Several species of oak sawflies commonly attack the foliage of oak trees. The scarlet oak sawfly, Caliroa quercuscoccineae, feeds on red and white oaks but is most common in Iowa on northern red oak and pin oak. Sawfly larvae damage the plants by feeding on the lower surface of the leaves. They eat the leaf material between the veins but leave the network of veins ("skeletonization"). The thin layer of the upper epidermis remains on the leaf and quickly dries and turns brown.
The oak sawfly is a wasp. However, the larvae are sometimes called "slugs" because they superficially resemble true slugs (shiny/slimy and nonsegmented in appearance, largest just behind the head and tapering toward the tail end). The scarlet oak sawfly larva is black to dark green and grows to a final length of approximately 1/2-inch. Larvae are present on the trees through much of the summer although evidence of feeding is most apparent in late summer.
This insect overwinters in a cocoon 2 to 3 inches below the soil surface. The adults (1/5 inch, fly-like, nonstinging wasps) emerge from the ground in May and June. The females insert eggs into the leaves of a host plant. The eggs hatch after 1 to 2 weeks and the young larvae feed on the leaves for about 4 weeks. When full grown they drop to the ground, burrow into the soil, pupate and emerge as adults that produce a second generation in late July or August.
Defoliation, which may range from spotty to complete, is not usually fatal to healthy, well-established trees. Small, newly transplanted and stressed trees may warrant protection from severe defoliation. Otherwise, control is probably not justified. It is typical to discover the damage after the larvae have finished feeding and dropped from the leaves. In this case, especially in late summer, it is too late to take any effective action.
Management of sawflies must commence while the larvae are still small. Oak sawfly larvae can be physically removed from infested trees if there are only a few on small trees. If handpicking is not practical, chemical control may be warranted, but only when large numbers of larvae and a serious damage potential exist. Sprays applied after larvae have left the leaves do no good.
Most garden insecticides can be used to control sawfly larvae when control is warranted. Be sure the specific host is listed on the insecticide label before you use the product. Read and follow label directions.