Abiotic induction of susceptibility in insect-resistant willow

Plants are mosaics of resource quality to insect herbivores. Some leaves are superior to others, resulting in variable insect performance. Certain Salix viminalis L. (Salicaceae) genotypes show an unusually strong resistance to the gall midge Dasineura marginemtorquens Bremi (Diptera: Cecidomyiidae). A few leaves are frequently galled on such resistant willows, whereas on the great majority of leaves all larvae die before any galls are formed. We performed a series of greenhouse and field experiments in order to understand in more detail the observed intraplant heterogeneity. First, we tested whether random events (biotic or abiotic) can induce a susceptibility that is expressed later (next season), i.e., delayed induced susceptibility. Second, we performed a combined field and greenhouse experiment to test the possibility that susceptibility was induced by abiotic factors at the time of gall induction (within days), i.e., rapid induced susceptibility. We found support for rapid induced susceptibility, but not for delayed induced susceptibility. Although resistance remained strong, significantly more leaves with surviving larvae and galls were found on potted plants moved to the field compared with identical plants that remained in the greenhouse. Potted field plants were exposed to field conditions during a very short period in connection with gall initiation. Thus, we can exclude soil moisture, nutrient availability, and the history of the plant as factors explaining the induction of susceptibility. We tentatively suggest that radiation interferes with the expression of resistance. Our findings suggest that plasticity in plant resistance may be linked to ambient light conditions, and that heterogeneity in leaf suitability can be of ecological significance in plants that are considered resistant.