Combating a Treacherous Pest

Tackling Emerald Ash

Borer takes expertise and experimentation.

By Rachel Schnaitman

Maintaining the plant health care program throughout Longwood’s greenhouses, conservatories, and outdoor gardens is undoubtedly crucial to our Gardens … and, thankfully, our Integrated Pest Management (IPM) team is up to the task each and every day. This small-but-mighty team of four full-time staff serves as entomologists, plant pathologists, biologists, virologists, and general problem solvers—and, for many years now, they have been wearing all of those hats while working to solve the problem of Emerald Ash Borer (Agrilus planipennis), or EAB.

A green buprestid or jewel beetle that feeds on ash species, EAB—native to China, Mongolia, North Korea, South Korea, Japan, Taiwan, and eastern Russia—was first found in the United States in southeastern Michigan in 2002 and is believed to have arrived in wood-packing materials. Since that first US sighting, EAB has spread to 35 states and has decimated tens of millions of ash trees. It was first found in Pennsylvania in 2007 and is now in every county in the state. It arrived at Longwood in 2019.

Here at Longwood, our IPM approach does not only lie in addressing pests already seen on property—much of our approach pertains to developing strategies and tactics for tackling potential pests that may arrive in the future. While we did not see our first EAB in the Gardens until 2019, we had started planning for EAB in 2010. That year, staff evaluated ash species in our Gardens for their structural integrity, aesthetic quality, significance to Longwood, and interpretive value. We knew it was just a matter of time before we saw this pest in our Gardens; in preparation, we wanted to understand where we were most susceptible and begin to devise a strategy to mitigate EAB impact.

There are currently 16 species of ash tree that are susceptible to EAB, as well as the white fringe-tree (Chionanthus virginicus). The most common species we have at Longwood is Fraxinus americana, the white ash. We have more than 30 specimens in the Gardens proper that we are monitoring for EAB and over 65 in the natural areas. Signs

Opposite: (REF. #1) View of EAB tunnels (S-shaped galleries) and exit holes. Photo by Cliff Sadof, Purdue University; (REF. #2) Bolt preloaded with EAB larvae parasitized with Tetrastichus planipennisi. This larvae parasitoid comes inside of and symptoms of EAB include yellowing, thin, or wilted foliage, heavy woodpecker activity, D-shaped exit holes, suckering from the base of the tree, and S-shaped galleries under the bark. As infestation increases, the tree’s canopy will thin, advanced dieback will be seen and eventually, trees will die after 3 to 4 years of heavy infestation.

When developing the strategy around any of our pest management plans, our overarching goal is to define and minimize the negative impact of pest organisms on plants, infrastructure, and people—while holding to our strategy of using the “least toxic, yet most effective” means possible to ensure the safety of the environment, staff, and guests. Our IPM team considers several different treatment options that fall in the following categories: chemical treatment, biological control, mechanical control, or cultural modifications. For EAB, chemical, biological, and mechanical control are advised by industry experts. When developing our EAB management plan, we knew we needed to identify select trees for treatment, research available biological controls, and understand that eventually we might need to remove trees once they became affected by EAB.

Our initial survey in 2010 identified the ash trees that we wanted to protect from EAB’s eventual arrival. We began our chemical injection treatments for EAB in 2012 and have continued treating select trees—currently totaling 48 trees throughout our Gardens—every three years. In conjunction with those treatments, we continue to survey our trees with help from our arborist team, who were instrumental in finding our first adult EAB in 2019. Knowing that infested ash is weakened, we have also completed targeted ash removals near paths and roadways.

We’re not stopping there when it comes to our efforts to combat EAB in our Gardens. In addition to our chemical treatments, we continually monitor the latest research and science around emerging ways to mitigate EAB. As a result, this past year we introduced a new tactic to our management plan— biological control, by way of actually a wood ‘bolt’ prepared on a small ash tree. This temporary bolt is used as a home for the larvae predator of EAB. Once we receive this bolt, we can expect an adult Tetrastichus to emerge in a few weeks. releasing EAB parasitoid wasps into our Gardens. In the summer of 2022, we partnered with the US Department of Agriculture’s Agricultural Research Service to release three species of these wasps: Spathius galinae, Tetrastichus planipennisi and Oobius agrili. These parasitoids are very small, the adult insect is similar to the size of a mosquito. But don’t be alarmed, they are only targeting EAB and will not sting humans.

When we choose our release sites—such as the area in which we released the EAB parasitoid wasps—we take into account a number of factors. We chose release-site trees that were not being chemically treated for EAB and trees that were not in serious decline as we wanted to give the EAB parasitoids several years to become established. Releasing them on declining trees would mean that there could be a potential for the tree to deteriorate before the parasitoids could reproduce. We also released the wasps in different life stages. The S. galinae are released as adults. The O. agrili are released on EAB eggs that they have parasitized. The parasitoids take a few weeks to emerge then they begin their journey to find EAB eggs in the gardens that they can parasitize. Finally, we released wood bolts that have been infested with EAB larvae parasitized with T. planipennisi These parasitoids are natural enemies of EAB and have shown promise in reducing infestations in environments where they have been introduced. We’re hoping that these parasitoids will establish healthy population levels in the next three years and help to diminish EAB throughout our forested areas. IPM is a multi-faceted process rooted in longevity. Going forward, we will continue to survey and scout our ash trees for EAB. Each year, staff from our IPM, arborist, and Land Stewardship and Ecology teams will inspect our trees and decide if any should be added to our chemical treatment schedule, or if a tree should be evaluated for removal. We will also closely watch the parasitoid wasps introduced into our Gardens to determine how quickly they establish as we continue our fight against the Emerald Ash Borer.

Borer exit hole found in 2019, indicating the pest had arrived at Longwood; (REF. #4) Release of EAB parasitoid Spathius galinae, summer 2022. Photo by Becca Manning; (REF. #5) EAB parasitoid release, summer 2022. All three parasitoids are released together using different methods. This allows multiple predators to attack each life stage of the EAB. In this picture you see the adult wasps released in plastic cups and the larvae predator is inside the wood bolt at far left. Photo by Becca Manning.