Upon inspecting a skeletonized Alternate-leaved Dogwood (Cornus alternifolia) recently, I found the culprits responsible. Wasp-like insects called Dogwood Sawflies (Macremphytus tarsatus) were eating every vestige of leaf (except for the midveins) on the tree. Although called sawflies, they are not true flies — they are in the same order (Hymenoptera) as ants, bees and wasps. The first part of their common name comes from the saw-like appearance of the females’ ovipositor which they use to cut into plant tissue and lay eggs.
Dogwood Sawfly eggs are laid in dogwood (any Cornus species) leaves in the spring. A translucent yellow larva hatches from each egg. As a larva feeds on dogwood leaves, it grows and molts several times. After molting a second time the larva is covered with a powdery, white waxy coating which makes it look like a bird dropping (a survival technique used by several species of insects). After the final molt it is yellow below with a single row of black spots above (see photo). In late summer or early fall, Dogwood Sawfly larvae seek shelter in logs or other decaying wood, and pass the winter in a pre-pupal state in cocoons made of rotted wood. They pupate in the spring, emerge as adults, mate and lay eggs.
Although it looks like a caterpillar, this larva is not going to metamorphose into a butterfly or moth. This is because it is a sawfly larva, and is closely related to bees and wasps. (It gets its name from the adult female’s saw-like, egg-laying ovipositor that opens like a jack-knife from the tip of her abdomen.)There are several ways to distinguish between these two types of larvae (sawflies and butterflies/moths). While both have three pairs of true legs on their thorax, caterpillars (larvae of moths and butterflies) have up to five pairs of prolegs (fleshy structures that resemble legs) located on their abdomen behind their true legs, while sawfly larvae have six or more pairs. A closer look at the tips of the prolegs on caterpillars will reveal tiny hooks called “crochets,” which are lacking on sawfly larvae prolegs. Sawfly larvae also exhibit distinctive behavior. If you see something that looks like a caterpillar feeding along the margin of a leaf and it rears up its hind end when disturbed (perhaps to frighten predators), chances are great that you are looking at a sawfly larva.