Monday, October 1, 2012

Scarlet Oak Sawfly

skeletonized oak leaf
“What’s up with the oaks?” or “Are my oak trees are dying?” were common questions from residents in the Hilltown/Perkasie/Bedminster area this year. Symptoms included whitening in the tops of pin oaks and, on closer examination, leaves that had been “skeletonized”. This means that all of the soft parts of the plant tissue were gone but the “bones’ remained… just the skeleton.

You might think, as I did originally, that the insect called Oak Leaf Skeletonizer was the culprit. But a conversation with Penn State entomologist Greg Hoover led to the conclusion that this was another insect, Scarlet Oak Sawfly. Both insects skeletonize leaves but Scarlet Oak Sawfly does not leave tell-tale pupal cases on the leaves as does the Oak Leaf Skeletonizer.

pin oak injury from sawfly feeding
 The clincher was speaking with arborists Craig Brooks (Bedminster) and Bob McMullin (Doylestown). Both reported diagnosing the problem in the past two years. Craig and Bob are International Society of Arboriculture (ISA) certified arborists and excellent tree men. If you are looking of tree care, an ISA certified arborist is a really good idea.

So, what will become of oaks infested with Scarlet oak sawfly? PSU’s Greg Hoover thinks that natural predators and parasites will begin to take control and reduce sawfly populations to levels that are almost harmless. Defoliation is the issue here. The question is: “How much foliage can a tree stand to lose?” Answer is: "Some, not too much, not too often." It is not a black and white situation. Many factors such as overall tree condition, amount of foliage loss and site factors come in to play. It is unlikely that partial defoliation in one year will be a life or death situation for an oak.

sawfly larva feeding
 However, major defoliation in consecutive years is a problem. Think about gypsy moth damage. Similar scenario here. Except that Scarlet oak sawfly has two generations, one in early summer and one in late summer. Insecticide options exist, but the bugs habit of feeding from the tops of trees downward can present logistical problems... how to get good spray coverage at the top of a 50 foot tree? If necessary, arborists have the tools to do the job. It is not a do-it-yourself situation.

Personally, I have my bet on Mother Nature to come to the rescue. Sure, She can be unpredictable and is habitually late, but population spikes of one bug eventually result in their enemies coming along to even things out.