Severe Defoliation of American Elm by the invasive insect Elm Zigzag Sawfly (EZS)

by Owen Clarkin, OFNC Vice-President & Chair, Conservation Committee

The Ottawa Field-Naturalists’ Club has been tracking Elm Zigzag Sawfly (Aproceros leucopoda) since its recent (2020) detection in North America near Montreal. This includes making the first observations of EZS in Ontario (2020), the city of Ottawa (2021), the state of Vermont (2022), and the city of Toronto (2023).Our focus until recently has been detection: when we are at new locations, we have been searching for EZS to confirm whether it is present (easy, due to distinctive feeding traces in the leaves), or apparently absent (feeding traces cannot be found with effort). This insect has been documented to cause severe damage to Elm trees in Europe and in the southern United States as an invasive species.

A typical feeding trace of Elm Zigzag Sawfly, showing the origin of the name. The sawfly larva is visible at the top end of the feeding path. This one is on a Slippery Elm (Ulmus rubra) leaf, which so far has been uncommon. Gray’s Creek CA, 13 August 2023. Owen Clarkin photo.

In Ontario, we have observed that EZS has been generally easy to find toward the southeast (e.g. Cornwall), and more difficult to find or apparently absent to the northwest (e.g. Palmerston Canonto Conservation Area , Driftwood Provincial Park). We also have noticed that EZS has become increasingly easy to find and with greater defoliation effect each year from 2021-2023.

There is suddenly a lot more blue showing in this canopy, as this American Elm (Ulmus americana) has been fully defoliated by EZS. Gray’s Creek CA, 13 August 2023. Owen Clarkin photo.

On August 13th in Cornwall at Gray’s Creek Conservation Area, we noticed for the first time entire trees with severe defoliation impact from EZS. To our knowledge, this is the first time EZS has been shown to have severe impact in Ontario. American Elms (Ulmus americana) in particular were affected, often being severely to almost completely defoliated. Slippery Elm (Ulmus rubra) in our experience has rarely had EZS feeding traces found, but at the same site today EZS traces were present and conspicuous, but thankfully without significant effect.

Our third native species Rock Elm (Ulmus thomasii) was not found at the same site today, and prior to 2023 we had never seen an EZS feeding trace on a Rock Elm leaf. Unfortunately, as of August 17th, Rock Elms at several other Eastern Ontario sites were confirmed to be hosting EZS larvae. Like Slippery Elm, the effect on Rock Elm is not significant yet, but it did take several years before the severe damage to American Elm was detected. This means that while we hope for the best, EZS is now another confirmed invasive pressure on all three native elm species.

The damage caused by EZS at this site is so severe, leaves are being fully skeletonized, leaving only this “fishbone.” Gray’s Creek CA, 13 August 2023. Owen Clarkin photo.

It is our opinion that the level of impact observed to American Elms in Cornwall on this date is noteworthy, and EZS may be poised to further devastate a species and genus already profoundly endangered by Dutch Elm Disease since the 20th century. Other naturalists and citizens are encouraged to search for Elm Zigzag Sawfly to help discern the range already invaded, and the degree of impact to Elms.

Links to some of the iNaturalist observations of EZS from August 13th

The impact to some of these trees is shocking.

Links to some of the iNaturalist observations of EZS from August 13th

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