California oak moth caterpillars eat all the leaves on an oak, leaving a brown skeleton. Then they rappel down on a strand of silk, twirling and swinging. If you were enjoying the shade, good luck getting out of their way. For the oak, the caterpillars are a bigger deal -- will the tree survive?
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It’s an event that scientists still can’t explain. Every few years, a light-brown moth native to California seems to appear out of nowhere up and down the state. These California oak moths surround a few trees, usually coast live oaks. Females beat their wings frantically to attract a mate. Once they lay their eggs, the tree is in trouble.
Bright-yellow, orange and black caterpillars, known as California oakworms, cover its leaves after a few months.
“They will completely devour one tree, while the tree right next to it seems completely untouched,” said Peter Oboyski, executive director of the Essig Museum of Entomology at the University of California, Berkeley. “That suggests that the females are all laying their eggs on the same tree. Is it because they’re attracted to each other? Or is it because this tree is particularly yummy and the tree next to it maybe has too many defensive chemicals? A lot of what’s interesting about this story is all the mystery around it.”
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--- What are those clouds of moths around that oak?
In the San Francisco Bay Area, California oak moths emerge in mid-June and in early to mid-October. As the sun sets, you’ll see clouds of them fluttering around an oak, often a coast live oak. Moths lay their white and red eggs on the oak’s leaves or on plants below the tree.
The moths that fly in June lay eggs from which caterpillars hatch in early July. The moths that fly in mid-October lay eggs that hatch at the end of the month. These caterpillars develop very slowly over winter and turn into the pupae from which moths will emerge in mid-June. That’s why California oak moths often lay their eggs on coast live oaks so that the evergreen trees can provide foliage for their hungry caterpillars through the winter.
--- Should you get rid of California oak moths?
Even though it’s harrowing to watch caterpillars defoliate an oak, Oboyski said that healthy trees usually survive the onslaught and grow back their leaves.
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