In this post John Grunseich, a Masters Student in the Helms Lab talks about his new paper Risky roots and careful herbivores: Sustained herbivory by a root‐feeding herbivore attenuates indirect plant defences out now in the latest issue of Functional Ecology.

About the paper 

What’s your paper about?  

In this paper, we investigated how olfactory cues from plant roots affect the behavior of belowground insect herbivores and their natural enemies, and how these interactions change over time.  

We found that roots of cucumber plants emit a burst of volatile organic compounds following short-term herbivory by cucumber beetle larvae and that these volatiles attract entomopathogenic nematodes that kill the beetle larvae. This is similar to what we see for aboveground plant tissues, like leaves, where these kinds of interactions have been well studied. For example, we know that when insects chew on leaves, or even when your lawn mower trims blades of grass, these wounded plants release olfactory cues that recruit natural enemies like predatory insects or parasitoids that kill the feeding herbivores and offer a form of indirect protection for the plants.  

While these findings were not unexpected, we also found something in this paper that surprised us. When the beetle larvae fed on our cucumber plants continuously for a week, the plant roots stopped producing the olfactory cues we initially saw after herbivory. We also found that once these volatiles were gone, the plants were no longer able to recruit beneficial nematodes as bodyguards against the larvae. This suggests that the beetle larvae might be able to suppress plant defenses for their own benefit, which is something we are continuing to study in on-going projects. There have been many other examples documented of insect herbivores hijacking plant physiology to overcome plant defensive strategies and gain the upper hand in the co-evolutionary arms race between plants and herbivores. Our study reveals yet another possible example, in this case belowground, and it presents us with an exciting opportunity to uncover the underlying mechanisms. 

What is the background behind your paper? 

I (John) have always been a proponent of collecting as much quality data as possible when doing an experiment. This paper was initially conceived after I analyzed some preliminary data on cucumber root volatiles for a different project. During my initial experiments, I noticed significant temporal variation in the root volatile profiles, depending on how long the beetle larvae had been feeding on the roots. This led us to wonder how such variation might influence the interactions among cucumber plants, the root-feeding beetles, and their nematode natural enemies. 

How is your paper new or different from other work in this area and does this article raise any new research questions? 

We noticed from previous research with western corn rootworms feeding on maize roots, that both the rootworms and their nematode natural enemies were attracted to the same olfactory cues from herbivore-damaged maize roots. We thought this presented an interesting ecological conflict, so we set out to investigate whether this pattern also exists in other multi-trophic systems. Because we found evidence of temporal variation in the root-produced olfactory cues, this added a new dynamic that had not been previously considered in other belowground interactions.  

Who should read your paper? 

We hope this study will be interesting for a broad range of ecologists and plant biologists, from researchers studying trophic interactions and plant-herbivore or predator-prey dynamics, to chemical ecologists studying the roles of chemical cues in ecological interactions, and even to researchers studying the mechanisms and timing of plant defense responses.  

About the research 

What is the broader impact of your paper (outside of your specific species/study system) 

Through this research, we are adding to the current knowledge of volatile-mediated multi-trophic interactions, especially within belowground systems where our knowledge remains limited.  

Why is it important? 

This research has important implications for biological control of pest insects in agroecosystems. Here we found that entomopathogenic nematodes, which are commonly introduced to soil environments for biological control (and can be used to control cucumber beetles), can effectively use olfactory cues from plant roots to locate their insect hosts. Thus, providing a nice defensive benefit for plants and farmers. However, some root-feeding herbivores, like the cucumber beetle larvae, can potentially thwart this line of plant defense and the nematodes’ host-finding ability by suppressing plant production root olfactory cues. This means that timing of nematode release could be very important for optimizing the effectiveness of biological control against certain herbivores. 

Did you have any problems setting up the experiment/gathering your data? 

Studying multi-trophic interactions always presents experimental challenges because we have to manage three (or more) biological organisms, such that they are all available in the correct life stages when the time comes for an experiment. Another challenge comes with troubleshooting behavioral assays. Sometimes it is difficult to figure out how to set up a behavioral experiment with the correct conditions so that organisms will behave as naturally as possible. The experiments for this study truly offered an eye-opening perspective on the difficulties of managing multiple organisms and conducting behavioral assays.  

About the author  

How did you get involved in ecology? 

As an undergraduate, my studies were focused on plant-insect interactions. Throughout my degree, I tested many waters, ranging from population-dynamics of the sugarcane aphid to endophyte-plant-herbivore interactions and finally belowground tritrophic interactions. Through all of these experiences, I have been able to better appreciate and become more involved with my studies.  

What are you currently working on? 

I am currently working on my master’s degree in entomology, where I am focusing on plant-herbivore-parasitoid interactions and the chemical cues that mediate these interactions. 

What’s your current position? 

I am currently a master’s student at Texas A&M University studying Entomology. 

What project/article are you most proud of? 

I am most proud of the work that went into this study as it is my first empirical article. It was a great undertaking, with many great experiences! I was able to collaborate with some amazing people and work towards the goal of conducting the experiments and publishing this work!  

What is the best thing about being an ecologist? 

The best thing in my eyes is that there are many questions to be answered and that once you’ve answered one question there are always three or four more that spring up due to finding that previous answer. There is always something to study and there is always a new topic to learn that provides insight into the topic as a whole. 

What is the worst thing about being an ecologist? 

I say this lightly with a touch of humor, but the worst thing about being an ecologist is, that just as mentioned above, answers often lead to more questions which often grow in complexity as they progress. This complexity in combination with the need to either maintain, rear, or locate study organisms that do not always want to cooperate makes life as an ecologist very difficult from time to time, but I would argue that these difficulties also make the work that much more rewarding!  

What do you do in your spare time? 

In my spare time, I collect and rear various insects, centipedes, and tarantulas and I am currently working on culturing a self-sustaining aquatic environment for various aquatic insects and plants. 

Read Risky roots and careful herbivores: Sustained herbivory by a root‐feeding herbivore attenuates indirect plant defences in full here.