Problems and Solutions

Where Have All the Larvae Gone?

Walking around campus in spring, you may have noticed fewer tussock moth caterpillars dangling down from the oaks. You may have asked yourself, "Where have all the larvae gone?"

In the Grounds Department, we count larvae on selected oak trees once a week throughout the spring. The numbers are indeed down, and in some parts of campus, there are virtually no larvae.

Several factors could account for the quiet spring. In 1997 we stopped annual spraying of the oaks with pesticides, and adopted an Integrated Pest Management approach instead. We now monitor, or count larvae on eight random samples, on trees throughout campus. When the count is higher than a set tolerance level, different intervention options are considered. We also note beneficial insects that feed on insect pests. We have been finding higher numbers of these, such as parasitic wasps, lacewings, and ladybird beetles.

It could be that a balance between pests and beneficial insects has been reestablished. Spraying kills both good and bad insects indiscriminately, but enough time has passed that both good and bad insect populations may have rebounded in such a way as to balance one another. All are part of the naturally occurring oak woodland ecosystem that was here before the urban development of the area.

One of our interventions has been releases of beneficial insects, such as green lacewing larvae, in 1999 - 2000. We continue to find eggs, larvae and adults that indicate lacewing populations exist in many parts of campus.

Another effective intervention option has been the power washing of egg and cocoon masses from tree trunks, buildings and other structures in the fall. In areas where there has been much larval feeding in the spring, there are masses of cocoons on trunks where larvae congregate to pupate. The adult females that hatch from these pupae are flightless. They emit pheromones that attract flying males, and then mate and lay eggs in virtually the same spot they emerged. This limits the mobility and spread of the population and creates localized hot spots. On trees where there have been many larvae, we have used water under high pressure to wash off egg masses from tree trunks in the fall. Thus decreases the number of larvae that hatch the following spring, but allows enough to remain in the upper canopy to provide food for birds, spiders and beneficial insects. When we power wash hot spots, we have found much lower numbers of larvae the following spring.

Another likely but little understood reason for the low number of larvae this spring is the cyclical nature of insect populations. Insects reproduce in large numbers and can quickly build up the size of their population over one to several years. However, when they exceed the carrying capacity of their site, their population can crash, and it takes time to rebound again. The crash can be caused by lack of food, a boom in the predator population, weather phenomena, or viruses that appear, spread and disappear rapidly. It may be that we are on the downhill slope or low point of the population wave. As we gather more information each spring, we are better able to predict and answer the question, "Where have all the larvae gone?"