A tree in the jaws of a moth – temporal variation in oak leaf quality and leaf-chewer performance

Authors

  • Tomas Roslin,

  • Juha-Pekka Salminen


T. Roslin (tomas.roslin@helsinki.fi), Dept of Biological and Environmental Sciences, PO Box 65 (Viikinkaari 1), FI-00014 University of Helsinki, Finland. – J.-P. Salminen, Dept of Chemistry, FI-20014 University of Turku, Finland.

Abstract

The extent to which individual host trees maintain their relative quality over time may affect patterns of abundance, distribution and microevolution in herbivorous insects. In this study, we explore temporal consistency in the quality of oak Quercus robur foliage, using leaf-chewing larvae of the moth Amphipyra pyramidea as our model herbivores. By utilising an artificial diet, we are able to isolate the impact of chemical contents from physical attributes, and thereby to ask to what extent purely chemical parameters create tree-to-tree differences in host quality, how consistent such differences are among trees between different parts of a single growth season, and to what extent individual moth larvae are able to compensate for chemical variation in food quality. We find that with physical traits controlled for, chemical traits suffice to create strong differences in larval growth rates between trees, and between larvae fed on young and mature foliage. Nevertheless, these initial differences are efficiently compensated for the fact that larvae with lower growth rates continue to grow for a longer time, and thereby end up at the same size as larvae with high growth rates. At the pupal stage, we could no longer detect differences between either larvae fed foliage from different trees or between larvae fed young versus mature foliage – despite notably little variation among individuals within each group. Such compensatory responses were also reflected in patterns of consistency. The intraclass correlation for larval weight was relatively high (ρ=0.45), but lower for development time (ρ=0.26), and non-existent for pupal weight (ρ=0.00). These results suggest that in terms of pupal mass, A. pyramidea is able to compensate more or less completely for differences in resource quality, that patterns of consistency vary with the specific trait examined, and that the net effect of spatiotemporal variation in host plant quality on herbivore fitness should be dissected by experiments aimed at different life history traits. If slow growth comes with high mortality, spatiotemporal patterns in resource quality may have a major impact on herbivore fitness; if not, the patterns may be nullified by efficient compensatory mechanisms.

Ancillary