Brendan Randall
Research
Microbe-plant-insect interactions
My fascination centers on the mysteries that remain regarding the role of biodiversity and the environment in shaping species interaction dynamics. My dissertation work at UMD with Dr. Karin Burghardt aims to determine whether symbiont diversity or drought is the more important driver of tripartite interactions.
The key aims of my dissertation are to understand:
1. If rhizobial strain genotypes differentially alter plant and herbivore responses to abiotic and biotic stress
2. How changes in access to rhizobial partner diversity shape plant-partner outcomes
3. How variation and severity of abiotic stressors impact soybean production and biological communities across spatiotemporal scales
Goal: Develop targeted rhizobial inoculants for growers that harness beneficial effects to plants under stress.
In my first chapter, I found interactive effects between genetic variation within rhizobial strain populations and drought on soybean plant trait expression (Fig. 1) and that this has consequences for feeding by insect herbivores (Fig. 2). Additionally, I found interactive effects between rhizobial strain identity and drought on caterpillar growth rates. This work shows that the microbial partner a plant associates with has consequences for insect feeding and growth at higher trophic levels. This work is currently being prepared for publication and will be available soon! Stay tuned for more work coming soon from my other chapters!
Cannibalism and Chemical Ecology
Before graduate school, I was an undergraduate research assistant for the Wetzel lab at Michigan State University (Dr. Wetzel is now at Montana State University) from 2019-2021. I led and completed an independent research project examining the nutritive mechanisms that govern diet switches to cannibalism in Cabbage looper (Trichoplusia ni), a lepidopteran herbivore. When caterpillars were given unbalanced ratios of two important macronutrients, protein and carbohydrates, they switched their diet to cannibalism earlier and more frequently than caterpillars given a more balanced diet (Fig. 3). We also found fitness costs to cannibalism, as survival of cannibals was extraordinarily low compared to caterpillars given a balanced diet. If you're curious about insect cannibalism or want to see some rather gruesome photos of caterpillars eating each other, check out my first-author paper in Ecological Entomology!
In addition, I also completed a project with my mentor, Kayleigh Hauri, where we found that herbivory was reduced in tomato plants planted in diverse, clumped chemotype plots relative to monoculture and alternating diverse chemotype plots while natural enemy abundances increased in divers, clumped chemotype plots. If you're curious and want to learn more about how the spatial arrangement of chemical diversity in fields shapes herbivore pest-feeding behavior and natural enemy recruitment, check out my co-authored publication in Journal of Applied Ecology!
Figure 1. (A) Variation in average multivariate plant trait expression in response to rhizobial strain and herbivory treatments. (B-D) Variation in average multivariate plant trait expression and multivariate trait dispersion in response to rhizobial strain and watering environment. The trait space is replicated from (A). Colored dots represent an individual plant’s collection of traits in a specific strain and a specific environment. The origin of ellipses represents the weighted averages (95% CI) of every plant inoculated with one of three example rhizobial strains (3B, Strain #2; 3C, Strain #14; 3D, Strain #24) in well-watered and drought treatments (N = 12 plants/strain/watering treatment). All plots are derived from the same RDA model. Figure adapted from Randall et al., in review
Figure 2. Rhizobial strain influences drought-mediated herbivore growth rates. Reaction norms represent the mean ± standard error (SE) effect of watering (well-watered or drought) on caterpillar relative growth rate. Each line represents one of 24 rhizobial strain genotypes, in which 12 plants were inoculated in either well-watered or droughted conditions.
Figure 2. Cannibalism increased in caterpillars given an unbalanced diet of either carbohydrates or protein, with earlier cannibalism observed in caterpillars given larger amounts of carbohydrates relative to protein. Figure adapted from Randall et al., 2023
