By David Showalter

In just the last decade, the emerald ash borer (EAB) has killed tens of millions of ash trees in North America and is a growing threat to European ash. However, Asian ash species are not affected by the EAB. As a part of an effort to save the multi-billion dollar ash resources, scientists at The Ohio State University are launching a crowdfunded project to discover the genes that allow Asian ash trees to resist the insect.

David Showalter

Little Beetle, Big Problem

The emerald ash borer is an alien, invasive, wood-boring beetle that has killed tens of millions of North American ash trees since its accidental introduction from Asia and subsequent detection in 2002. Small metallic green EAB adults lay their eggs on the bark of ash trees. Out of those eggs, EAB larvae tunnel through the bark, excavating feeding galleries that disrupt the connections between the leaves and the roots of the trees. The larvae grow and pupate to emerge from the tree as adults the next year, leaving telltale D-shaped exit holes.

Extracting an EAB larva from its feeding gallery in a North American ash. Photo by David Showalter.

As a result of larval feeding damage, North American ash decline and die over a period of several years. Trees in 23 U.S. states and two Canadian provinces have so far succumbed to the beetle. The infestation is still spreading and threatens the existence of North American and European ash species. With the deaths of the trees we will lose billions of dollars in commercial and ecological value.

Peeking at the Playbook

Unlike North American ash, Asian ash species have lived along with EAB for millennia and have evolved an unknown set of genes — a metaphorical playbook — which allows them to resist the beetle’s attacks. Discovering those genes is our goal in a crowdfunded research project, with the ultimate objective of breeding North American and European ash trees resistant to EAB.

To identify the genes that allow certain ash trees to resist attack by EAB, we will compare coevolved (resistant Asian trees) and non-coevolved (susceptible North American trees) ash species. We’ve collected samples immediately before and after attack by the EAB larvae (the life stage that actually kills the tree by feeding under the bark).

Fall colors on resistant Asian ash and susceptible North American ash during an experiment. Photo by Daniel Herms.

We will profile gene expression in resistant and susceptible trees, before and after attack, using a next-generation-sequencing technique called RNA-Seq in order to identify genes associated with resistant trees. Using a second approach called quantitative polymerase chain reaction (qPCR), we will more accurately validate the association and identify candidate resistance genes to be used in further investigations.

For more information see the video below from our crowdfunding campaign. The first part is a summary of the current EAB problem. The second part, beginning at 2:06, uses a football metaphor to describe our approach to solving it. We invite you to join the effort and share with your friends. Be sure to check our Lab Notes page where we will be posting periodic updates.

David Showalter is a PhD student in plant pathology at The Ohio State University, advised by Dr. Pierluigi (Enrico) Bonello. His research explores how ash trees respond chemically to EAB attack and the effects of defensive chemicals on the development of EAB larvae. His interests extend more broadly to the use of innate plant resistance mechanisms to protect forests and crops from insects and disease. Follow him on twitter @showalterdn and get more information on his research.