What You Don't Know About Campus Caterpillars

Barry Grivett

Editor's note: Originally published in 2007, this story comes back around every spring. Just like the caterpillars.

Each spring, campus plants drip with caterpillars. They dangle from tree limbs, plop on sidewalks, land on unsus­pecting picnickers. But last year the critters arrived in such huge numbers they disrupted campus events and sparked a student debate in the Stanford Daily about appropriate remedies. We asked Karen Stidd, supervisor of horticultural support for grounds services, for the lowdown on larvae.

They’re hairy little buggers.
Three species of caterpillar are found on the Farm. The most common is the Western tussock, which looks like a miniature cactus in bloom. Although it hatches black, the tussock develops distinctive red and yellow spots, four tufts of dorsal hair, and spiky clusters of fuzz that can leave a rash on crawled-upon skin. “If you handle them, use gloves,” Stidd advises. The less prominent fruit tree leafroller and California oakworm are green, smooth and kinder to the touch.

Metamorphosis is not for wimps.
The Western tussock female lays its eggs in late spring, in clusters attached to trees. There the eggs must survive months of rain, winter cold and parasitic wasps (more on those later), before they hatch the follow­ing spring. The larvae gorge for a month or two and when sufficiently fat look for a suit­able spot to spin their cocoons. If trees are over­crowded, some will drop to the ground and look for another tree, notes Stidd, thus the occasional cater­pillar on laps and shirt collars. “They’re look­ing for a place to pupate.” (Just not on the lapel of the visiting dignitary, if you don’t mind.)

Spam, spam, spam, oak leaves and spam.
The Western tussock isn’t especially picky, but has a special fondness for oak leaves, which probably explains its attraction to Stanford, Stidd says. “Sometimes they drop out of oaks and start crunching manzanitas. We’ve even found them on rose bushes.”

Perhaps it helps to think of them as “pre-butterflies.”

Loved by schoolchildren and Disney animators, caterpillars are nevertheless treated as pests by horticulturists. “They aren’t quite so lovable when they’re eating your trees,” Stidd says. Left unchecked, they can seriously damage flora. Fruit trees are especially vulnerable. A healthy oak tree can survive even a radical infestation, but weaker ones may not, and the denuded canopy left behind is unattractive. “The real concern is aesthetic.”

Call in the egg eaters.
Stidd and her colleagues practice “integrated pest management,” which is another way of saying “no chemicals.” In bad years, trees may be power washed to destroy the eggs, or natural controls introduced in larger numbers. One of those natural controls is Telenomus californicus, a parasitic wasp that injects its eggs inside caterpillar eggs. When the baby wasps hatch, they eat their way out, consuming the caterpillar larvae. They are one reason Stanford avoids poison sprays. “The insecticide kills the wasps along with the caterpillars, and long term you may make it worse than it would have been otherwise,” Stidd says.

What’s up with the Plague of 2006?
No one knows. Stidd speculates that campus construction may have affected predators’ habitats, but whatever the reason, she acknowledges the campus was pelted with caterpillars. Although estimating numbers is difficult, “it had to be in the thousands, maybe tens of thousands,” Stidd says. It got so bad she and her colleagues became a kind of caterpillar SWAT team, called in to perform “emergency treatments” at campus events. This usually involved clearing picnic tables of a carpet of caterpillars. Nature has a way of balancing things out, though. “We can expect out­breaks periodically, but when that happens the predators profit, and the following year is usually back to normal.”