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Citation. Brown, D. G.; Weis, A. E. 1995. Direct and indirect effects of prior grazing of goldenrod upon the performance of a leaf beetle. Ecology 76:426-436.   [1607  CC]

Abstract. A plant's grazing history can influence the nutritional quality of its leaves and directly affect herbivore performance by influencing food consumption. We examined the direct and indirect influences of prior grazing of Solidago missouriensis upon leaf nutritional quality and the growth and fecundity of Trirhabda canadensis (Coleoptera: Chrysomelidae). We fed leaves from both previously grazed (during 1988 and 1989) and ungrazed goldenrod plots derived from a single S. missouriensis close to larvae and adults of T. canadensis in laboratory performance trials. We computed standard insect performance indices and used structural equation modeling and path analysis to interpret the results of these trials. Prior grazing of goldenrod had a direct negative impact upon the growth of T. canadensis larvae in spring of 1990; a concomitant reduction in foliage consumption accentuated this effect. Larvae fed leaves from grazed plots accumulated biomass and N more slowly, used them less efficiently, and reached a lower final mass than did larvae fed leaves from plots without prior folivory. Goldenrods produced a new flush of leaves following larval grazing in the spring of 1990. Therefore, adults feeding in plots defoliated plots had direct positive impacts upon the growth and egg production of adult T. canadensis. Adults converted these young leaves into biomass with greater efficiency and preferred them to older leaves from initial ungrazed plots in choice tests. However, negative indirect effects of prior grazing upon initial adult mass and total foliage consumption negated any positive influence that these more nutritious leaves had upon growth, and made the overall impacts of food availability and quality must be considered jointly to understand how prior grazing influences herbivore performance.

Keywords. goldenrod, insect growth and reproduction, insect herbivory, nutritional quality, path analysis, prior grazing, Solidago missouriensis, structural equation modeling, Trirhabda canadensis

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