Introduction (written for students)

The interactions between herbivores and their host plants are often complex, involving plant chemical and physical defenses, herbivore foraging behaviors, and many other factors. Most plants are attacked by several to many different types of herbivores. Each herbivore may feed in a different manner or on different plant tissues, causing different types of feeding damage. One of the more unique plant-herbivore interactions is the formation of galls. Galls are modified plant tissue stimulated by the oviposition and feeding activities of certain insects and spider mites. They result when the cells around the damaged area grow larger or divide more often than normal cells. As the insect feeds on the plant, it becomes surrounded by this abnormal plant growth. The insect continues to feed from within the gall, which protects it from many (but not all!) of its natural enemies. Other organisms, including viruses, bacteria, fungi, nematodes, and mites, may induce plant galls, but insects are the most common gall formers.

Galls can be used to test a number of interesting ecological and evolutionary questions about plant-herbivore interactions. The hypothesis that host plant quality affects herbivore densities and community structure was tested by Fritz et al. (1987b). As predicted, both densities of individual sawfly species and the relative abundances of these species varied among clones of arroyo willow. Additional data showed that shoot size is an important plant trait affecting gall densities: larger shoots have higher sawfly densities (Fritz et al. 1987a.) Since galls act as nutrient sinks (Nakamura, et al. 2003, Price et al. 1987), larger galls should provide more nutrients and therefore increase the success rate of the galling insect. Investigating the relationship between gall size and gall success (e.g., percent emergence) would provide a test of the generally supported hypothesis that plant galls are adaptive for the galling insect. The mechanisms through which habitat affects the density of galling insects were investigated by Fernandes and Price (1992). Lower rates of parasitism and fungal attack of galls may be at least partly responsible for higher gall densities in xeric (dry) environments compared with mesic (moist) habitats.

Willow trees (genus Salix) are attacked by several gall-forming herbivores. Gall midges form galls on buds, and sawflies form galls on leaves and shoots. Studies for this lab will be conducted at Engelhorn Pond on the Central Washington University campus where many of the willows have leaves with elongate, reddish capsules emerging from the leaf surface (Image of Gall – upper surface; Image of Gall – lower surface). These are caused by sawflies of the genus Pontania. Sawflies are not actually flies but relatives of bees and wasps (Order Hymenoptera). Adult females oviposit (lay eggs) into the leaf tissue. The egg hatches into a larva, which feeds on the leaf tissue while enclosed in the gall. When the larva has completed its development, it chews a hole in the gall and departs. See weblink in References to “Forest and Timber Insects in New Zealand” for pictures of egg, larva, pupa, and adult Pontania.

Willows are also eaten by a variety of free-feeding invertebrate herbivores. Lace bugs suck sap from leaves, spider mites chew leaves, and flea weevils chew on leaves and new shoots. You may find other insects feeding on the willows at the study site.

During this lab, you will (collectively) test a number of hypotheses about the gall-forming sawflies on willows. Particular questions chosen by student groups, in consultation with the instructor, may include:

In the process, perhaps you will become expert cecidologists (students of plant galls)!



Materials and Methods (written for faculty)

Study Site(s):

We are fortunate in being able to walk to our study site, a very small reserve (<1 ha) across the street from the Biology Building at the western edge of Central Washington University’s campus. Engelhorn Pond formed as a borrow pit in the 1920’s when gravel was excavated for use in the construction of the Interstate 90. The pit filled with water from runoff and groundwater and vegetation (including willows) colonized the site. Although very small, and nearly surrounded by university buildings, this site offers an urban refuge for ducks and other wildlife. As a wetland it garnered the attention of The Nature Conservancy, which purchased the site and donated it to the Biology Department. The pond is the dominant feature of the site and is surrounded by willow trees (Pacific Willow, Salix lasiandra). Gall densities vary from year to year, but usually galls are fairly abundant on the leaves. Numerous other sites are possible inside the city limits where willows grow along streams and irrigation canals. At your location, any site where willows grow and you can easily find galls would be appropriate. You might also consider sites with other plant species that harbor galls (such as poplars, goldenrod, maple trees).

Overview of Data Collection and Analysis Methods:

Introduction to the Study System

When we first get to the field site, I show students galls on the willows. Have a few students carefully open the galls (with a pocket knife or thumbnails) to find a sawfly larva. You could collect a few larvae in advance of the lab, and let students view them under a microscope during the lab introduction, or take a portable dissecting scope or simple hand lens with you to the field.

Hypothesis 1: all teams

Additional Hypotheses

In addition, each team will choose one of the other hypotheses described below, or can design a new hypothesis, to test. Read through these hypotheses to see which interests you most, or discuss ideas with your instructor. You may come up with interesting hypotheses based on your initial observations of the trees and galls, or even by reading the titles of some of the journal articles in the References section. Alternatively, your instructor may assign a hypothesis to each team to be sure each hypothesis gets tested by your class. See Appendix [PDF] (211 KB) for suggestions on random sampling, descriptive statistics, and statistical tests. When you “sample” leaves, please avoid removing them from the branch so that other teams may sample the same plants and we leave the willows as undisturbed as possible.

Hypothesis 2

Hypothesis 3

Hypothesis 4

Hypothesis 5



Questions for Further Thought and Discussion

Conclusions about hypothesis 1:

For additional hypotheses tested by individual teams:



References and Links

Web Links



Tools for Assessment of Student Learning Outcomes

Informal Assessment

While students are deciding which hypothesis to test, I wander among groups and ask them some simple questions to assess whether they are grasping the main concepts and hypotheses.

While they are collecting and analyzing data, I informally walk among teams of students and ask them simple questions such as:

We discuss the results of hypothesis 1 as a class, after putting the means and standard deviations on the board. I ask students whether trees vary in the extent of herbivory by galling sawflies.

Formal Assessment

I assess student learning primarily by requiring a formal oral report (similar in style to paper presentations at ESA meetings). I give students a guideline (below) for what material they should include in the talk, and give them the grade sheet I use to grade their oral report (see Oral Report Grade Sheet). Assessment of their data analysis is via written data summary and analysis they hand in at the time of the oral report. Students also evaluate oral presentations by other teams, providing me an opportunity to see if they understand what other teams found.


Your team will present a short, concise (5-10 min.), well organized oral presentation (in PowerPoint) based on the additional hypothesis you tested.

Use the following outline to prepare for your talk:

Your grade will be based on the criteria listed in the Oral Report Grade Sheet.