It’s easy! Count the prolegs.

Prolegs are the fleshy leg-like structures on the caterpillar which occur on the abdomen behind the actual front legs. True caterpillars (larvae) of butterflies and moths (Lepidoptera) have 3-5 pairs of these prolegs. (See the illustration of the Cecropia moth caterpillar.)

There is another group of insects called sawflies, whose larva resemble hairless Lepidoptera caterpillars, however they exhibit 6 or more pairs of prolegs. Sawflies are related to wasps and bees (which the adults resemble); their larvae are typically pests of woody plants. Next time you see a “caterpillar” check out its prolegs!

To help you identify sawfly larvae pests, check out this publication: Sawflies of Trees and Shrubs from the University of Minnesota