mcmahon ’24 studies tritrophic interactions in singer lab

Every year, the Bailey COE awards fellowships to fund summer, fall and spring research opportunities for Wesleyan students across all majors and class years. Biology major and environmental studies and Jewish studies minor Ben McMahon ‘24 spent his summer examining the tritrophic interactions involving white oak, phloem feeding insects, caterpillars and ants, and the ecological impact the different trophic levels have on one another. 

Hi, Ben! Would you tell me a bit about yourself, and your academic interests? 
I’m Benjamin McMahon, you can call me Ben! I’m from Boston. I’m a second semester senior here at Wesleyan. I’m a biology major, environmental studies minor, and Jewish studies minor. One of the reasons I came to Wesleyan was so that I had the freedom to pick which classes to take. In terms of biology and environmental etudies, I think they overlap very well. A lot of the issues that those fields are facing most prominently are interdisciplinary issues. I think you need a lot of biology to apply solutions to the environment, and vice versa: You need to understand the environment to understand sort of why biotic mechanisms or phenomena are so important. 

What has your experience been like working with the Singer Laboratory?
I joined the Singer Laboratory, headed by professor of biology Michael Singer,  my sophomore year. The Singer Laboratory here is an evolutionary ecology lab, with a focus on entomology. What’s nice about that is that it’s a biology lab, but it’s a biology lab that’s in the field. It allowed me to do field research, which is something I’m really interested in.

During the school year, I wasn’t conducting active research. Instead, I was assisting other projects that had been started by previous students and had been running for years prior. Then, the summer after my sophomore year, I came back to work during the field season, which is when we do most of our research. That’s from late May through June into early July. I was doing active research, but I wasn’t guiding my own research. It was awesome because I got a lot of experience, and I had the opportunity to learn what it means to do field research as opposed to research in the lab.  My junior year, I had the desire to come back and actually conduct my own research. So, I applied for a Bailey COE Summer Research Fellowship.

What research did you conduct with the help of the Bailey COE summer grant?
I was able to  design my own  research question, design my own experiment based off of previous experiments in the lab, and analyze my own research question. It was focused on understanding what factors regulate caterpillar communities on white oak trees in Connecticut forests. The lab specifically focuses on tritrophic interactions, which are interactions across three different trophic levels.We specifically focus on plants, herbivorous insect interactions, and then the predators and enemies of these herbivorous insects. 

The primary trophic level would be the producers, in this case, the host plant, the white oak. Secondary would be the primary consumers of the white oak—herbivores such as caterpillars, and then also other herbivores like phloem sap feeding insects like treehoppers. The third trophic level would be enemies and predators of these, like ants or birds. The research mainly is focused on understanding how  the effects of the different players in this system  interact, structure, and regulate the caterpillar community. 

How did you study the different trophic interactions?
For this experiment that I was running after my junior year, we were trying to understand  how the presence of phloem feeders, these phloem sap feeding insects, and the presence of ants  affects and regulates the caterpillar community.  White oak and other plants have defense mechanisms against  things that are trying to harm them, whether it be leaf-eating insects like caterpillars, or whether it be sap-eating insects like treehoppers, and so they can defend against these natural enemies via the production of these phytohormones, endogenously within the plants that then trigger a cascading defense mechanism downstream.

What kinds of defenses does the plant use?
The plant can produce hormones that defend against things attacking the leaves or things attacking their sap. There’s a lot of different mechanisms by which it works. They could make it more difficult to feed on them, like through increasing the toughness and thickness of the leaves. Or they could release hormones that attract natural predators of the caterpillars like parasitoids. Or they could release chemicals within the sap or within fluids in the plant that are unfavorable to whatever’s feeding on the plant material.

What we understand about plants is that they have these two pathways: one protects the leaves and one protects the sap, but the plant can only do one or the other. So if it’s defending against the caterpillars, it can’t defend against the treehoppers and vice versa. 

