Grateful for a reprieve from essential worker duties, I left the sterile walls of my pathology lab to go on a hike. This wasn’t your typical walk in the woods. I was working to save some of the largest members of a forest from some of the smallest invaders—by utilizing an even smaller predator.
I drove west towards the City of Oxford, Ohio, home to my alma mater, Miami University. Just a few miles from town, I met landowners John and Bobbi Kinne, as well as collaborator Steven Sullivan from Miami University’s Hefner Museum of Natural History. We each grabbed that morning’s cargo delivery, put on our masks, and walked into the nearby forest.
At every turn, someone in the group pointed out something of interest, from plants and seeds to fungus and soil. My specialty is animal science and medicine, but I reentered academia as a biology graduate student and found myself eager to absorb the botany and ecology knowledge my companions shared. With each new tidbit, I felt as though I was uncovering a dormant passion for nature, long buried from years of the daily grind in laboratory science.
Not long after entering the woods, we came across a fallen ash tree displaying lace-patterned grooves under her bark, the reason for the giant’s demise. Emerald ash borers—or more specifically, their larvae—voraciously eat through the inner phloem of ash trees, creating grooves that destroy what are essentially the blood vessels of the tree. Since their appearance in Michigan in 2002, the emerald-green beetles have spread to 35 U.S. states by either flying to those places themselves or by hitching a ride on wood products. In the borer’s native range of eastern Asia, the local trees tolerate their presence, whereas North American ash succumbs to death within six years.¹ When once-abundant ash trees fall, invasive plants frequently take their place, such as the bush honeysuckle, which stretches branches across the new canopy gap left by a fallen giant. Ash seedlings struggle to survive in honeysuckle shade, and, consequently, ash trees are dying faster than they can regrow. Though the forest that we were walking through was clearly at risk, I was in awe to find living, thriving ash trees in the canopy. This made our quest seem even more imperative.
To save the living giants, we hiked deeper into the forest to release a predator of emerald ash borers, one from their own native range within Asia. Only the size of a gnat compared to the near penny length of the borer, the non-stinging wasp Tetrastichus planipennisi pierces the bark of young ash trees to lay eggs on borer larvae. Her young live inside and feed on the borers as they develop, eventually putting an end to the invader’s life cycle. Since she only targets emerald ash borers, there’s no threat to native insects.²
T. planipennisi are produced and supplied by the USDA EAB Parasitoid Rearing Facility in Brighton, Michigan, which was the origin of today’s precious cargo—a dozen clear cups filled with thousands of wasps who were eager to lay their eggs. At a suitable grove of ash trees, our group stopped to open a cup, hoping to see a tiny swarm of the prismatic-winged dots fly to the aid of the ash trees. To our disappointment, the predators did not immediately take to the sky; instead, many chose to mill around in the cup, likely acclimating to their new surroundings. The wasps are sensitive to environmental changes, such as temperature and wind, so I rested the open cup in the shade of the forest, jotted down relevant data, and continued hiking, spreading cups of wasps in suitable groves along the way.
Upon the final release, we headed back to home base, satisfied with our work and hopeful the wasps would establish and disperse to infested trees nearby. We stopped periodically to identify other trees, such as red and white oaks, various maple, red cedar, and a towering shagbark hickory. This forest is home to many species of trees, but almost a quarter of the woods are ash. Their loss, as a cornerstone species, could trigger a cascade of impacts amongst other plants, bugs, and even vertebrates like birds. As if on cue, the sound of a woodpecker echoed close by, and I envisioned him preying on a tasty emerald ash borer. Ohio woodpecker populations have increased thanks to such invasive bounty, but they alone have been unable to control the borers’ expansion.³
Returning home after the hike, I drove past the familiar red brick dormitories of the university, noticing the unnatural void of students due to the coronavirus pandemic. They weren’t the only missing residents in town. Glimpses of empty space where large ash trees once lined the city sidewalks and shadowed campus courtyards reflected the borers’ intrusion. I wondered if I, unlike the woodpeckers and nuthatches, made a difference in saving the species that day.
My time in the forest left me inspired by plants, even after years studying zoology. Memories of the many campus trees I rested or walked under resurfaced, along with an overwhelming sense of stewardship towards them. Stepping out of the laboratory and into the forest was a first step into conservation action. I wish to take another, and another, so that the next time I walk in the woods or stroll through town, ash trees will be basking in the sun, casting their shadows.
¹ K. S. Knight, J.P. Brown, and R.P Long, “Factors Affecting the Survival of Ash (Fraxinus spp.) Trees Infested by Emerald Ash Borer (Agrilus planipennis),” Biological Invasions 15, no. 2 (2013): 371-383. DOI: https://doi.org/10.1007/s10530-012-0292-z.
² L.S. Bauer, J.J. Duan, J.R. Gould, and R. Van Driesche, “Progress in the Classical Biological Control of Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) in North America,” The Canadian Entomologist 147, no. 3 (2015):300-317. DOI: https://doi.org/10.4039/tce.2015.18.
³ W.D. Koenig, W. D., A.M. Liebhold, D.N. Bonter, W.M. Hochachka, and J.L. Dickinson, “Effects of the Emerald Ash Borer Invasion on Four Species of Birds,” Biological Invasions 15, no. 9 (2013): 2095-2103. DOI: https://doi.org/10.1007/s10530-013-0435-x.