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

Authors: Reeves, Justin; Blumenthal, Dana; Kray, Julie; Derner, Justin

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2015
Publication Date: 2/28/2015
Citation: Reeves, J.L., Blumenthal, D.M., Kray, J.A., Derner, J.D. 2015. Increased seed consumption by biological control weevil tempers positive CO2 effect on invasive plant (Centaurea diffusa) fitness. Biological Control. 84:36-43.

Interpretive Summary: Invasive plants may become more successful as atmospheric carbon dioxide (CO2) levels and temperature continue to rise. Because of this, controlling invasive plants may become a more pressing issue into the future. Biological control, the use of natural enemies (often insects) to control invasive plants, can be an effective plant control method. In this field experiment, we examined how increases in CO2 and temperature affected diffuse knapweed (Centaurea diffusa), a problematic invasive plant in much of the western United States. During the experiment, the lesser knapweed flower weevil (Larinus minutus) independently infested the diffuse knapweed (we did not release the weevil), which gave us the opportunity to examine which plants among those grown under elevated CO2 alone, elevated temperature alone, and elevated CO2 and temperature together the weevils preferred. Under elevated CO2, the plants had increased numbers of seed heads and seed production, while under both elevated CO2 and temperature, the plants matured more quickly. The more successful plants grown with elevated CO2 were preferred by weevils, which led to the weevils greatly reducing (although not completely eliminating) benefits to the plant. Weevils seemed to prefer the plants grown with elevated CO2 because they matured more quickly, meaning that the timing of plant growth and weevil activity may be better aligned with predicted changes in climatic conditions. Because of this, these weevils might be an effective means to control diffuse knapweed in the future.

Technical Abstract: 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 the future. Here, we show that free air CO2 enrichment increased the fitness of Centaurea diffusa Lam., a problematic invader in much of the western United States. However, CO2 enrichment also increased the impact of the biological control agent Larinus minutus (Coleoptera: Curculionidae) on C. diffusa fitness. Centaurea diffusa plants flowered earlier and seed heads developed faster with both elevated CO2 and increased temperature. 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. Elevated CO2 increased both 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. 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.