Note to caterpillars dangling under the oaks: Meet the beetles
Western tussock moth caterpillars create their cocoons in the oaks where they hatch and feed. But they descend on silk threads to find another place to pupate when trees get too crowded, which is very much the case this spring. Grounds Services is taking an integrated pest-management approach by releasing microscopic worms and spined soldier beetles, as well as power-washing trees and surfaces where eggs are laid and cocoons are left.
The Grounds Services department is taking several steps to battle a boom in the number of those fuzzy caterpillars dripping from oak trees throughout campus, including the release of predatory spined soldier bugs. Reinforcements will be sent in later in the season, in the form of predatory nematodes.
An especially large infestation last spring has groundskeepers stepping up pest-control efforts now to avoid another outbreak next season, according to Karen Stidd, a horticulturalist and certified arborist in Grounds Services. Stidd said the department takes an integrated pest management approach, which calls for chemicals as a last resort.
"Natural cycles in insect populations rise and fall, and as the population of natural enemies increases, we can expect the tussock population to decline to a more normal level over the next few years," Stidd said.
The majority of the caterpillars on campus are Western tussocks, which typically hatch around the end of February, Stidd said. The larvae then gorge on oak leaves until it's time to spin their cocoons and morph into moths. But when a tree becomes too crowded, the pests drop to the ground to crawl up another tree—unless they land on you first.
The caterpillars are usually gone by Commencement. But according to Herb Fong, Stanford's head gardener, the season may sometimes extend into summer, especially when there are several cycles of caterpillars pupating. So in addition to releasing predatory insects, groundskeepers are currently power-washing caterpillars off trees and will do subsequent washings later in the season when eggs are laid.
Healthy trees can usually withstand the damage caused by caterpillars and grow new leaves, Fong said. But an all-out attack on weaker trees can leave them unattractively stripped of their canopy or, if infested over several seasons, dead. Furthermore, the hair of the tussock caterpillar can trigger allergic reactions, either in the form of a rash or, if inhaled, respiratory problems.
Neither the nematodes—microscopic, non-segmented worms—nor the spined soldier bugs are harmful to humans, Fong said. Other natural predators of the tussock caterpillar on campus include several species of wasps, assassin bugs, lacewings and birds.
Although less common, two other types of caterpillars found on campus are the fruit tree leafroller and the California oakworm. Both are smooth and kinder to the touch. The prickly tussock, on the other hand, has red and yellow spots, spiky clusters of fuzz and four white tufts of back hair.
Fong said the caterpillars are especially abundant in landscaped areas, so people should inspect benches and picnic tables before sitting there. And for those walking or biking, Fong said a good idea might be to look up occasionally.