Direct and Indirect Effects of Prior Grazing of Goldenrod upon the Performance of a Leaf Beetle

Abstract

A plant's grazing history can influence the nutritional quality of its leaves and directly affect herbivore performance. In addition, leaf nutritional quality can indirectly 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 clone 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 by larvae ate younger leaves than those feeding in undefoliated plots. Leaves from defoliated plots had direct positive impacts upon 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 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 impact of prior grazing upon egg production negative. These results indicate that impacts of food availability and quality must be considered jointly to understand how prior grazing influences herbivore performance.