Phenotypic Plasticity in Foraging Behavior of Sawfly Larvae

Abstract

To maintain relatively stable growth rates over gradients of resource quality, insect herbivores may adjust their consumption, or search for different quality forage. Using a total of six early-, mid-, and late-season species of sawflies consuming mountain birch foliage, we studied whether and how larval growth rate is buffered against low-quality foliage by plasticity in foraging behavior. We measured species-specific patterns in foraging behavior, the costs and benefits of behavioral plasticity, and postingestive physiological efficiencies along a leaf quality gradient. We conducted the experiments on 20 individual mountain birch trees, which were known from previous tests to vary in quality for herbivores. To our knowledge, this is the first study testing the relationship between compensatory consumption and dispersion of feeding bouts. In spite of species-specific variation in the level of postingestive efficiency, leaves from the same poor-quality trees were hard to process physiologically by all the sawfly species. However, sawfly species differed dramatically in the ways their larvae dispersed their feeding sites and consumed leaf mass on poor-quality trees. Larvae displayed two different responses to low-quality food that were almost mutually exclusive and corresponded to the larval feeding phenology of the herbivore species. Species showing dispersion of feeding sites on poor-quality trees were early-summer species; their larvae generally consumed young and nutritious foliage heterogeneously distributed within canopies. In contrast, late-summer species responded to poor leaf quality by increasing their consumption rate. Behavioral traits and physiological efficiencies did not vary independently, but responded as an integrated hierarchical trait network to variation in leaf quality, which generated relatively stable growth rates. Compensatory consumption was beneficial to larvae because it improved larval performance on poor-quality trees. Dispersion of feeding sites may have been advantageous because larvae are able to find higher quality leaves within the heterogeneous mountain birch canopy. This study shows the power of comparative multivariate analyses in understanding complex intercorrelated plastic responses.