What was your hypothesis?
We think that having treehoppers present or treehoppers absent will affect the caterpillar community because the tree in one case will be able to defend directly against the caterpillars and in the other case it would have to choose between the treehopper and the caterpillar defenses. That is further complicated, because treehoppers feed on the sap, but they secrete this material called honeydew, which is essentially this sugary fluid that attracts ants. So what that means is that when treehoppers are present, we assume  the tree will have more caterpillars because the tree is only defending against the treehoppers, therefore not defending against the caterpillars, and the caterpillars can thrive.

But, the treehoppers being present also attracts ants, which are natural predators to the caterpillars. So it’s this regulating hypothesis wherein the treehoppers being there allow there to be more caterpillars, but they also attract ants that will take care of the extra caterpillars, so the caterpillars don’t overrun the tree. Those are the two hypotheses that  explain why we’re doing this; we’re trying to understand how these players affect each other. 

Could you describe your research process? 
Essentially we have to find white oak trees with accessible branches that have both phloem feeders present, and also have ants present. We want all of our branches to have the same initial conditions. We take those branches and we remove all of the insects, all of the arachnids. We beat the branch so everything comes off and it’s just a bare branch. 

Then we assign the branch four different treatments. So it’s either going to have phloem feeders or not have phloem feeders, and it’s going to have ants or not have ants. We allow ant access or exclude ant access by putting sticky resin around the branch so the ants can’t enter. In terms of phloem feeder presence, we either remove the phloem feeders and don’t put them back, or we add them back so they’re there. We add back two caterpillars. We’ll flag the branch so we can find it, and then we’ll come back to that branch and sample it, after a week and two weeks. Then, we’ll collect all of the caterpillars and the whole arthropod community and basically see the effects. 

On branches that had phloem feeders removed, were there more or less caterpillars in general? On branches that had ant access, were there more or less? We can also compare their effects on  different types of caterpillars by breaking them up both in terms of size and diet breadth.

What were your results and conclusions?
So, there’s interesting findings for this summer in particular. In general, we found that phloem feeders and ants did not affect caterpillar abundance. This contradicts a lot of the past data. So we were trying to figure out why that happened, and we have a couple different hypotheses. One is that the data from this summer was particularly lacking in caterpillar count. This is largely due to the environmental conditions and the climate of that summer in the summer before. So what we discussed in the lab was that there was a noticeable drought, which likely would have affected the caterpillars’ parent generation. So what that means is, overall, our caterpillar counts were significantly reduced from this summer compared to other summers, and so we just lacked quantity of data.

But that being said, we did find some interesting data  using other models. We found that there was a statistically significant difference in year, which indicated there’s some truth to that lower caterpillar count from the summer after the drought.

We also compared phloem feeder and carpenter ant as variables instead of just presence. So what that means is instead of, in our analytical models, asking: Does presence or absence of ants affect the caterpillar community?  In this model, we asked: Does the number of ants and number of phloem feeders affect? We found that phloem feeder abundance reduced the density of small specialist caterpillars. That suggests potentially density dependent effects. That would be something that our experimental design isn’t ideal for understanding. In the future, we would need a different experimental design because our experimental design pertains to the presence or absence of phloem feeders, not varying abundance. We also think that the effects of ant predation might be less noticeable on the scale of just a branch; it might be somehow distributed across the entire plant.

How do you plan to expand upon this research in the future?
Overall, it seems that ant predation, from these results, is more insignificant than we previously thought. Our previous data shows that bird predation is a far more significant effector of the caterpillar community, which means that this experiment would be great to reproduce, but instead of one of the variables being ant exclusion, it would be bird exclusion. I actually applied to come back for the fifth year and do master’s research on that.

Is there anything else you would like to share with readers?
I had a great experience in this lab! Professor Singer is very knowledgeable, not just on this subject matter, but also regarding Wesleyan resources and grants.  I’d say if anybody is interested in field research, feel free to reach out to me, or to Professor Singer, because he is open to having more people join the lab! I would definitely recommend checking out ecology research. It’s a lot of fun.