Which Way Out--A Study of the Exit Tunnels Made by the Goldenrod Gall Fly, Eurosta solidaginis
Part of the Young Naturalist Awards Curriculum Collection.
On a warm summer day five years ago, my friend and I set out on an expedition. We pretended to be Native Americans gathering provisions to bring back to our teepee. No nuts or berries grew in sight, so we had to settle for "onions." These were no ordinary onions, though; these strange spheres grew on goldenrod plants. We spent hours collecting them in the field until we were called inside for dinner. Hanging our heads, we walked slowly back to the house, hoping we'd be back later.
Background
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It was not until years later, as I began my research project for the Young Naturalist Awards, that I discovered what actually lay inside what we called "onions." Hidden within these plant growths were tiny goldenrod gall fly larvae. The adult goldenrod gall fly (Eurosta solidaginis) is the size of a small housefly and has dark markings on its wings. These little creatures undergo a complex and intriguing life cycle. It begins when the female injects her egg into a terminal bud on a goldenrod plant in late spring. The egg hatches in about a week, and the larva journeys down into the meristematic tissue (a plant's growth cells) within the stem. There it creates an opening as it feeds on plant matter. The goldenrod plant reacts to the gall fly by making a sphere-shaped gall around the insect. The larva will call this gall home for 11 months (Sandro 2006).
In autumn, the gall larva hollows out an exit tunnel for escaping from the gall after pupation in the spring. All it leaves at the end of the tunnel is the epithelium, or outer layer of the gall. As the days grow colder in late autumn, the gall larva prepares to spend the winter in the gall by increasing its levels of glycerol and sorbitol to create a natural antifreeze (Storey). The insect stops eating and enters diapause (growth and development is suspended) in preparation for the cold after slowing its metabolic rate (Sandro 2006). The larva pupates in spring and then emerges as an adult fly after about two weeks. In order to break free of the gall, the goldenrod gall fly crawls through the exit tunnel it made in the fall and anchors itself to the end. It sends body fluids to its head, which bursts the outer layer of the gall, freeing the fly. On the outside of the gall, it rests and dries its wings. After mating, the life cycle begins anew (Collicut).
Starting from Scratch—Generating a Project Idea
My interest in gall flies began when I spoke with Phil Stephens, a naturalist at the Chippewa Nature Center in Midland, Michigan. Since I live in Michigan, and I was beginning my project in December, he suggested studying insects in winter. This was something I hadn't thought about before, so I decided to pursue his idea. He recommended reading A Guide to Nature in Winter and A Guide to Insect Lives, both by Donald Stokes.
As I read these books, I realized that I knew very little about insect activity in winter. I was fascinated by the survival techniques of insects in the cold. Of course, I couldn't leave my investigation as broad as "insects in winter." I had to pick a certain insect on which to focus my attention. Rachel Larimore, another naturalist who works at the Chippewa Nature Center, e-mailed me ideas for narrowing down my research. After looking over my options, I decided that the goldenrod gall fly would be an intriguing topic.
Establishing a Research Question
While studying the life cycle of the gall fly, I realized that this insect was the hidden secret inside the "onions" my friend and I collected as 9-year-olds. The exit tunnels they make caught my attention. As I learned about their escape passages, one question came to mind: Do goldenrod gall flies have a directional preference in their exit tunnel placement?
To help answer my question, I divided the gall into two hemispheres, the upper and the lower. Four quadrants were in the top hemisphere and four were in the lower. Each set represented the north, south, east and west. In a study done by the University of Kansas, the effect of gravity on monarch butterflies (Danaus plexippus) was tested at the International Space Station. They observed them in a microgravity environment from the caterpillar stage to adult butterfly. The results concluded that the species did depend on gravity for orientation ("Caterpillars 'lost' in space without gravity," 2010). The gall fly's decision when planning its escape could also be affected by gravity, which would help it determine up from down. Sadly, I did not have access to the ISS for my research, but based upon their experiment with monarch butterflies, I hypothesized that the goldenrod gall fly would prefer the upper hemisphere because of its sense of gravity. This would make it much easier for the gall fly to rest and dry its wings on the top of the gall after escaping through the exit tunnel.
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In another study of the monarch butterfly done by Jason Etheredge, the insects were tested in different magnetic field scenarios. His results indicated that the monarch butterfly has an internal magnetic compass used for migration to the southwest (Etheredge 1999). This brought about the question of whether the gall fly has an internal magnetic compass. I knew this was only a slight possibility, but I felt it was a question worth answering. The larva is in an enclosed space, so it can't use the Sun for directional purposes. An internal compass could assist in the placement of their exit tunnels.
If a pattern exists for the goldenrod gall fly's exit tunnels, this could mean that, like the monarch butterfly, it also uses an internal magnetic compass. If this were true, the southward direction seemed to offer the biggest benefit. When the insect comes out of the exit tunnel facing south, the Sun's rays would be the most intense on that side, drying the fly's wings more quickly. I hypothesized that if the goldenrod gall fly uses magnetic information to orient itself, then its directional preference would be toward the south quadrants.
