Indirect Effect of Elevated CO2 on Population Parameters and Growth of Agasicles hygrophila (Coleoptera: Chrysomelidae), a Biocontrol Agent of Alligatorweed (Amaranthaceae)

Alligatorweed, Alternanthera philoxeroide (Mart.) Griseb. (Amaranthaceae) is an invasive weed in China that is often kept under control by the alligatorweed flea beetle, Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae) introduced into China from Argentina in the 1980s. Elevated CO2 levels have been shown to have a direct effect on Ag. hygrophila. In order to fully evaluate the indirect effects of three different atmospheric concentrations of CO2 (420, 550, and 750 ppm) on the population parameters of Ag. hygrophila reared on Al. philoxeroides, we collected life table data for Ag. hygrophila using the age-stage, two-sex life table method. In general, there were no significant differences in the lengths of the preadult parameters among the three treatments. The adult duration and total longevity of males, however, did increase as CO2 increased in concentration. Although the adult preoviposition and total preoviposition periods decreased, the fecundity, oviposition days, eggs per oviposition day, net reproductive rate, intrinsic rate of increase, and finite rate of increase all increased significantly at the high CO2 concentration. Consequently, we determined that the Ag. hygrophila population size will potentially increase rapidly over a short period of time at elevated CO2 concentrations. Our results suggest that 550 and 750 ppm CO2 may also cause physiological changes in Al. philoxeroides that, in turn, provide enhanced nutrition for increasing reproduction in Ag. hygrophila by accelerating maturation of their reproductive system. These results indicate that the efficacy of Ag. hygrophila as a biological control agent against Al. philoxeroides will likely be increased at 550 and 750 ppm CO2.