Plant physiological responses to simultaneous above- and belowground herbivory in the field are seldom measured. Linking these responses to plant fitness could improve predictions of the efficacy of biological control agents for use on invasive plants, ultimately improving our understanding of variable success in controlling populations of invasive non-native species by providing a mechanism of the biological control. In an infestation of spotted knapweed (Centaurea stoebe) located in the Colorado foothills, plants received a natural gradient of herbivory by the small, stem and foliar-feeding Larinus minutus and the much larger root-feeding Cyphocleonus achates, with some plants receiving no damage from either. We asked three questions: 1) What is the relationship between spotted knapweed physiology, plant biomass and potential reproductive output? 2) How does shoot herbivory by L. minutus and root herbivory by C. achates affect plant physiology, plant biomass and potential reproductive output? 3) Which herbivore most negatively affects knapweed and do the two species interact? We measured plant physiological traits as well as above- and belowground biomass, flower production, herbivore damage and insect population size in June and July 2009.

Results/Conclusions

Net C assimilation rate and transpiration increased as L. minutus herbivory intensity increased, then declined as the level of herbivory increased, while water use efficiency increased 61% over the same time period. In short, intermediate levels of L. minutus herbivory were associated with higher C assimilation rates which declined as herbivory increased. The overall negative effects of herbivory on plant physiological responses were correlated with eventual declines in plant performance measures including aboveground biomass and flower production. The decrease was likely due to chronic aboveground herbivory by L. minutus, while the effects of C. achates root damage to plant physiology were only marginally significant. Surprisingly, the effects of these two species were not interactive. Rather, the lesser of the two weevils, L. minutus, reduced spotted knapweed physiological parameters and potential reproductive output more than C. achates, even when both insects were present. Previous studies have shown C. achates root-feeding to have large negative effects on population densities of spotted knapweed. The addition of L. minutus to the suite of insects used in biological control of spotted knapweed should facilitate reduction in knapweed densities through reductions in photosynthetic rates and subsequent declines in plant growth and reproduction rather than exclusively through seed head herbivory.