Procedures
I wanted to focus my research within a five-mile radius of my hometown, Evart, Michigan. I obtained permission to look in backyards, roadside ditches, and a large camping area. With a field journal in hand, I set out for the second time to hunt for "onions." Once I found a site that had at least 12 galls, I used a compass to determine the direction north, and marked the upper north quadrant of the gall with a paint pen. I then picked each specimen using clippers and put the galls into a numbered egg carton to keep them separate from one another. After finishing the procedure with one gall, I walked to the next at a minimum of four feet away. Once I had collected 12 galls, I moved on to another site at least 100 feet away. Looking alongside the road in ditches was my least favorite place to look; many people I knew would pass by, wondering what I was doing wading in a foot of snow snipping off goldenrods. Of course, the weather was completely different from the summer five years ago, but I still greatly enjoyed being outside. The beauty of the outdoors in wintertime, with the Sun sparkling on the snow's surface, made an irresistible scene. The galls were collected during the months of January and February. Altogether I collected at 13 different sites.
Now that I had collected the galls, I needed to analyze the placement of the exit tunnels. Before I opened my specimens, I used a different colored paint pen to mark a vertical line down each gall so I would know where the north side was when I cut them in half. Also, I used a caliper to measure the diameter of each gall. With a knife, a small probe, and a steady hand, the galls were opened at the midline and explored. If all went well, I would find a plump white larva (Eurosta solidaginis). However, frequently the gall often contained other, unwanted creatures. Using the guide found in "Goldenrod Gall Size as a Result of Natural Selection," I identified what I observed (Colvard 1998). The obtuse wasp (Eurytoma obtusiventris) is one parasitoid of the gall fly. It causes the larva to pupate earlier than normal before feeding on it. If this was the scenario, I saw a small brown puparium. Another parasitoid of the goldenrod gall fly is the giant wasp (Eurytoma gigantea). The white larva of this insect was occasionally present, or I saw only a blackened chamber.
One other insect I saw was a beetle larva (Mordellistena unicolor). Sometimes the gall was empty because birds, such as the black-capped chickadee (Parus atricapillus) or the downy woodpecker (Picoides pubescens) had already opened the gall to devour the larva (Colvard 1998). All these different creatures made opening the galls a fun experience because I never knew exactly what I would find inside them.
After observing the inside of a gall, I marked the end of the exit tunnel with a pin if a gall possessed an exit tunnel. Then I recorded the hemisphere it was in. I printed out a diagram of the quadrants including the degrees, placing the gall above the diagram. I then lined up the paint mark with north and recorded the quadrant of the exit tunnel. Altogether I analyzed 156 galls, but only 50 of them actually contained gall fly larvae with exit tunnels. I collected multiple times because the number of galls with parasites was very high. I needed enough galls to make a sufficient sample size. Some galls had been pecked open by birds in the winter. Therefore, the exit tunnel was present, but I didn't include them in the total of galls with exit tunnels. I choose to focus my attention on the galls that had an actual gall fly with an exit tunnel to increase the accuracy of my study.
Results
In the galls I collected, 78% of the exit tunnels were in the first four quadrants (the upper hemisphere). According to my data, I concluded that the goldenrod gall fly seems to prefer the upper hemisphere to build its exit tunnel. This confirmed my hypothesis that the goldenrod gall fly would prefer the upper hemisphere because of its sense of gravity. To further confirm my hypothesis, it would be interesting to study tunnel placement in a microgravity environment and see if the results were the same. The upper hemisphere has advantages for the gall fly; along with being able to rest and dry its wings easier on the top of the gall, this could "be a better vantage point for the first flight" (Larimore).
Unfortunately, there didn't seem to be a significant preference for any single direction (north, south, east or west). From these results, I concluded that preference of direction is mostly random. This means that they most likely do not have an internal compass that could assist them when placing their exit tunnels. My hypothesis that the directional preference would be the south quadrants was incorrect. When I realized this, I tried looking at the average gall diameter in each quadrant to see if there was a relationship between the placement of the exit tunnel and the size of the gall. Again, it was random.
As I marked the galls with the paint pen in the field, I tried to accurately place the line on the north side, but this could have been a source of error. Also, a larger sample size would help in improving the accuracy of my research.
Secrets Under the Snow
After finishing my study, I knew I needed to do something more. I spoke with a teacher at the elementary school in Evart, and I was given the opportunity to teach a fourth grade class for an afternoon. When I was 9, I knew absolutely nothing about the gall fly inside my treasures. I wanted to share the things I had discovered about the goldenrod gall fly and insect activity in winter with other kids. In the classroom, I discussed the winter habits of the black-and-yellow mud dauber wasp (Sceliphron caementarium), the cattail moth (Lymnaecia phragmitella), snow fleas (Achorutes nivicolus), the Eastern tent caterpillar (Malacosoma americana) and, of course, the goldenrod gall fly (Eurosta solidaginis). We looked at a PowerPoint presentation and passed around specimens of each of the insects. I included a take-home packet with a magnifying glass, a pencil, and a nature guide that I created so that they could get out and look for insects in the winter themselves. They all seemed to enjoy looking at the different creatures under their new magnifying glasses and writing about their discoveries in the guides. It was exciting to share some secrets that lie under the snow with the class.
Conclusion
William Lawrence Bragg once said: "The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them." Through this research project, I did find new ways of thinking about the goldenrod gall. Five years ago, all I knew about these strange plant growths was the fun I had collecting them. Now I know that inside our "onions" were tiny insects with a captivating life story. The research I have done has motivated me to make further in-depth studies about the gall fly and other insects in the future. Maybe one day I will have the opportunity to send a goldenrod gall to the space station, but for now I will start with my feet on the ground. One thing I could study in more detail is the parasites that made my research so challenging. I found a lot more parasites than I expected within the galls. Is this typical for all areas of Michigan? Or is it particularly high in this area? These questions are left unanswered … for now. I am looking forward to studying the tiny creatures emerging from the galls in the spring. After all my research, this is something I certainly do not want to miss.
Bibliography
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