First generation scarlet oak sawfly (Caliroa quercuscoccineae) larvae are munching oak leaves in southwest Ohio. The larvae are currently in the 1st and 2nd instar stages and a little less than 1/8" long. Despite this sawfly's common name, larvae may be found feeding on a wide range of oaks including pin, black, red, and white oaks as well as its namesake oak.
The larvae feed gregariously side-by-side on the lower leaf surface consuming everything except the veins and upper leaf epidermis. Initially, the upper epidermis has a faded, whitish appearance. Eventually the epidermis dries out, turns brown, and drops from the leaf leaving behind the veins to produce the characteristic skeletonizing symptom associated with this sawfly.
The larvae have semi-transparent bodies that are flattened towards the front and tapered towards the back. The flattened area is trimmed in yellow with the visible gut contents making it appear a greenish-black line is running down the middle. The larvae glisten in the sun and appear slug-like. This is due to their interesting habit of covering themselves with their own excrement that helps them stick to leaves and presumably dissuades predators. Their slimy appearance gives rise to another common name: oak slug sawfly. Late instar larvae develop two peculiar black markings on their anterior end that makes them look like they are wearing sunglasses.
The sawfly spends the winter as late instar larvae inside cocoons in the leaf litter. Development is completed in the spring. Once the black, fly-like females are mated, they use their saw-like ovipositors to insert eggs in rows along major leaf veins. There are 2 - 3 generations per season in Ohio; consequently, damage tends to escalate as the season progresses.
Thus far, first generation populations appear to be small and randomly scattered in southwest Ohio. However, populations of each succeeding generation should be closely monitored because this sawfly has a history of producing significant defoliation on oaks in Ohio forests and landscapes.
Heavy defoliation of oaks occurred during “outbreaks” of this native sawfly in southern Ohio in 1997 and 1998. In 2011, Erik Draper (OSU Extension, Geauga County) reported heavy defoliation of oaks in the northeast part of the state. However, as is common with native general defoliators, years with high populations are usually followed by many years with almost no evidence of leaf damage allowing trees to recover without the aid of insecticides to suppress sawfly populations. Indeed, southern Ohio has not experienced widespread defoliation from this sawfly since 1998 and no significant tree loss was associated with that outbreak.