Triangulating from the cairns…
Explores how alpine plants like Cardamine cordifolia deploy chemical defenses — including endophytes, volatile compounds, and hormone signaling pathways — against specialist insect herbivores, using genomic and bioassay methods to uncover the molecular basis of plant-insect coevolution.
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Plants cannot run from the insects that eat them, so they fight back with chemistry. Across the meadows and forest understories around Gothic, Colorado, a quiet chemical war plays out between native plants and the insects, microbes, and other organisms that exploit them. Studying this war — known as plant-insect chemical ecology — reveals how mountain ecosystems assemble, why certain plants thrive in certain spots, and how evolution shapes the diets of insects over millions of years. Much of this work at RMBL has centered on bittercress (Cardamine cordifolia), a native mustard that grows along a striking light gradient: from sunny meadows where insect damage is heavy to deeply shaded forest understories where it is light.
Several concepts are essential for understanding the findings below. Bittercress, like other mustards, defends itself using glucosinolates — chemical compounds that break down into toxic mustard oils when leaves are damaged. Plants regulate these defenses through hormones, especially jasmonic acid (which is triggered by chewing insects) and salicylic acid (triggered by many leaf-colonizing bacteria). Crucially, these two hormone pathways suppress one another, a phenomenon called jasmonic acid-salicylic acid crosstalk. As a result, defending against an insect can leave the plant vulnerable to a microbe, and vice versa — a pattern researchers call cross-resistance (or cross-susceptibility) when prior attack by one enemy alters the plant's response to another.
A few other ideas recur throughout this work. Endophytes are microbes that live inside plant tissues, sometimes helping defend the plant and sometimes exploiting it. Leaf mining is a specialized feeding behavior in which insect larvae tunnel between the upper and lower skin of a leaf, a hallmark of the herbivorous fly Scaptomyza nigrita. The shade avoidance syndrome describes how plants growing in low light stretch their stems and petioles to reach brighter conditions. And because light availability changes so dramatically across short distances in subalpine landscapes, the light gradient itself becomes an organizing axis for the chemistry, ecology, and evolution of the entire community.
Choice feeding trials where individual grasshoppers are offered leaves from different plant treatments to measure relative herbivory rates. Herbivory ...
Time-calibrated phylogenetic analysis using multiple genes and both maximum likelihood and Bayesian approaches to estimate evolutionary relationships ...
Illumina GAIIx RNA sequencing of larval samples from different plant treatments followed by differential expression analysis to identify genes respons...
Pre-treatment of bittercress plants with jasmonic acid to simulate herbivore-induced plant defenses, followed by controlled bacterial inoculation to t...
Systematic field survey of natural herbivore damage patterns across different leaf positions on host plants. Quantifies within-host foraging preferenc...
Canopy cover measurement using densitometer to classify habitats as sun (5% shade), deep shade (95% shade), or ecotonal (5-95% shade) environments.
Simultaneous or sequential attack by herbivores and microbes is common in plants. Many seed plants exhibit a defence trade-off against chewing herbivo...
Plant distributions can be limited by habitat-biased herbivory, but the proximate causes of such biases are rarely known. Distinguishing plant-centric...
The research program took shape in the 1980s and 1990s with a series of careful field experiments on bittercress. Louda (Louda, 1984) used insecticide-exclusion experiments to show that a single leaf beetle could reduce bittercress leaf area by more than 80%, suppressing plant height and fruit production even at relatively modest defoliation levels. Louda and Rodman documented that glucosinolate concentrations varied with habitat and that the herbivore community pressed hardest on plants with weaker chemical defenses (Louda & Rodman, 1983), and later demonstrated that insect herbivory itself helped explain why bittercress is largely confined to shaded streamside habitats (Louda & Rodman, 1996). Shading experiments by Collinge and Louda (Collinge & Louda, 1988) confirmed that leaf-miner damage was reduced in shade, tying herbivory directly to the light gradient.
