Buildings & Grounds Maintenance

A Tool for Pest Management at Stanford University

The blooming plants of late winter to early spring are not only beautiful to see, but they can tell us when troublesome plant pests may appear. For example, when the purple-leaf plums are in full flower, caterpillars of the western tussock moth will hatch from overwintering egg masses and begin to feed on new plant growth. This type of correlation between a plant and an insect is called a coincident bloom, and the collection of such observations over time is an important part of our Integrated Pest Management (IPM) program in the Stanford Grounds Department.

We check certain plants on a weekly basis and look at leaf and flower growth. We note when 50% of flowers are at each of the following stages: FB=flowers beginning, FU=flowers unfolding, and FF=full flower. We also make observations of leaves: LS=leaves sprouting, LU=leaves unfolding, LF=leaves full. These coincident bloom monitoring records are then compared to our pest monitoring records, which are generated from our weekly inspections of selected oak trees.

One of the most common caterpillars we find is the western tussock moth (Hemerocampa vetusta). This insect spends most of the year in egg form. In early spring, the eggs hatch into hungry larvae that grow as they feed on the new spring growth. After several weeks, the larvae spin cocoons and pupate. In early summer adult moths emerge from these pupae and live only long enough to mate and lay eggs. This completes the cycle until the following spring. Because the larvae are a nuisance and can cause considerable leaf damage to oaks, it's beneficial to be able to predict their appearance and duration on campus. This varies from year to year, and this is where our coincident bloom monitoring is a useful tool.

Our coincident bloom monitoring signals when to begin our weekly monitoring of tussock larvae in late winter and when to anticipate potential damage to our plants. This is possible because both insect and plant development are functions of temperature and daylength. When the weather is cold, spring flowering and insect hatching are delayed until conditions are more favorable. The converse is also true. The data we've collected over the past four years now serves as a guideline for when the larvae hatch, how long they'll be feeding, and when they will disappear. Here's what to expect this upcoming spring and summer:

Tussock caterpillars will hatch from overwintering egg masses and begin to feed on new plant growth when the following plants are in full flower:

Large tussock caterpillars will begin to disappear into cocoons, either singly or in clusters, when the following plants are in full flower:

Adult moths will hatch, mate and lay eggs when the following plants are in full flower:

The above lists are just a few of the observations we've recorded. We are looking into expanding our program to track other campus pests. By keeping accurate records of plant and pest development from year to year, we have created a pest management tool which helps to manage our workload and to let others know how long to expect problems with western tussock moth and other pests to be present.