Biological control under a changing climate: the efficacy of the parthenium weed stem-galling moth under an atmosphere enriched with CO2.
Parthenium hysterophorus (Parthenium weed) is a highly invasive species that has become a major weed in Australia and many other parts of the world. The present study reports on the effect of atmospheric CO2 enrichment upon the performance and effectiveness of one of its biological control agents, Epiblema strenuana (stem-galling moth). Parthenium weed plants, when grown under an elevated CO2 concentration (550 µmol mol-1), produced a significantly greater dry biomass (38%), produced more branches (35%) and seeds per plant (37%), than plants grown under an ambient CO2 concentration (380 µmol mol-1). Galls produced by E. strenuana significantly reduced the height (46%) and dry biomass (45%) of parthenium weed under the elevated CO2 concentration. In the presence of E. strenuana, the total number of cypselae produced by plants was 60 or 32% less at ambient or elevated CO2 concentrations, respectively. X-ray revealed that E. strenuana had a significant negative impact upon cypsela fill under elevated CO2 concentration with about 50% not being filled. Gall induction by E. strenuana stimulated lateral branching and this was observed in plants grown both under elevated and ambient CO2 concentrations. Under elevated CO2 and in the presence of E. strenuana, net photosynthesis and water use efficiency were decreased by 25 and 28%, respectively. Despite parthenium weed producing more biomass and seed, this study indicates that the efficacy of E. strenuana as a biological control agent of parthenium weed is likely to be retained in a future climate with an elevated CO2 concentration.