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

Success of biological control of invasive weed programs is typically quite variable across diverse landscapes due to shifting abiotic conditions that can affect the strength of biotic interactions across the range of the invasive weed.  Soil resource availability is one factor that is likely to vary across a landscape and affect the impact of weed biological control agents because vigorously growing plants are better competitors and can withstand more herbivore damage.  We tested these predictions to determine whether they may help explain the spatially variable impact of the classical biological control agent Mecinus janthiniformis Toševski & Caldara of the Eurasian Dalmatian toadflax (Linaria dalmatica (L.) P. Mill.), an herbaceous and noxious weed of rangelands in northwestern NA. We manipulated density of weevil mating pairs and soil nitrogen content in a common garden experiment to assess the interactive effects of resource conditions and herbivory on host plant performance. As expected, we found soil nitrogen levels to moderate the effects of M. janthiniformis herbivory on plant growth and reproduction.  In general, increasing soil nitrogen levels positively affected plant performance (growth and reproduction), while weevil density only negatively affected stem growth.