Interactive effects of Mecinus janthiniformis herbivory and varying soil resource conditions on the performance of Dalmatian toadflax, Linaria dalmatica

It is predicted that insect-plant interactions vary with resource conditions. The outcome of these interactions can affect size, biomass and reproductive output of the host plant and may explain the spatially variable impact of the biological control agent Mecinus janthiniformis Toševski & Caldara sp. n. (Coleoptera, Curculionidae) on the Eurasian herbaceous Dalmatian toadflax (Linaria dalmatica (L.) P. Mill.), a noxious weed of western rangelands. The Eurasian stem mining weevil M. janthiniformis was first released in the U.S. in 1995. Since then it has reached outbreak densities at many release sites in the northwestern U.S. and is considered the most effective biocontrol agent for Dalmatian toadflax. We manipulated weevil density and soil nitrogen in a common garden experiment to assess the interactive effects of resource conditions and herbivory on host plant performance. We found that soil N moderated the effects of herbivory by M. janthiniformis on plant growth and reproduction. In general, the effects of soil N were greater on plant performance than weevil density. The data will help improve the understanding of what mechanisms may moderate biological control impact on Dalmatian toadflax across the Northwestern U.S.