Top Photo: Sawfly larva on oak leaf (note eight pair of prolegs).
I walk by the tree numerous times a day. I knew it was a white oak and I knew it had some sort of leaf miners or skeletonizers actively feeding on the leaves. The leaves were turning a lighter shade of pale from their centers outward.
I was tempted to find out what was going on with the tree but didn’t act on it. I didn’t act on it, that is, until Ranger Brooke gave me a little nudge when she mentioned to me she thought some sort of small caterpillar, “inch worm,” I think she said, was eating the leaves, and did I know what it was.
So, naturally, I had to sort this out. I first had to figure out what kind of oak tree leaves the little inch worms were eating. And then, I had to find out what the little inch worms actually were.
The tree’s leaf shape, the fact that the leaves are dark on top and pale below (aside from the eaten portions of the leaves), and the peeling bark on the branches convinced me the tree is a swamp white oak. Swamp white oak is essentially a mid-western if not northern oak. We’re near the southern limit of its range here in Durham.
The tree grows in wet woods and swamps and, like all oaks, can hybridize with other oaks. This one seems to have all the basic characteristics of a genuine swamp white oak (Quercus bicolor).
I knew what the tree was, now, I had to figure out what was eating the leaves. I grabbed a few leaves from the tree. One of the leaves had two larvae clinging to its underside, but I couldn’t tell whether they were caterpillars or some other insect larvae. The larvae were 7 mm and 10 mm in length.
A few close up photos revealed the larva had three pairs of true legs and eight pairs of prolegs. Being insects, the three pairs of true legs at the anterior end of the larva was as expected. The eight pairs of prolegs indicated the insects were not caterpillars but a sawfly larvae.
True legs continue on with the insect into adulthood, prolegs are only worn during the larval stage and are used for grasping onto a substrate, a twig, leaf edge, or any other object the larva wishes to remain attached to.
Lepidoptera, butterflies and moths, have five or fewer prolegs in their larval stage. If a larva shows six or more pairs of prolegs, its not a butterfly or moth larva. In this case, with eight pairs of prolegs, it’s a sawfly larva.
Sawflies are related to bees and wasps. They’re stingless wasps. In the larval stage they consume vegetation. It seems there’s a sawfly species for each plant type or family. I had to find out what sawfly specializes in oaks.
The larvae were only eating the surface of the leaves and only on the underside of the leaf. I quickly focused on the slug or scarlet oak sawfly (Caliroa quercuscoccineae). Like the scarlet oak sawfly larvae, my larvae were yellowish in color, had black true legs and face, and their feeding behavior was as described, they graze on the surface of the underside of oak leaves, red or white oaks. If this is not Caliroa quercuscoccineae it’s certainly a species within the genus Caliroa.
The sawfly larvae are essentially tiny eating machines, in one end, out the other.
The adult sawfly lays eggs in rows along the midrib of the leaf. Upon hatching, the larvae feed on the lower surface of the leaf leaving the veins intact. The upper surface of the leaf dries and eventually falls off. This makes the leaf transparent, or at least translucent. The sawfly larvae can completely defoliate a tree, from top to bottom.
When the larvae have eaten their fill, they drop off the leaf and onto the ground, form cocoons in which they overwinter in the leaf liter beneath the tree, pupating the following spring.
Sawflies are named for a saw-like organ at the tip of their abdomens used for cutting into vegetation for oviposition (laying eggs).
Thanks Ranger Brooke, for making me actually act on what I was already wondering about. I learned a lot in the process. Follow the science.