What is happening to the pin oak trees? In many areas of Ohio, the pin oak leaves look very brown due to damage caused by scarlet oak sawfly.
The scarlet oak sawfly is a native insect and is one of several in this genus that are commonly found in the eastern half of North America.
Pin oak and scarlet oak are the most preferred host, but will feed on a wide range of oaks including red, black, and white oaks.
The three-eighths-inch long, black fly-like adults emerge in mid-May to early June after the oak leaves have fully expanded. The females insert their tiny, oval eggs in rows by slicing through the leaf epidermis.
Upon hatching, the larvae crawl to the leaf undersurface where they begin to feed.
The sawfly larvae will reach a maximum size of about a quarter inch. Their semi-transparent bodies are flattened toward the front and tapered toward the back. The flattened area is trimmed in yellow with the visible gut contents making it appear greenish-black to black.
Gross, right? It gets worse.
The larvae glisten in the sun and appear sluglike. This is due to their interesting habit of covering themselves with their own excrement, which helps them stick to leaves and presumably dissuades predators.
Their general shape and slimy appearance cause this type of sawfly to be called a ''slug sawfly.''
The larvae feed gregariously side by side consuming everything except the veins and upper leaf surface / 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 skeletonizing symptom associated with this sawfly.
The sawfly spends the winter as inside cocoons in the leaf litter. Development is completed in the spring when the adults emerge.
How can something so little cause so much damage? Well, one reason is that there can be two or three generations of this pest each summer. One generation can occur in 35 to 40 days. Consequently, damage tends to escalate as the season progresses.
Natural enemies normally control insect numbers. As with most native forest insect pests, populations can naturally rise and fall dramatically from year-to-year. The good news is that since most of the damage is caused late in the summer, the impact on healthy, established trees is minimal. Small, newly transplanted or stressed trees are most at risk. Repeated defoliation, however, may decrease growth, decrease vigor or kill trees.
Apparently, the scarlet oak sawfly liked the hot, dry weather this summer. Let's hope that the natural predators catch up next year.
Mary Smallsreed is a member of Trumbull County Farm Bureau and grew up on a family dairy farm in northeast Ohio.