—Dr. Raymond Cloyd
Caterpillars are the larval stage of butterflies and moths; whereas the larval stage of sawflies is greasy looking and slug-like with the adults resembling wasps. Remember, caterpillars are in the insect order Lepidoptera (butterflies and moths) whereas sawflies are in the order Hymenoptera (ants, bees, and wasps). There are a number of caterpillars and sawflies that feed on horticultural crops. Common caterpillar pests include bagworms, eastern tent caterpillar, fall webworm, mimosa webworm, yellownecked caterpillar, walnut caterpillar, cutworms, European corn borer, and tomato/tobacco hornworms. Sawflies that feed on plants include the European pine sawfly, brownheaded ash sawfly, rose sawfly, and scarlet oak sawfly. Sawfly larvae resemble caterpillars; however, there is a difference.
There are two ways to distinguish between caterpillars and sawflies. First, sawfly larvae have prolegs (stubby-looking legs) on every segment of the abdomen whereas caterpillars are typically missing prolegs. In Figure 1, a caterpillar is on the top and sawfly on the bottom. Second, caterpillar larvae have hairs or crochets on their feet while sawfly larva do not have hairs or crochets on their feet, which is shown in Figure 2, with the caterpillar prolegs on the top and sawfly prolegs on the bottom. Why is it important to know the difference between caterpillars and sawflies? Well, one of the common insecticides recommended for use against young caterpillars is Bacillus thuringiensis subsp. kurstaki that is sold under many trade names including Dipel and Thuricide. This is a bacterium that must be ingested or consumed by the target insect pest, in this case, caterpillars, in order for death to occur. However, the insecticide has no direct effect on sawfly larvae. Therefore, it is important to correctly identify the “caterpillar-like” insect before selecting an insecticide. Specimens may be sent to Kansas State University, Department of Entomology (Manhattan, KS) or the Kansas State University Diagnostic Clinic in the Department of Plant Pathology (Manhattan, KS).