Don’t let the oak slug sawfly haunt you

by | Aug 30, 2017

Skeletons are becoming a familiar sight in Durham this summer, and it’s not from overly excited fans of Halloween décor. These skeletons are of once-green oak leaves, the remaining veins after the oak slug sawfly has had its fill. This insect is responsible for the sudden transformation of oaks in and around downtown Durham from lush green trees to gray shadows of their former selves.

In a process called skeletonization, the oak slug sawfly feeds on the green tissues of oak leaves, leaving the leaf veins behind. Image: J. Schelp, Durham resident.

The oak slug sawfly, also known as the scarlet oak sawfly, is a primitive wasp whose immature larval stage feeds on oak tree leaves. They feed on the green, pulpy part of the leaves, leaving only leaf veins behind. This results in the leaf framework being left behind, a feeding habit called skeletonization. The leaves look almost transparent as a result.

While this skeletonization is not Halloween-related, it can be every bit as spooky. Trees appear unhealthy and homeowners often wonder if this spells doom for their yard trees.

In most cases, trees damaged by the oak slug sawfly recover the following spring and suffer little if any long-term health problems. Natural enemies and diseases typically keep the sawfly populations down and reduce outbreaks after a year or two. However, if defoliation repeats itself for two, three or more years, tree health may decline, causing it to become more susceptible to secondary pest agents. Essentially, repeated defoliation lowers the ability of the tree to fight off other pests or stressors, potentially causing mortality. Because of this, and the oak slug sawfly’s prevalence in the urban environment where high-value trees are common, homeowners may assess their situation and decide to treat their trees. There are several insecticides available for use and interested homeowners should consult a certified arborist and always follow pesticide label directions.

So don’t let these leaf-eaters bother you. In a couple of short years, they should be down to a skeleton crew of a population again thanks to natural enemies.

When an oak tree is heavily attacked by the oak slug sawfly, it can take on a haunting appearance. Image: J. Schelp, Durham resident.