Oakworms, Again.

TOP PHOTO: Early instar orange-striped oakworm caterpillars go to work on red oak leaves.

It’s that time of year again when oakworms do their best to defoliate the red and willow oaks here at the museum. There may be thousands of these voracious moth larvae in the oak trees here at the museum but they never do as much damage as to harm the trees in any great manner. The trees seem to thrive regardless of the devouring hordes of caterpillars relentlessly munching on their leaves.

It often comes up in the lepidopteran literature about these moths, orange-stripped oakworms, that the onslaught of their masticating caterpillars is late in the season. The harm done to the trees is minimal. Most of the tree’s growth for the season has already taken place before the masses of caterpillars take over.

Here at the museum, I start seeing the adult moths in late May to late June. I begin seeing tiny frass on hard surfaces beneath the trees the moths have laid eggs on in mid to late July. The caterpillars become quite noticeable due to their size, the volume of their frass, and the missing leaves in the trees by the end of July and the first few weeks in August.

Adult male moth.
This looks like a female moth.
Recently hatched eggs.
Hours old, these caterpillars are about 3/8″ long.
Frass (caterpillar poop) on top of trash recepticle.
Close of frass.
Frass increases in volume as the caterpillars grow and munch.
This is about as large as they will grow (about 2 1/4″).

It’s also around this time, the first few weeks in August, that we start seeing the caterpillars hiking across the path seeking a place in the nearby woods to pupate. They pupate underground and will emerge about nine moths later as adult moths in late May to late June, when the whole process begins anew.

Marching off to pupate.
This one didn’t make it and will become food for larval yellowjackets.
This one made it to the leaf litter.

I will most likely be reminding you of the life cycle of this persistent species of moth around this time next year when I again start seeing the frass collect beneath the trees, the chewed leaves, and the caterpillars hiking across the path on their way to pupate.

2 responses to Oakworms, Again.

  1. Avatar
    P Hanlon says:

    How long do the oakworms stay on the Willow Oak raining their frass on my back deck? My willow oak is at least 50 years old and 50+ feet tall. It does’t seem to be affected by the caterpillars though I do notice some dead branches in the canopy. Tips are healthy. It’s a big nuisance to constantly be trying to clean up the frass. I have to move all furniture out of reach of their nightly bombardment. Just want to know how long the blitzkrieg will last?

    • gregdodge
      gregdodge says:

      When the bombing ends depends on where you’re located. Here at the museum it’s just about over. I’ve been seeing lots of caterpillars hiking across the path on their way to pupate. If they’re no longer in the trees they’re no longer eating and frass stops raining down. The entire larval stage lasts for more than a month, so if you’re seeing small frass and no larvae on the ground you may have weeks of bombardment left. Good luck.

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