Cimbex americana Leach is the largest sawfly that I have ever seen and apparently the largest in North America. Named for its predilection for American elm, a once common street tree in the pre-Dutch elm disease era, the elm sawfly also feeds on willows and the leaves of a variety of other hardwood trees. Although Dutch elm has yet to ravage the elms of Edmonton, none grow near the spot we found this large, wasp-like not-wasp: Elk Island National Park. Perhaps this monster is able to fly long distances, but a more reasonable assumption is that its larval stages were spent on some native like the willows that grow so abundantly in the park. The pale caterpillar like grubs have a black dorsal stripe and grow to 5cm (2 inches) in length, so if you have them on your trees, you are likely to notice them.
Our elm sawfly very obligingly posed for a few pictures in hand before we returned it to a leaf. Many people would probably respond with a bit of fear and loathing to such a large and scary looking ‘wasp’, but this attractive insect has no real way to do more than pinch your finger with its mandibles. The wasp-like show is just that – all show and no sting (but, of course, a saw instead). BugGuide.net has a lot of pictures of the adults of this highly ornamented species that seems to mimic different wasps in various parts of its broad range. There are about 15 recognized species in the genus including the intriguingly named Button Horn Sheet Wasp (C. femorata). I wonder how it got that name?