The Elm Sawfly is the largest sawfly in North America, measuring up to an inch long. They may look like a fly (and have “fly” in their name), but they are actually in the order Hymenoptera (wasps, bees, and ants). The larvae look very similar to caterpillars, but they’ve got a little extra in the proleg department, the “false” legs along the abdomen that look like suction cups. Butterfly and moth caterpillars have no more than five pairs of prolegs (there’s always an exception — flannel moth caterpillars), while sawfly larvae have more than five pairs. Like caterpillars, most are herbivores. This particular one fell from a willow tree, where we assume it was feeding. Besides willow (Salix), larvae enjoy foliage from elm (Ulmus), maple (Acer), birch (Betula), and basswood (Tilia). Larvae overwinter in cocoons in plant litter or just below the surface of the soil, pupate in the spring, and emerge as adults in May or June.
Photo by Carmen Smith on 9/4/20 in Norris, MT