An experimental test of hybrid resistance to insects and pathogens using Salix caprea, S. repens and their F1 hybrids.

The aim of this study was to assess the responses of herbivores and pathogens to hybrid plants under controlled conditions. F1 hybrids and parental species, produced by hand-pollinating willows in the field, were potted and kept in an experimental field under controlled conditions. In 1997, plant growth and survival were measured along with densities of insects and the degree of pathogen infection on the willows. The survival rate was higher for S. repens than for the hybrids and lowest for S. caprea. Densities of the sawflies Pontania pedunculi and P. brigmanii and the leaf-galling midge Iteomyia capreae were higher on hybrids and on S. caprea than on S. repens. The densities of Crepidodera fulvicornis (Chrysomelidae), chrysomelid larvae and the bud-galling midge Dasineura rosaria did not differ between any of the plant categories. Hybrids were more severely infected by rust (Melampsora sp.) than S. caprea and the totally resistant S. repens. Densities of herbivores on hybrid willows were consistent with the dominance hypothesis (i.e. herbivore densities were similar to densities on one of the parental species) or supported the no-difference hypothesis. Furthermore, herbivore densities on hybrid plants were most similar to densities on the more susceptible parent. The breakdown in rust resistance in hybrid plants suggests that resistance traits are severely disrupted by the genetic re-arrangement in hybrids and that this increased susceptibility could select against hybridisation.