Scarlet Oak Sawfly
Recently, the scarlet oak sawfly has caused the canopies of oaks to turn brown in several parts of Kentucky (ex. Magoffin and Letcher Counties in 2018 and 2019). These insects are native throughout the eastern United States but rarely cause significant defoliation except in occasional outbreak years. As the larvae feed on the leaf tissue on the underside of the oak leaves, they skeletonize the leaves and cause them to turn brown. Fortunately, while this may look bad, scarlet oak sawfly rarely causes any serious damage to healthy oak trees, so management of these insects is typically unnecessary.
What is it?
Scarlet oak sawflies (Caliroa quercuscoccineae) are insects that, as larvae, feed primarily on the leaves of a broad range of oak species (ex. scarlet, red, pin, black, and white oaks). These sawflies overwinter as larvae in cocoons in the leaf litter. In May or June, adults emerge and mate. Although they are a type of non-stinging wasp and not true flies, adults resemble flies and are black with yellowish legs and very small. After mating, female scarlet oak sawflies lay their eggs in rows along the midribs of the underside of oak leaves. These eggs hatch within a few weeks and the larvae begin to feed on the lower surface of the leaves, leaving only a skeletonized network of veins. The larvae can grow to about half an inch in length and are typically yellowish-green. Because these larvae often use a slimy substance to help them adhere to the underside of the leaves, they are sometimes called the oak slug sawfly. These larvae feed on oak leaves for about four weeks, then drop to the ground and pupate. There is often more than one generation of scarlet oak sawfly per year, so a new generation of adult scarlet oak sawflies often appears in July or August, mating and causing another wave of larvae to skeletonize the leaves.
What does it do?
The larvae skeletonize oak leaves by feeding on the lower surface of the leaves, eating the leaf material between veins, but leaving the network of veins intact. Oak sawflies often leave the thin upper epidermis of the leaf intact as well, but this quickly dessicates and falls from the leaves, creating the characteristic skeletonized look.
If a large number of larvae are present, the canopy of affected trees can suddenly turn brown. Browning and defoliation of oak trees typically begins in the upper canopy and progresses downward throughout the summer. Because multiple generations of this insect can occur each year, defoliation of the canopy often progresses in waves as each new generation of larvae feeds.
How can it be managed?
Management is typically not warrented for scarlet oak sawfly. Because most outbreak years are followed by years with relatively little damage, oak can recover from the defoliation and damage by this insect is thought to be minor in the long-term.
Typically, the presence of these sawflies is not noticeable until late summer when the feeding larvae have already caused defoliation, so it is difficult to preemptively manage these insects. However, defoliation is rarely fatal to healthy oak trees, so maintaining healthy forests is often the best management option. Following proper silvicultural practices in forested areas can help create a healthy stand of trees. Landowners can thin overstocked stands to proper stocking levels, remove invasive species, and use crop-tree release to favor healthy oaks and improve the overall health of the stand. In most cases, natural predators and other defenses help to keep the scarlet oak sawfly in check. During outbreak years, however, some active management plans can be used if desired. Using chemical controls such as insecticides can help limit the population if chemical sprays are administered while the larvae are still feeding on the leaves. However, as with any chemicals, insecticides should be used with caution. Instructions on the insecticide label must be read and followed, and proper safety practices must be observed. Contact your county extension office for more information on types and amounts of insecticides that can be used.