Be A Better Gardener: Progress in the Battle Against A Tree-Killing Pest

by Thomas Christopher

At the age of 70, I can remember too many epidemics of tree-killing pests sweeping through the Northeastern landscape. I was a boy when Dutch Elm Disease killed most of the stately American Elms that shaded our streets. Spongy moth caterpillars (then known as “gypsy moths”) stripped the foliage of our oaks, killing many of them. Hemlock woolly adelgid reduced too many groves to ghost forests of standing dead timber. In the past decade, the emerald ash borer has killed most of the ash trees, which formerly constituted an estimated 3 percent of the forest trees in southern New England and as much as 5 percent in Vermont.

With the exception of the spongy moth, all of these pests share a common origin: they all came to North America as accidental imports from Asia. Their success in making this transition derives from the fact that 200 to 300 million years ago North America and Asia were parts of a common landmass sharing a single flora before they drifted apart as separate continents. Eastern North America and Eastern Asia still share remarkably similar forest floras. Because of this shared flora, many of the tree pests that evolved in Asia can also prey on related species they find in the eastern United States. Whereas the Asian trees over millions of years of coexistence with the pests have evolved robust resistance to them, the related American trees often have none. The results are typically disastrous.

Fortunately, scientists are developing new and more effective techniques for blunting the impact of newly arrived pests. A good example of how this can work has been the campaign against the emerald ash borer (“EAB”), which was described to me recently by Dr. Claire Rutledge of the Connecticut Agricultural Experiment Station.

Dr. Rutledge began studying this pest in 2004, just two years after it was first observed infesting North American ash trees in southeastern Michigan. Researchers at Michigan State University subsequently found evidence that the beetle had been at work in the state for at least a decade previous to its discovery. That results from its larvae tunneling through the trees’ live wood, the phloem, just underneath the bark.  This hidden attack is only noted, generally, when the tree fails. This highlights a major obstacle to programs to limit EAB damage. By this point the EAB population has commonly exploded locally, leading to mass attacks that have overwhelmed not only the ash population but also local arborists when tasked with removing tens of thousands of dead street and forest trees all at once.

Dr. Rutledge drafted a native wasp to help with the EAB watch. The female wasps of this species collect wood-boring beetles to bring back to its nest as food for its young. The wasps, Dr. Rutledge found, particularly like to dig their nests in the sandy, compacted soil of baseball infields. So she enlisted citizen scientist volunteers all over Connecticut and equipped them with nets to watch the baseball fields and collect the prey of the females returning to their nests. In this way, Dr. Rutledge could note the arrival of the EAB almost as soon as they arrived in a community.

Armed with this knowledge, she and her colleagues were able to attack local EAB populations while they were still small. They released three species of parasitic wasps that the U.S. Department of Agriculture researchers had identified in the EAB’s northeast Asian homeland, wasps that preyed only on the EAB and had been extensively tested to ensure that they wouldn’t also attack native North American insects.

These releases ensured that EAB populations no longer exploded but instead were maintained at a steady low level. This reduced pressure on ash trees, so that roughly 40 percent of the mature white ashes (Fraxinus americana) survived the onslaught, whereas previously virtually all had died.  

Much work remains to be done, according to Dr. Rutledge. Green ash (Fraxinus pennsylvanica) and black ash trees (Fraxinus nigra), for example, do not seem to possess the resistance of their white ash relatives and will need further help. However, Dr. Rutledge is optimistic that our native ashes are on a better trajectory. I’m hoping that this model of a faster, more effective treatment of an invasive pest may lessen my experience of watching one tree species after another disappear from my woods. 

To listen to my conversation with Dr. Claire Rutledge, log onto the Berkshire Botanical Garden’s “Growing Greener” podcast at www.berkshirebotanical.org.

Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, located in Stockbridge, Mass. Its mission, to provide knowledge of gardening and the environment through a diverse range of classes and programs, informs and inspires thousands of students and visitors each year. Thomas Christopher is a volunteer at Berkshire Botanical Garden and is the author or co-author of more than a dozen books, including Nature into Art and The Gardens of Wave Hill (Timber Press, 2019). He is the 2021 Garden Club of America's National Medalist for Literature, a distinction reserved to recognize those who have left a profound and lasting impact on issues that are most important to the GCA. Christopher’s companion broadcast to this column, Growing Greener, streams on WESUFM.org, Pacifica Radio and NPR and is available at berkshirebotanical.org/growinggreener.