Increased seed consumption by biological control weevil tempers positive CO2 effect on invasive plant (Centaurea diffusa) fitness

Background/Question/Methods

Predicted increases in atmospheric CO₂ and temperature may benefit some invasive plants, increasing the need for effective invasive plant management. Biological control can be an effective means of managing invasive plants, but the varied responses of plant-insect interactions to climate change make it difficult to predict how effective biological control will be in the future. Field experiments that manipulate climate within biological control systems could improve predictive power, but are challenging to implement and therefore rare to date. Here, we show how free air CO₂ enrichment (to 600 ppmv CO₂) and warming (1.5/3°C day/night) in the field affected the interaction between Centaurea diffusa Lam., a problematic biennial invasive forb in much of the western United States, and one of its key biological control agents, Larinus minutus (Coleoptera: Curculionidae). Natural dispersal of L. minutus into the experimental plots provided a unique opportunity to examine weevil preference for and effects on C. diffusa grown under the different climate change treatments.

Results/Conclusions

Elevated CO₂ increased C. diffusa fitness (seeds/m² produced). However, CO₂ enrichment also increased the impact of L. minutus on C. diffusa fitness by increasing the proportion of seed heads infested by L. minutus and, correspondingly, the amount of seed removed by weevils. This reduced, but did not eliminate, the positive effects of CO₂ on C. diffusa fitness. Plants flowered earlier and seed heads developed faster with both elevated CO₂ and increased temperature. Correlations between plant development time and weevil infestation suggest that climate change increased weevil infestation by hastening plant phenology. Phenological mismatches are anticipated with climate change in many plant-insect systems, but in the case of L. minutus and C. diffusa in mixed-grass prairie, a better phenological match may make the biological control agent more effective in future climates.