Aphid individual performance may not predict population responses to elevated CO2 or O3

Changes in atmospheric composition affect plant quality and herbivore performance. We used the Aspen Free Air CO2 Enrichment (FACE) facility to investigate the impacts of elevated carbon dioxide (CO2) and ozone (O-3) on the performance of the aphid Cepegillettea betulaefoliae Granovsky feeding on paper birch (Betula papyrifera Marsh.). In Year 1, we simultaneously measured individual performance and population growth rates, and in Year 2 we surveyed natural aphid, predator and parasitoid populations throughout the growing season. Aphid growth and development (relative growth rate (RGR), development time, adult weight, embryo number and the birth weight of newborn nymphs) were unaffected by CO2 and O-3. Aphid fecundity decreased on trees grown at elevated CO2, O-3 and CO2+O-3. Neither nymphal performance nor adult size were reliable indicators of future fecundity at elevated CO2 and/or O-3. Aphid populations protected from natural enemies were unaffected by elevated CO2, but increased significantly at elevated O-3. Individual fecundity in elevated CO2 and O-3 atmospheres did not predict population growth rates, probably because of changes in the strength of intraspecific competition or the ability of the aphids to induce nutrient sinks. Natural aphid, predator and parasitoids populations (Year 2) showed few significant responses to CO2 and O-3, although CO2 and O-3 did affect the timing of aphid and natural enemy peak abundance. Elevated CO2 and O-3 affected aphid and natural enemy populations independently: no CO2 x O-3 interactions were observed. We conclude that: (1) aphid individual performance did not predict population responses to CO2 and O-3 and (2) elevated CO2 and O-3 atmospheres are unlikely to affect C. betulaefoliae populations in the presence of natural enemy communities.