Willow Oak planted 10 years ago is being defoliated by caterpillars of 2 types

Asked August 7, 2016, 4:59 PM EDT

Last year there were some caterpillars on this tree. I had been checking the tree periodically, but this weekend I realized that it was heavily infested. About 1/3-1/2 of the leaves are gone or damaged. One caterpillar I saw looked like the typical "tent" caterpillar I am familiar with, I don't have a picture of that one. Today I found only another, larger caterpillar with black and orange stripes. This orange one I photographed and attached to this e-mail. Over that last 5-10 years we discontinued using any pesticides in our yard and it seems to have coincided with a return of the bees in numbers. Is there something I can treat this with, with a minimal disruption of other insects? Thank you very much, Frederick, Maryland

Frederick County Maryland pest control willow oak orange striped oakworm

2 Responses

This looks like the orange striped oakworm. This is a late season pest that feeds on oak and other tree species. Defoliation usually occurs one branch at a time when populations are small.
Mature larvae are about 1 1/2 inches long. They are black with eight orange stripes and two black spines behind the head. Look for clusters of caterpillars feeding near defoliated areas. Jar the branches and some may drop to the ground making collection and identification easier. The caterpillars feed in clumps on the ends of branches. Older caterpillars are defoliators. You can prune out branches or knock them down with a pole pruner to make collections easier. Drop into a bucket or soapy water.
It is late in the season and usually by this time of year the trees have stored enough energy to be able to take the defoliation in stride. There is an ichneumonid wasp that parasitizes this caterpillar. No control is recommended.
Next season start monitoring for this pest from June through September looking for defoliation. You can manually destroy young larvae on small trees. If you have a large infestation, you can spray with Bacillus thuringiensis, a biological control that affects only the caterpillars and not humans or non targets that are not caterpillars. The caterpillars need to be under an inch for good control.

Thank you very much, that is very informative and helpful!