As the old saying goes: if it looks like a duck, swims like a duck, and quacks like a duck, it’s probably a duck. Following that logic: if it looks like a caterpillar, walks like a caterpillar, and feeds on plants like a caterpillar, it’s probably a caterpillar—right?
This isn’t always the case and one important example would be the sawflies. Sawfly larvae look an awful lot like true caterpillars (which turn into moths or butterflies), but these creatures are actually related to ants, bees and wasps. In contrast, adult sawflies have a distinct wasp-like appearance which hints at the true evolutionary relationships of these creatures.
With a little know-how, you can learn to tell apart sawfly larvae from true caterpillars. In addition to three pairs of true jointed legs on the thorax, true caterpillars possess 4-5 pairs (or fewer for “inchworms”) of stubby, blob-like prolegs on their abdomen—each tipped with tiny velcro-like hooks (crochets) that can be seen under magnification. Sawflies have more pairs of prolegs (7 pairs) and lack crochets.
There are dozens of sawfly species in the Midwest, some of which can be considered plant pests, while others can go unnoticed due to their small size or cryptic habits. The commonest sawfly species can be pests of pines and other conifers, elms, birches, and other hardwoods, and ornamental flowers and shrubs such as dogwoods, roses, and columbines. If you’ve ever spent time thumbing through a caterpillar guide book and can’t seem to find a match, there’s always a chance you might be looking at a sawfly.
To further explore the world of sawflies, check out this article from the Wisconsin Master Gardener Program.