Wednesday, February 13, 2013

You've got a lot a gall....

Wouldn't it be great if a teacher and students could study a plant, its herbivore, and the predators of the herbivore?  Ideally you'd want to study these three trophic layers of the system in a single class session, or perhaps two.  Of course, as an educator, you'd want all of this to happen very cheaply, or better yet, for free.

Warren G. Abrahamson, his colleagues, and a host of students have been working with just such a system for many years.  They have generously provided all of the tools necessary for teachers to share this system with their students.

Goldenrod species are found in most places in the United States and in many locations they are unwitting hosts to the goldenrod gall fly.  Female flies lay eggs into the growing tip of the goldenrod.  Chemicals released by the hatchling larva cause the plant to form a corky spherical gall about the diameter of a quarter coin.  The larva spends about 50 weeks in the gall feeding on plant tissue, growing, and pupating, before emerging as an adult to complete the life cycle.

Students in my evolution class collect a sample of about 250 galls during the first week of class in mid January.  For two years in a row now we have completed the task without the need for snowshoes.  We number and then measure the diameter of each gall using a plastic artist's templates designed for drawing circles of increasing diameters.  Next we carefully split the galls using pruning shears and use Dr. Abrahamson's online key to determine the outcome of the gall fly's efforts. I would say that the pruning shears are perhaps the most important tool for success with this research project; asking 20 students to cut hard spherical objects using knives might be a recipe for disaster.  As much as I respect and admire the work of the Saint Michael's College Rescue Crew, I'd rather not set myself up to need their services.

Gall fly larvae represent a high-protein meal when most other insects are underground.  A number of predators and parasitoids take advantage of the bounty.  Woodpeckers, chickadees, beetles, and wasps all partake in the fly larva feast.  In our most recent foray into the world of gall fly biology, we observed that already by January, 70% of the galls no longer housed gall fly larvae.

The entire sample of galls serves as a 'before predation' example; the subset that still contains viable fly larvae represents the 'after predation' survivors.  The data set is perfectly amenable to graphing using histograms and statistical analysis.

As predicted in the published literature, my students found that 2 bird species accounted for most of the larger galls, and many of the smaller galls hosted wasp rather than fly larvae.  The student data set shows strong evidence of stabilizing selection.  It seems that it is in this case better to be average than to be exceptionally large or small. 

By coincidence, my daughter's first-grade class used galls as a study system for life cycles and food webs during the same time period.  The first graders did not want to kill the larvae and so I was assigned to release them to 'the wild'.  The larvae sat in my car in a paper cup during a day when the temperature dipped to several degrees below freezing.  The larvae were as soft and squishy as when they were removed from the galls, despite the fact that my water bottle in the same car was frozen solid.  It was a nice illustration of the antifreeze properties of the larval tissue.

We will revisit the gall fly population on campus late in the semester to see if additional months of exposure to bird predation has further reduced the proportion of surviving flies. Dr Abrahanson's generosity in sharing what he has learned serves as an example to all of us academics studying our sometimes obscure topics.  We have each in our own way figured out how to wring data from all sorts of bizarre systems.  There are tricks that we have learned from colleagues or gleaned from conferences.  We should share these tips! 

The images used on this page are from Wikimedia Commons: