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      Bergamot versus beetle: evidence for intraspecific chemical specialization

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          Abstract

          Many plant-eating insects have developed the ability to eat plants that synthesize toxins which they use to defend themselves against herbivores. While these specialized insects are good at dealing with specific plant toxins, plant species with highly variable chemistry can present a challenge. This study tested for reciprocal effects of a specialist tortoise beetle feeding on a host plant with individuals contained two different essential oil toxins. Overall, beetles showed higher survival, growth, and preference for one of the plant's essential oil types. Thus, host plant species with variable chemistry could change the long-term relationship between plant and herbivore.

          Abstract

          A large proportion of phytophagous insects show host plant specificity (monophagy or oligophagy), often determined by host secondary chemistry. Yet, even specialists can be negatively affected by host chemistry at high levels or with novel compounds, which may manifest itself if their host species is chemically variable. This study tested for reciprocal effects of a specialist tortoise beetle ( Physonota unipunctata) feeding on a host plant ( Monarda fistulosa) with two monoterpene chemotypes [thymol (T) and carvacrol (C)] using a controlled field experiment where larvae fed on caged plants of both chemotypes, haphazardly collected natural plants with and without beetle damage, and growth chamber experiments where larvae that hatched and briefly fed on one chemotype were reared on either chemotype. In the field experiment, plant chemotype did not affect larval weight or length, but did influence larval survival with almost 8.3 % more surviving on T plants. Herbivores reduced seed head area (86.5 % decrease), stem mass (41.2 %) and stem height (21.1 %) of caged plants, but this was independent of host chemotype. Natural plants experienced similar reductions in these variables (74.0, 41.4 and 8.7 %) and T chemotypes were more frequently damaged. In the growth chamber, larval relative growth rate (RGR) differed for both feeding history and year. Larvae from T natal plants reared on T hosts grew at almost twice the rate of those from C and reared on T. Larvae from either T or C natal plants reared on C plants showed intermediate growth rates. Additional analyses revealed natal plant chemotype as the most important factor, with the RGR of larvae from T natal plants almost one-third higher than that of those from C natal plants. These cumulative results demonstrate intraspecific variation in plant resistance that may lead to herbivore specialization on distinct host chemistry, which has implications for the evolutionary trajectory of both the insect and plant species.

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          Most cited references43

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          Specialist versus generalist insect herbivores and plant defense.

          There has been a long-standing hypothesis that specialist and generalist insects interact with plants in distinct ways. Although many tests exist, they typically compare only one species of each, they sometimes confound specialization and feeding guild, and often do not link chemical or transcriptional measures of the plant to actual resistance. In this review, we synthesize current data on whether specialists and generalists actually differ, with special attention to comparisons of their differential elicitation of plant responses. Although we find few consistencies in plant induction by specialists versus generalists, feeding guilds are predictive of differential plant responses. We outline a novel set of predictions based on current coevolutionary hypotheses and make methodological suggestions for improved comparisons of specialists and generalists. Copyright © 2012 Elsevier Ltd. All rights reserved.
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            Host Specialization in Phytophagous Insects

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              Herbivory reduces the strength of pollinator-mediated selection in the Mediterranean herb Erysimum mediohispanicum: consequences for plant specialization.

              In this study, I tested whether selection occurring on several morphological and floral traits in Erysimum mediohispanicum (Cruciferae) is modified by the effects of herbivores. Six plots were established in 1997 in the Sierra Nevada, Spain; three were fenced to exclude native ungulates, and the remaining were open to ungulates. I determined pollinator and ungulate preferences for plant traits and their effect on plant fecundity. Then I compared the selection regimes between plants excluded from herbivores and plants open to them. When ungulates were absent, I found significant selection on flower number, reproductive stalk height, basal diameter of the stalks, petal length, and inner diameter of the flowers. When ungulates were present, selection on floral traits completely disappeared, and selection strength on flower number and morphological traits decreased. This effect was due to the ungulate preference for larger plants and the phenotypic correlations between plant size and floral traits. Results suggest that pollinator-mediated selection can be disrupted by conflicting effects of plant enemies acting during or subsequent to pollination. An accurate picture of the pollinator role as selective pressure requires the consideration of the entire life cycle of the plant as well as the ecological scenario in which the interactions occur.
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                Author and article information

                Journal
                AoB Plants
                AoB Plants
                aobpla
                aobpla
                AoB Plants
                Oxford University Press
                2041-2851
                2015
                17 November 2015
                : 7
                : plv132
                Affiliations
                [1 ]Department of Ecology and Evolutionary Biology, University of Colorado , N122 Ramaley CB 334, Boulder, CO 80309, USA
                [2 ]Present address: Department of Entomology, University of Wisconsin, 1630 Linden Drive, Madison, WI 53706, USA
                Author notes
                [* ]Corresponding author's e-mail address: ken.keefoverring@ 123456wisc.edu

                Associate Editor: Colin M. Orians

                Article
                plv132
                10.1093/aobpla/plv132
                4683979
                26578745
                19812b24-448b-4143-b0b5-3c0ade6b827b
                Published by Oxford University Press on behalf of the Annals of Botany Company.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 15 July 2015
                : 29 October 2015
                Page count
                Pages: 10
                Funding
                Funded by: University of
                Funded by: Colorado
                Funded by: Museum of Natural History
                Funded by: University of Colorado Boulder http://dx.doi.org/10.13039/100007493
                Categories
                1023
                Research Articles

                Plant science & Botany
                carvacrol,herbivore specialization,monarda fistulosa,physonota unipunctata,secondary chemical variation,thymol

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