Abstract
From the perspective of an herbivorous insect, conspecific host plants are not identical, and intraspecific variation in host nutritional quality or defensive capacity might mediate spatially variable outcomes in plant-insect interactions. Here we explore this possibility in the context of an ongoing host breadth expansion of a native butterfly (the Melissa blue, Lycaeides melissa) onto an exotic host plant (alfalfa, Medicago sativa). We examine variation among seven alfalfa populations that differed in terms of colonization by L. melissa; specifically, we examined variation in phytochemistry, foliar protein, and plant population genetic structure, as well as responses of caterpillars and adult butterflies to foliage from the same populations. Regional patterns of alfalfa colonization by L. melissa were well predicted by phytochemical variation, and colonized patches of alfalfa showed a similar level of inter-individual phytochemical diversity. However, phytochemical variation was a poor predictor of larval performance, despite the fact that survival and weight gain differed dramatically among caterpillars reared on plants from different alfalfa populations. Moreover, we observed a mismatch between alfalfa supporting the best larval performance and alfalfa favored by ovipositing females. Thus, the axes of plant variation that mediate interactions with L. melissa depend upon herbivore life history stage, which raises important issues for our understanding of adaptation to novel resources by an organism with a complex life history.
Highlights
Dietary niche breadth, or the number and type of resources consumed by an organism, drives numerous ecological and evolutionary processes, from mediating the coexistence of competitors [1, 2] to predicting geographical range size [3]
Herbivorous insects are useful in the study of dietary niche breadth because their lives are often so closely tied to their host plants that any change in diet can affect multiple aspects of the insect’s life history [4, 5, 6]
Dietary niche breadth of herbivorous insects is necessarily dependent on host plant variation, the importance of which has typically been explored in a comparative fashion across plant taxa
Summary
The number and type of resources consumed by an organism, drives numerous ecological and evolutionary processes, from mediating the coexistence of competitors [1, 2] to predicting geographical range size [3]. Dietary niche breadth of herbivorous insects is necessarily dependent on host plant variation, the importance of which has typically been explored in a comparative fashion across plant taxa. This comparative work has laid the foundations of plant defense theory and has generated many hypotheses for how and why insects use a particular plant species (e.g. physiological efficiency [5, 13], neural limitation [14], and enemy-free space [15]). We explore the consequences of intraspecific host variation in the context of an ongoing expansion of dietary niche breadth using the butterfly Lycaeides melissa and its introduced host, alfalfa (Medicago sativa)
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