Question of the Week: August 5, 2021

Sawfly larvae have terrorized this Shumard red oak branch.

“Why are the leaves of my oak tree like parchment paper?”

Sawfly larvae have fed on the bottom sides of these leaves. I get this question frequently from mid-summer through frost, and it can apply to many species of oaks.

Sawfly damage to bur oak leaves.

I see the damage every summer high up on a bur oak outside my home office window. People ask about their red oaks and live oaks, and I see it every once in a while on Chinquapin oaks.

Continued Below

What you should know about sawflies…
There are many species. Each carries the name of its host tree in its own name. So, you have “oak sawflies,” “elm sawflies,” etc.

The adult is a short-lived and almost unnoticeable wasp-like insect that lays her eggs on the leaf surface after using a part of her body to cut a slit into the leaf tissue, hence the name “saw” fly.

I downloaded this great photo from a university website a few months ago but now can’t find the source in order to give credit. I just spent another 20 minutes trying to find its source. This is what sawfly larvae look like as they feed.

The small larvae are gelatinous and harmless looking at first, but as they go through successive instars (phases), they get larger and their damage becomes more noticeable.

You can see the parchment-like look of the leaves and the network of veins.

The larvae feed on the bottoms of the leaves, skeletonizing them and leaving just a network of veins.

There is usually only one generation per year. The larvae fall to the ground with the dead leaves at the end of the season and emerge the next spring to start the cycle over again.

By the time the damage has become evident it’s too late for much control. However, if you catch them early enough general garden insecticides (not including B.t.) will usually stop them.

The good news is that their damage is only temporary. Affected branches will leaf out again the following spring.

Here is a very thorough explanation of sawflies from the University of Wisconsin Master Gardeners program.

Posted by Neil Sperry
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