Direct effects of a biocontrol agent are greater than indirect effects through flower visitors for the alien plant Dalmatian toadflax (Linaria dalmatica: Scrophulariaceae)

Herbivory and pollination are important determinants of female reproductive success in flowering plants. Plants must interact with herbivores and flower visitors simultaneously and interaction with one may alter the outcome of the interaction with the other. These indirect effects can have dramatic impacts on plant fitness. The purpose of this study was to examine whether the stem-boring weevil Mecinus janthiniformis (Curculionidae: Coleoptera) affects flower visitation rate and seed set of the exotic plant Dalmatian toadflax (Linaria dalmatica (L.) Mill. Scrophulariaceae). We compared the flower production, flower morphology, visitation rate, fruit production, and pollen limitation on Dalmatian toadflax plants with and without larval feeding by M. janthiniformis. Feeding by M. janthiniformis reduced the number of flowers and per plant visitation rate, and there was a significant interaction between herbivory and flower number suggesting that the change in visitation rate was not solely a function of a reduction in flower abundance. Herbivory also had direct negative impacts on the reproductive success of Dalmatian toadflax. Total flower and fruit production decreased by over 30 % in plants attacked by M. janthiniformis. However, plants with M. janthiniformis were not more pollen-limited than those without M. janthiniformis. This suggests that herbivory had primarily direct effects female reproductive success.