April, 2018

Elm Sawfly Larva

Cute little caterpillar, right? First, this insect is not exactly little; it is over two inches long. Second, while it is technically correct to call it a caterpillar, it will not become a butterfly or moth. This is a larva of the elm sawfly (Cimbex americana), a type of non-stinging wasp. Due to their white color and formidable size, elm sawfly larvae are frequently noticed around this time of year, as they feast on the young foliage of their favorite host, elm. After it finishes eating and growing, the larva will pupate in a tough cocoon. The adult is a large, glossy black insect, but they are not very easy to find and I've only seen one.

People often ask how to tell a sawfly from a moth or butterfly caterpillar. The most objective way is to count the number of prolegs, which are the suction-cup extensions on the abdomen that help the little beast hold onto branches while it eats. All insects have three pairs of real legs that are located right behind the head. The prolegs are the ones to the rear of those. A butterfly has five pairs of prolegs and a moth caterpillar can have from two to five pairs. Many sawflies, though, have seven pairs, so if you start counting and are not finished when you reach five, then you know you've got a sawfly.