Due to differences in genetic makeup and exposure to environmental factors (such as soil moisture and nutrient levels and exposure to plant pathogens and herbivores), plants vary in their chemical and physical traits. This can cause differences in susceptibility to herbivory or differences in nutritional quality that attract herbivores. Therefore, one might expect to find differences among plants in the number of herbivores that feed on them, the ways herbivores select feeding and oviposition sites, and the success of these herbivores. In this observational experiment, students will conduct investigations of sawfly galls (Hymenoptera) on willow (Salix) trees to examine some of the ecological and possible evolutionary consequences of plant-herbivore relationships to each of the interacting species. Galls make great sampling units for investigating herbivory because they are discrete (each gall contains one herbivorous larva), quantifiable (easy to see and count), and indicative of insect preference (since each gall represents one successful oviposition by a female sawfly) and performance (successful larval development is indicated by an emergence hole). During a single lab period, students will become familiar with the plant-herbivore system and work in teams to collect data to test a general hypothesis proposed by the instructor. Teams then choose a second instructor-generated hypothesis or develop their own to test. Examples of these are (1) The level of herbivory by sawflies varies among willow trees, and (2) Galls on leaves with competing galls are less successful than single galls on leaves. Outside of lab time, students will analyze their data statistically, and prepare a formal oral report on their investigation.


Kristina A. Ernest

Department of Biological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926-7537, ernestk@cwu.edu



One 3-hour lab period, plus approximately an hour of another lab or lecture period for oral reports. Additional lab time for data entry into a spreadsheet (requires computer access), and statistical analysis.


Two to three hours for statistical analysis and preparation of oral report


  • data set for hypothesis 1
  • data set for additional hypothesis
  • formal oral report based on investigation
  • peer evaluation of other oral reports


The field work is conducted at any site with several willow trees that have galls on the leaves. This investigation works only in the fall, when galls are fully developed and easily visible on the leaves.


I use this lab activity in a junior-level general ecology course for all biology majors. I have 20-24 students per lab section.


Public, primarily undergraduate university of 8500 students, with a small masterís program


This lab should be transferable to other types of institutions. It may be used for sophomore to senior levels, primarily for biology majors. Both plant and herbivore have broad distributions and are speciose. More than 200 species of sawflies form galls on willow (Salix) species (Nyman et al. 1999). Salix occurs in every state of the U.S. (see USDA map for Salix: http://plants.usda.gov/cgi_bin/topics.cgi? earl=plant_profile.cgi&symbol=SALIX), and Pontania sawflies have a broad distribution in North America as well as Europe and Asia. However, Pontania galls may not be present or common at all sites where willows grow. Euura is another sawfly genus that forms galls on willow, but on petioles or stems rather than leaves. The hypotheses could be adjusted to address the distribution of galls on stems or branches. Alternate host plant-herbivore systems are available in most sites, but it may take a bit of searching to find an appropriate system. Consider galls on maple (Acer) leaves (commonly caused by eriophyid mites), goldenrod (Solidago) stems (caused by Epiblema caterpillars or the tephritid fruit fly, Eurosta) or leaves (Asteromyia gall midges), oak (Quercus) stems and leaves (primarily caused by cynipid wasps), hackberry (Celtis) leaves (by jumping plant lice, psyllids), or creosotebush (Larrea) stems and buds (20+ species of the cecidomyid genus Asphondylia).


I have known a number of people who conducted research on gall-forming insects, and indirectly inspired the idea of this lab, including Tom Whitham, Peter Price, and their numerous graduate students and postdocs at Northern Arizona University. Karen Clancy kindly answered questions about Pontania gall life history and provided several photographs for this experiment. Daniel Beck has used this activity several times in general ecology and provided critical feedback; the written and oral report evaluation sheets are modified from his ideas. This contribution to TIEE benefited greatly from detailed comments by Bruce Grant and an anonymous reviewer.


Kristina A. Ernest. April 2005, posting date. Testing Hypotheses on Plant-herbivore Interactions Using Sawfly Galls on Willows. Teaching Issues and Experiments in Ecology, Vol. 3: Experiment #2 [online]. http://tiee.ecoed.net/vol/v3/experiments/sawfly/abstract.html

Sawfly galls (Pontania sp.) as they appear on the under surface of willow (Salix lasiandra) leaves in central Washington.
Photo by K. Ernest.
full size image