Parallel foundational work in the same Colorado meadows examined a very different system: the lycaenid butterfly Glaucopsyche lygdamus, whose caterpillars secrete substances that attract protective ants. Pierce and Mead (Pierce & Mead, 1981) showed experimentally that ant-tended caterpillars suffered far less parasitoid attack than untended ones, a result that helped establish parasitism as a major selective force shaping insect-ant mutualisms (Pierce & Easteal, 1986). Later work revealed that braconid parasitoids can themselves exploit these ant associations (Fiedler et al., 1995).
A central thread of the bittercress work has been understanding the chemical conversation between plant, insect, and microbe. Treating bittercress with jasmonic acid raises glucosinolate levels and reduces feeding by adult Scaptomyza nigrita by more than half (Humphrey et al., 2016), but it also slows plant growth substantially (jasmonate-induced defenses cut leaf growth rates by more than 20%, (Kim, 2010)). Salicylic acid treatment has the opposite effect on insects, making plants more attractive to Scaptomyza (Villalobos, 2011). Within a single plant, lower and older leaves carry higher glucosinolate concentrations, and flies preferentially attack those leaves — choosing exactly the most chemically defended tissues (Humphrey et al., 2016). This counterintuitive attraction to toxic compounds is one of the more striking results in the system.
The interaction reaches beyond plants and insects to include leaf-dwelling bacteria. Surveys and experiments showed that herbivore-damaged leaves carry far more bacteria than undamaged leaves, with putatively pathogenic Pseudomonas syringae particularly amplified (Humphrey et al., 2014). A later study using absolute-abundance sequencing confirmed bacterial loads several doublings higher in damaged tissue and demonstrated that co-occurrence of insects and pathogens is clustered across plants, correlating with roughly 50% lower reproductive success in affected individuals (Humphrey & Whiteman, 2020). Consistent with hormone crosstalk, prior infection by Pseudomonas reduced subsequent leaf-miner damage on systemic leaves (Ortiz, 2011), and larvae preferred bacteria-infected leaves in choice tests (Villalobos, 2012).
Across the light gradient, bittercress populations have diverged. Reciprocal transplants showed that plants from sunny, high-herbivory sites invest more heavily in defensive chemicals and show stronger petiole elongation in shade, while shade-source plants flower later, defend less, and have an attenuated stretching response (Humphrey et al., 2018); (Faries, 2015)). Meanwhile, evolutionary work on the herbivore itself has shown that the transition to plant feeding in Scaptomyza, which occurred roughly 10-16 million years ago, involved hundreds of stress-response genes (Whiteman et al., 2012), loss of yeast-detecting odorant receptors inherited from microbe-feeding ancestors (Mitchell et al., 2015), and recruitment of an ancient detoxification pathway to handle mustard oils (Gloss et al., 2014).
Early work in the 1980s and 1990s established that insect damage and chemical defense structure bittercress populations along the light gradient. Studies in the 2010s expanded the cast of characters to include leaf bacteria and used genomics to reconstruct how a yeast-eating fly became a mustard specialist. Research since 2020 has moved toward mechanism at finer and finer scales. Humphrey and Whiteman (Humphrey & Whiteman, 2020) introduced quantitative sequencing methods that pin down absolute, not just relative, microbial abundances on leaves, opening the door to epidemiological models of plant disease in wild populations. Complete reference genomes for bittercress-associated Pseudomonas strains (Baltrus et al., 2020) now allow strain-level tracking of how bacterial communities respond to herbivory. On the insect side, Peláez et al. (Peláez et al., 2022) traced the evolution of the plant-piercing ovipositor in Scaptomyza to a small number of genetic variants, including one near a neural-development gene, showing how a key morphological innovation enabled herbivory.
The trajectory is toward integration: combining field experiments with genomics, microbiome sequencing, and quantitative genetics to ask how multiple interacting partners — plant, herbivore, pathogen, and beneficial microbe — co-evolve and co-occur in real landscapes. Long-read sequencing, microbial isolate libraries, and reciprocal transplants across the Gothic light gradient now form a connected toolkit unavailable to the founding generation of researchers.
