Timing of growth determines fitness and performance of a galling insect on willow

Abstract

Characteristics that determine a plant's quality as herbivore food exhibit within-plant heterogeneity. Most models suggest negative effects from secondary chemicals. Less work has focused on plant growth dynamics that might also be important in creating heterogeneity in the distribution of food resources among leaves within the plant. Gall-forming insects are sessile during their feeding stage and are therefore of particular interest when assessing the relative importance of growth and non-growth processes in plants. Galls act as sinks for photoassimilates, and successful redirection of these resources requires that gall formation takes place in plant modules that are in an active growth phase. The gall midge Dasineura marginemtorquens infests leaves of the fast-growing Salix viminalis. Several young leaves per shoot are used as oviposition sites during any single egg-laying occasion. This study investigates the extent to which growth in leaves that are apparently suitable for gall initiation varies along shoots of S. viminalis, and tests whether or not such variation affects the fitness and performance of D. marginemtorquens. The relative position of the galled leaves along a shoot was found to determine the success of the gall midge in terms of larval survival, larval developmental time, and adult size. Leaf growth dynamics, but not leaf size, was associated with the variation in insect fitness and performance. Thus, when considering habitat quality for a sessile insect like D. marginemtorquens, the length of time that the galling site acts as a photoassimilate sink is more important than the final size of the plant module.