This article is the fourth in a series of featured stories this spring/summer with our partners at Bartlett Tree Research Laboratories. This partner series aims to educate the DC region about its insect inhabitants and provide practical tree care advice to residents.
In my role with the Bartlett Tree Research Labs, I spend a lot of time in landscapes throughout the Mid-Atlantic trying to figure out what is going on with ailing trees and shrubs. What part of a tree is dying back? How long has it been happening? Has it been getting too much water or not enough? Is there any evidence of an insect or disease? If you have plant material in your yard or home, you probably have to ask some of these same questions! For those reading who want to sharpen their diagnostic skills, North Carolina State Extension has a phenomenal website that will have you thinking in a systematic approach throughout the plant diagnostic process.
Unfortunately, our patients don’t generally tell us exactly what is wrong, and that can make the diagnostic process particularly challenging. Honeydew, the sweet, sugary by-product produced by crapemyrtle bark scale, is also produced by many other bugs with piercing/sucking mouthparts (e.g., scale, aphids, lanternfly). While its presence could therefore be a helpful sign of an insect pest, we would have to dig deeper to figure out the specific causal agent and the appropriate approach to management. Sometimes, however, we in the plant healthcare space get lucky, and a new pest will have such a unique appearance or cause such distinctive damage that you just can’t miss it. Our critter for this installment does just that, the elm zigzag sawfly.
The elm zigzag sawfly (EZS) is an invasive Hymenopteran (the same insect order that contains bees, wasps, and ants) that has only been observed in North America over the past few years. Since its first detection in Canada, the sawfly (or the damage it causes) has been confirmed across a multitude of counties from Vermont out to Ohio and all the way down to North Carolina. The larval sawfly itself is fairly unremarkable, but the damage that this life stage causes to most, if not all elm species, is unmistakable. During my recent travels, I happened to stumble upon the characteristic feeding pattern on some elms in an urban plaza in Washington County, PA and then some nursery elms in Montgomery County, MD. In both cases, there was no evidence of the insect, just the damage left behind.
As of today, the concern around this new invader is low to moderate, in part because there remain many unknowns surrounding its biology and ecology in this new range. At low population densities, some defoliation of a healthy elm tree is far from a death sentence. Even severe defoliation as has been seen in North Carolina is unlikely to kill a tree outright if it is a one-time event. Should EZS continue to spread and persist in landscapes year after year, the concern grows that native, non-native, and hybrid elms could all be at risk of a significant challenge in the future. As many tree nerds and casual plant enthusiasts alike are no doubt well aware of, elms, and the American elm in particular, have already experienced a difficult past century (see: Dutch elm disease) and do not need another stressor in the mix. The National Mall, with its population of hundreds of American elm trees in varying health conditions, comes to mind as a local area that stands to be impacted by the spread of this new insect.
Fortunately from a management standpoint, sawflies are not brand-new pests in the Mid-Atlantic. Treatment strategies that work against common foes like rose slug sawfly or European pine sawfly stand a good chance of being efficacious against this new pest, but further research is needed to confirm this. A good integrated pest management (IPM) program begins with proper identification, so be on the lookout for feeding damage or the larvae themselves (but note that caterpillars and sawflies are readily confused!). Report sightings to your local natural resource management entities/authorities and consider contacting an International Society of Arboriculture certified arborist to get assistance with EZS management, particularly on larger trees.
