This distinctive pattern on an American Elm leaf is made by the Elm Zigzag Sawfly, a newly arrived species which is spreading rapidly. As far as is known, it was first discovered in Quebec last summer, and since then has spread to many other areas and just recently has been found with the City of Ottawa boundaries. It is yet another threat for the poor elm trees which already have to contend with other serious problems.

Linda is right, there doesn't seem to be a predator for these sawflies, yet anyway.