Among various theoretical models that predict the outcomes of plant defense, tolerance, or compensation for tissue damage, the limiting resource model (LRM) proposes that the focal resource limiting to either producer or consumer regulates plant-herbivore interactions. In light of the LRM, experiments in progress demonstrate a probable basis for the conflicting outcomes of biological control efforts on populations of the invasive C. stoebe. We measured the extent to which a seed head weevil (Larinus minutus), gall flies (Urophora spp.) and the root-feeding weevil (Cyphocleonus achates) could impact spotted knapweed growth, seed production, and densities in the Colorado Front Range. We tested the hypothesis that C. stoebe success metrics will vary depending on combinations of herbivory, soil resources, and competition from other plant species, and that multiple herbivores under low resource conditions would be most effective in control. Using an exclosure experiment, we monitored the size and reproductive output of C. stoebe under various herbivory and resource treatments. In a field experiment, the intensity of seed feeding (Larinus minutus, Urophora spp) and root-feeding (Cyphocleonus achates) herbivores was allowed to respond to soil nutrient and plant competition manipulations.

Results/Conclusions

In the exclosure experiment, herbivory negatively affected plant success, with the most pronounced effects resulting from damage by multiple insect species feeding on both above and below-ground plant tissues. Herbivory and biomass reduction of fertilized plants outweighed the reduction for plants with lower soil nutrients. However, absolute biomass was equivalent across nutrient treatments, with insects reducing plant success regardless of the soil resource environment. In the field experiment, we found that seed-feeding insects were more reproductively efficient on fertilized plants with lower plant competition. Nonetheless, plants growing with reduced soil nutrients and greater plant competition had the least biomass and reproductive output. These results support the notion that C. stoebe is more vulnerable to the cumulative stress of multiple rather than single herbivores, and that enhancing plant competition may help to reduce infestations. Further, these herbivores may have greater impacts when target plants are limited by a focal resource other than soil nitrogen, such as tissue loss and the ability to fix carbon. C. stoebe now infests millions of hectares of rangelands, and ecologists and conservationists lack evidence to support a sustainable method to control this plant. Our experiments demonstrate the conditions where biological control may succeed as a sustainable method.