Several big questions remain. How does climate change — through earlier snowmelt, shifting canopy cover, and altered phenology — reshape the alignment between bittercress defense investment and the herbivores and microbes that actually arrive each summer? Can the laboratory-scale findings about hormone crosstalk and bacterial amplification predict disease outbreaks in wild plant populations, and could land managers use such predictions? What role do endophytes play in mediating plant defenses in the field, beyond the leaf-surface bacteria that have received most attention? And on evolutionary timescales, what genetic constraints have shaped — or limited — the repeated origins of herbivory in flies and other lineages, and what does the Scaptomyza system reveal about which evolutionary paths are open and which are closed? Answering these questions will require sustained long-term monitoring of the kind RMBL is uniquely positioned to provide.
Baltrus, D. A., et al. (2020). Complete genome sequences for Pseudomonas sp. strains 29A and 43A. Microbiology Resource Announcements. →
Barriers to gene flow across a light gradient in Cardamine cordifolia (2015). →
Biotic factors influencing feeding preferences of Scaptomyza nigrita (2011). →
Collinge, S. K., Louda, S. M. (1988). Herbivory by leaf miners in response to experimental shading of a native crucifer. Oecologia. →
Cross-resistance between Pseudomonas syringae and Scaptomyza nigrita (2011). →
Effects of bacterial endophytes on plant-insect interactions (2012). →
Fiedler, K., et al. (1995). Exploitation of lycaenid-ant mutualisms by braconid parasitoids. Journal of Research on the Lepidoptera. →
Gloss, A. D., et al. (2014). Evolution in an ancient detoxification pathway is coupled with a transition to herbivory in the Drosophilidae. Molecular Biology and Evolution. →
Humphrey, P. T., et al. (2014). Diversity and abundance of phyllosphere bacteria are linked to insect herbivory. Molecular Ecology. →
Humphrey, P. T., et al. (2016). Aversion and attraction to harmful plant secondary compounds jointly shape the foraging ecology of a specialist herbivore. Ecology and Evolution. →
Humphrey, P. T., et al. (2018). Heritable plant phenotypes track light and herbivory levels at fine spatial scales. Oecologia. →
Humphrey, P. T., Whiteman, N. K. (2020). Insect herbivory reshapes a native leaf microbiome. Nature Ecology & Evolution. →
Jasmonate-induced plant defenses hinder growth in Cardamine cordifolia (2010). →
Louda, S. M. (1984). Herbivore effect on stature, fruiting and leaf dynamics of a native crucifer. Ecology. →
Louda, S. M., Rodman, J. E. (1983). Concentration of glucosinolates in relation to habitat and insect herbivory for the native crucifer Cardamine cordifolia. Biochemical Systematics and Ecology. →
Louda, S. M., Rodman, J. E. (1996). Insect herbivory as a major factor in the shade distribution of a native crucifer (Cardamine cordifolia A. Gray, bittercress). Journal of Ecology. →
Mitchell, C. L., et al. (2015). Evolution of herbivory in Drosophilidae linked to loss of behaviors, antennal responses, odorant receptors and ancestral diet. PNAS. →
Peláez, J., et al. (2022). Evolution and genomic basis of the plant penetrating ovipositor: a key morphological trait in herbivorous Drosophilidae. Proceedings of the Royal Society B. →
Pierce, N. E., Easteal, S. (1986). The selective advantage of attendant ants for the larvae of a lycaenid butterfly, Glaucopsyche lygdamus. Journal of Animal Ecology. →
Pierce, N. E., Mead, P. S. (1981). Parasitoids as selective agents in the symbiosis between lycaenid butterfly larvae and ants. Science. →
Whiteman, N. K., et al. (2012). Genes involved in the evolution of herbivory by a leaf-mining drosophilid fly. Genome Biology and Evolution. →