1 Previous Address: ETH Zurich, Switzerland. Current Address: Plant and Food Research, Palmerston North
2 ETH Zurich, Switzerland
Elevated concentrations of atmospheric carbon dioxide (CO2), a consequence of anthropogenic global change, may profoundly interfere with tritrophic interactions. Such effects have rarely been investigated. We used the system composed of Brassica plants, the cabbage aphid Brevicoryne brassicae and the endoparasitoid Diaeretiella rapae to compare effects of elevated CO2 (800 ppm) versus ambient CO2 (400 ppm). Plants were exposed to the CO2 concentrations for up to 10 weeks, aphids for 2-3 generations, and parasitoids for 1 generation. Concomitant bioassays with herbivore-infested plants exposed to parasitoids were also conducted. A significantly lower proportion of aphids were parasitized under elevated compared to ambient CO2. Parasitoid progeny emerged earlier but offspring adults were shorter lived under elevated CO2. Plant glucosinolate concentrations were higher under elevated compared to ambient CO2. However, contrary to expectations, aphid glucosinolate concentrations were significantly lower under elevated CO2. Likewise aphid body mass remained approximately 20% lower under elevated compared to ambient CO2. Thus, elevated CO2 seems to have enhanced plant direct defense through an increase in natural plant defense compounds, but also led to a reduction in indirect defense through decreased parasitism and parasitoid longevity. Our results indicate, for the first time, a conflict between bottom-up and top-down control under elevated CO2.