Sudden Oak Moth (or, the benefits of staring at tree trunks)

After being re-inspired to take a close and studious look at the trees around me by revisiting the Bark book, I was peering carefully at a live oak trunk a couple days ago and found this little guy:

Chrysalis of a California oak moth, Phryganidia californica, found on a live oak tree near lab.

I’m not sure if this is a new chrysalis or one from last fall that managed to hang on to the tree all winter, but either way it’s from a California oak moth (Phryganidia californica, aka oakworm).  Last fall there were so many of these dotted all over the oak trunks and some of the nearby buildings that it looked like a plague of incontinent pigeons had been through.  And the ravenous caterpillars that produced them had eaten so much live oak foliage that the trees were no longer recognizable as evergreens.  By late October there were virtual clouds of moths flitting around the tree tops when I headed home in the evenings.  Recently, I’ve been seeing a few young caterpillars dangling from the upper branches by their silk threads, so this could be from an early bloomer.

Mature oak moths. Photo by Jack Kelly Clark, University of California Agriculture and Natural Resources.

Happily, with a heaping pile of March rain, the oaks have made a full leafy comeback, and are prepared to handle another onslaught of caterpillar consumption.

Last fall this live oak tree had been heavily defoliated by oak moth caterpillars, but with the late rains this spring it's back to being full of bright green leaves.

I have mixed feelings about the California oak moths.  On the one hand, it’s still a bit of a thrill to find chrysalises out in the wild, and there were so many of these that it was easy to bring home a handful of twigs to give to my neighborlings (SM, now 6, and HM, 5) to watch hatch, which was fun.  I also have a soft spot for things that are delicately pretty but not showy, and the black-and-tan chrysalises and softly, silvery brown-grey moths appeal, since they’re so easily overlooked in favor of bigger brighter monarchs and swallowtails.

But they’re insatiable little buggers, and the caterpillars can do in a poor oak tree in an infestation year (the moths themselves don’t eat, just mate).  Oak trees that have been damaged by draught, other pests, or illnesses like sudden oak death (caused by the single-celled eukaryotic Phytophthora ramorum, which is related to inoffensive brown algae and diatoms) are very vulnerable to oakworms’ voraciousness.  Santa Clara county, where Stanford’s located, has had bouts of sudden oak death, and it’s been a dry year, so I was happy to see that the trees had made such a robust recovery.  Since the weather has been quite different this year than last (60-80% of average rainfall this year vs. 150-175% last year), I’m not sure how the oak moths will respond.  I may do another post about them this fall if they reach near-apocalyptic proportions again.


One thought on “Sudden Oak Moth (or, the benefits of staring at tree trunks)

  1. Pingback: Compare and contrast: oak moth infestation vs. zombie apocalypse | A bouquet from Mendel

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