Problems and Solutions

The Caterpillar Story

You know it's spring at Stanford when you sit down after a walk on campus only to find caterpillars in your hair and on your clothes. These bright green, black and brown worms are actually the larvae of several moths that are common in California's oak woodlands. The rural Stanford University campus is unique in retaining pieces of this plant community and providing a refuge for the animals that call it home.

The most common caterpillars on campus are those of tussock moth, fruittree leafroller, oakmoth and plutellid moth. These moths spend most of the year as eggs, then hatch in late February into hungry larvae that feed on new spring growth. Some larvae, such as oakworms, feed on only one kind of plant, while others feed on many hosts, like the tussock larvae that feed on oaks, liquidambers, hackberries and manzanitas here. When disturbed, or perhaps to find a tree with less competition for food, the larvae descend on silks and dangle in the wind. We often carry them to another site unknowingly, either on our bodies or our cars. By late April, many larvae have grown quite large and go from feeding to seeking a good spot to pupate, or prepare to emerge as adults. Leafrollers pupate in curled leaves, while tussock moths can congregate on tree trunks (lampposts, garbage cans, etc.) and spin clusters of cocoons. By June, the inconspicuous moths are flying, mating and laying eggs that survive the dry summer and cold wet winter.

The Grounds Services Department takes an IPM (Integrated Pest Management) approach to caterpillar management. Employees monitor selected oak trees on a weekly basis. They take random samples from each tree, then count and record the type, the developmental stage, and the number of each insect seen. Trees are selected based on their location (high visibility or high nuisance potential) and history (past defoliation or complaints received in past).

Action is taken only if unusually high numbers of pests are present, as these moths are part of a community with its own natural enemies.

The oak trees and these moth larvae have coevolved; the larvae feed on the tender new growth, before the leaves have concentrated their tannins. The larvae are only active in the spring, and oaks usually put out a second flush of growth in the summertime that is rarely damaged. Larval populations appear to be cyclical; one year may be a bad year for tussock moth larvae while very few oak worms may be present. The following year the pattern may be reversed. Large populations of one kind of insects often crash, due to predators, fungal diseases or viruses that limit pest populations. Tolerating this temporary problem is sometimes the best solution.

Each year hot spots develop, where larvae eat the new growth and produce frass (droppings) in quantity, as well as crawl on (and into) nearby structures. When a quick fix is necessary, we wash off the larvae with water. This temporarily reduces the population. We have good results preventing such hot spots by washing the trunks and structures of trees with water under pressure in the summertime, removing freshly laid egg masses.

In the past, we released lacewing larvae from a commercial insectary just after the caterpillars hatched, to eat these tiny moth larvae. In the late summer, we release Trichogramma platneri wasps that parasitize egg masses. The tiny wasps (harmless to people) lay their eggs in moth eggs, and the developing wasps eat the contents of the moth egg, destroying the pest in the process.

Currently, we are surveying our existing natural controls—those insects or predators that keep the pest populations in check. A native parasitic wasp, Telenomus californicus, has been found in tussock egg masses throughout campus in the fall. In the springtime, we see many birds feeding in the canopy. Scrub jays, California thrashers, phoebes, American robins and bluebirds are especially active in March and April, when the larvae have grown to 0.5-1 inch in size. Nocturnal beetles (Calosoma spp) live in leaf litter by day but crawl into tree canopies at night to feed on caterpillars. Tachinid flies parasitize the larvae and pupae of various moths. In our monitoring, we routinely find predators: lacewings (adults and eggs), assassin bugs (eggs, nymphs and adults), Mirid bugs and spiders.