Timothy J Gibb, Department of Entomology, Purdue University
Caterpillars are the larval form of butterflies and moths, and belong to the order Lepidoptera. Sawfly larvae may look like caterpillars, but belong to an entirely different order of insects (Hymenoptera). In fact, sawflies are much more closely related to bees and wasps than they are to butterflies and wasps.
The photos shown of Mallow sawfly were taken by John Obermeyer and serve to illustrate a question that we get in the Insect Diagnostic Laboratory quite frequently. ……. Are these caterpillars or are they sawflies?
So, how can one tell the difference between a sawfly larva and a caterpillar - especially when both are found feeding on the foliage of trees and other plants?
The secret answer lies underneath the insect in the form of prolegs. Prolegs are stubby and fleshy, unsegmented legs, usually found in pairs on the 3rd through 6th abdominal segments of caterpillars. These are the structures that help them climb vertical stems or even crawl upside down on the undersides of leaves. Caterpillars may have up to five pairs of abdominal prolegs but never more. Sawfly larvae always have six or more pairs.
If there is still a question and a form of magnification is available, look for the presence of tiny hooks (called crochets) on the ends of their prolegs. Caterpillars have them - sawflies do not.
Knowing how to determining whether a specimen is a sawfly or a caterpillar may impress your friends but how important is it in real life?
Answer: It is of prime of importance when it comes to controlling them. Some pesticides (such as Bt, (Bacillus thuringiensis) kill caterpillars easily but are ineffective against sawflies.
On the other hand, pesticides that traditionally are effective against bees and wasps are also effective sawfly larvae.
The bottom line is that not all caterpillar-like insects are caterpilllars. You should always be certain that it is a caterpillar before you apply caterpillar controls.
Sawfly larvae can cause serious harm to certain plants. They most always feed in large groups and when they do so on evergreen plants such as pine trees, they can cause irreparable damage in a very short time.