but it very rarely kills Manchurian ash. There must be something else that differs between the two species that allows the beetle to devour one and leave the other almost untouched. What could it be?
While Justin, Enrico, and other EAB researchers ruminated on this revelation, Justin was busy setting up another experiment. He wanted to know whether he could apply a plant hormone called methyl jasmonate to jump-start the metabolic pathways in ash trees to help them produce chemicals that could enable them to resist EAB attack. Methyl jasmonate is known to function this way when applied to other plants, almost like giving a tree a flu shot. The application of methyl jasmonate boosts the tree’s internal immune-system.
Justin reflects fondly on the ownership he felt over this experiment. He spent three years working on it. It required dedication and diligent effort. He worked with the city arborist, David Bienemann in Bowling Green, OH (now the municipal arborist for the city of Hamilton, OH). Justin and David planted 120 trees representing five different ash species. Justin designed the layout for planting so that each species was planted randomly, yet evenly, throughout the common garden. Then he placed ash logs infested with EAB into the garden, which increased the number of EAB adults in the garden by about 3,000 insects! Justin enjoyed a day’s visit to the common garden, and he reflects fondly on the meditative quality of the work as “[he] would drive two hours to the site listening to music, and once there collect samples or apply methyl jasmonate carefully and methodically, get back in the car, turn on the music, and drive the two hours back to Columbus. There was comfort and reward in the simplicity of just doing the work.”
The experiment worked! North American ash species had miraculously become resistant to EAB when treated with methyl jasmonate. And, by analyzing the chemicals in those trees, Justin was able to identify three chemical components that were likely involved. As a next step, Justin has just begun collaboration with Joy Ward and me at the University of Kansas through funding from the National Science Foundation. With Justin’s expertise, we are studying whether amounts of these three compounds vary naturally among white ash from different populations originating from throughout the tree’s species range. The most promising compound identified, verbascoside, killed EAB immatures in an experiment when fed to them mixed with an artificial diet. We are now in search of trees that have naturally high levels of verbascoside.
Justin’s work boldly revealed that typically susceptible North American ash trees are able to change their bark chemistry to resist attack by EAB, they just do not seem to be doing it with any regularity (anywhere on the continent) without help (from something like methyl jasmonate). There is still much more to understand before these results can be incorporated into breeding programs for EAB-resistant ash trees. New genome-editing tools such as the CRISPR/Cas9 system can help to speed up this process when combined with conventional tree-breeding approaches. However, research efforts to date in the U.S. have not embraced these new technologies at the rate they have for agricultural crops. New genomic technologies offer more than a small sliver of hope that ash trees will persist in North America. Justin is hopeful that “we will eventually be able to produce EAB-resistant ash trees by borrowing from other ash trees that have natural resistance. Applying modern genomic technologies to conventional tree-breeding methods has the potential to preserve not only a single species, but a whole genus that is being impacted worldwide. Unlike conventional breeding methods that take generations, rely on backcrossing with resistant species, and result in a genetic hybrid, the incorporation of genomics has the potential to save a whole genus in a fraction of the time while preserving the genetic integrity of each individual species.”
Read more about Justin and his research at:
Laurel Haavik, Ph.D., is a postdoctoral researcher at the University of Kansas, where she studies the interactions between insects and the trees that they eat. Follow her on Twitter at @ljhaavik, and check out her blog Science Shapes Lives.