Research Article: Successional Change in Phosphorus Stoichiometry Explains the Inverse Relationship between Herbivory and Lupin Density on Mount St. Helens

Date Published: November 12, 2009

Publisher: Public Library of Science

Author(s): Jennifer L. Apple, Michael Wink, Shannon E. Wills, John G. Bishop, Jon Moen.

Abstract: The average nitrogen-to-phosphorus ratio (N∶P) of insect herbivores is less than that of leaves, suggesting that P may mediate plant-insect interactions more often than appreciated. We investigated whether succession-related heterogeneity in N and P stoichiometry influences herbivore performance on N-fixing lupin (Lupinus lepidus) colonizing primary successional volcanic surfaces, where the abundances of several specialist lepidopteran herbivores are inversely related to lupin density and are known to alter lupin colonization dynamics. We examined larval performance in response to leaf nutritional characteristics using gelechiid and pyralid leaf-tiers, and a noctuid leaf-cutter.

Partial Text: Predicting the spatial and temporal dynamics of consumer populations as a function of macronutrient and energy resources has a long history in theoretical and empirical ecology. Response to nitrogen (N) has been a particular focus for understanding the dynamics of terrestrial insect herbivores [1], [2], [3], [4], [5], and provides the mechanistic basis for some proposals that predict herbivore feeding patterns [4], [6]. For example, as predicted by one version of the plant stress hypothesis [7], boring insect guilds respond positively to drought stress, as do phloem-feeding insects when drought stress is intermittent [8], effects that occur because of enhanced N availability in drought-stressed plants. Likewise, tests for bottom-up control of terrestrial herbivore populations typically manipulate soil N availability, sometimes with dramatic enhancement of herbivore densities [9], [10], [11], [12], [13], [14], [15].

On Mount St. Helens’ Pumice Plain (∼1200 m elevation), adults of the leaf-tying/leaf-mining caterpillars Filatima loowita (Lepidoptera: Gelechiidae; [42]; misidentified as Chionodes spp. in Bishop [34]) and Staudingeria albipenella (Lepidoptera: Pyralidae) mate and oviposit in early June through mid-July, and larvae are active in July and August. Late instar larvae overwinter and pupate in spring. The two species (referred to hereafter as “gelechiid”, “pyralid”, or collectively as “leaf-tiers”) have similar feeding habits and produce indistinguishable damage patterns. As early instars, they mine individual leaflets, while later instars tie leaflets together into silken feeding tubes; individual plants (up to 40 cm diameter and 10 cm tall) may host dozens of larvae. Euxoa extranea (Lepidoptera: Noctuidae) (hereafter, Euxoa) is an external leaf feeder that mates and oviposits from mid-July until late August. Larvae develop through the fourth or fifth instar before winter diapause, then re-emerge and feed in early summer, passing through 7–8 instars. At Mount St. Helens, all three species appear to feed exclusively on L. lepidus, avoiding even adjacent L. latifolius. Photos of the species and experiments are provided in Appendix S1. All three species were used for an experiment examining growth on wild-collected leaves, while the gelechiid was used for two additional feeding experiments.

Pyralid and gelechiid leaf-tier abundance has been inversely related to host density at Mount St. Helens in each of the last 15 years (1993 through 2007) [33]. The same pattern has been documented for root-boring Lepidoptera, not included in this study, and for Euxoa cutworms in two outbreak years [31]. These guilds differ in many aspects of their biology, including the tissue they feed upon, their exposure to enemies, and in phenology, leading us to hypothesize that increasing patch density or age causes differences in plant nutritional quality that affect all of these guilds similarly. We therefore considered whether variation in leaf nutrient or quinolizidine alkaloid (QA) content might explain the inverse relationship to host density.



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