Population and leaf-level variation of iridoid glycosides in the invasive weed Verbascum thapsus L. (common mullein): im
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CHEMOECOLOGY
RESEARCH PAPER
Population and leaf-level variation of iridoid glycosides in the invasive weed Verbascum thapsus L. (common mullein): implications for herbivory by generalist insects Christina Alba • Ryan Prioreschi • Carolina Quintero
Received: 24 April 2012 / Accepted: 31 October 2012 / Published online: 16 November 2012 Ó Springer Basel 2012
Abstract Plant–insect interactions, which are strongly mediated by chemical defenses, have the potential to shape invasion dynamics. Despite this, few studies have quantified natural variation in key defensive compounds of invasive plant populations, or how those defenses relate to levels of herbivory. Here, we evaluated variation in the iridoid glycosides aucubin and catalpol in rosette plants of naturally occurring, introduced populations of the North American invader, Verbascum thapsus L. (common mullein; Scrophulariaceae). We examined two scales that are likely to structure interactions with insect herbivores—among populations and within plant tissues (i.e., between young and old leaves). We additionally estimated the severity of damage incurred at these scales due to insect chewing herbivores (predominantly grasshoppers and caterpillars), and evaluated the relationship between iridoid glycoside content and leaf damage. We found significant variation in iridoid glycoside concentrations among populations and between young and old leaves, with levels of herbivory strongly tracking leaf-level investment in defense. Specifically, across populations, young leaves were highly defended by iridoids (averaging 6.59 the concentration present in old C. Alba Graduate Degree Program in Ecology and Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523-1177, USA R. Prioreschi C. Quintero Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309-0260, USA C. Alba (&) Department of Invasion Ecology, Institute of Botany, Academy of Sciences of the Czech Republic, 252 43 Pru˚honice, Czech Republic e-mail: [email protected]
leaves, and containing higher proportions of the potentially more toxic iridoid, catalpol) and suffered only minimal damage from generalist herbivores. In contrast, old leaves were significantly less defended and accordingly more substantially utilized. These findings reveal that quantitative variation in iridoid glycosides is a key feature explaining patterns of herbivory in an introduced plant. In particular, these data support the hypothesis that defenses limit the ability of generalists to feed on mullein’s well-defended young leaves, resulting in minimal losses of high-quality tissue, and increasing performance of this introduced species. Keywords Aucubin Catalpol Plant–insect interactions Plant invasions Introduction The long-standing interest in how plant defenses influence herbivory and thus plant performance has become an important aspect of research on biological invasions. For example, the novel chemistry, evolution of increased competitive a
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