Individual growth rates do not predict aphid population densities under altered atmospheric conditions

Altered atmospheric composition, associated with climate change, can modify herbivore population dynamics through CO₂ and/or O₃-mediated changes in plant quality. Although pea aphid Acyrthosiphon pisum genotypes exhibit intraspecific variation in population growth in response to atmospheric composition, the proximate mechanisms underlying this variation are largely unknown. By rearing single (green, pink) and mixed (green + pink) pea aphid genotypes on red clover Trifolium pratense at the Aspen Free Air CO₂ and O₃ Enrichment (Aspen FACE) site, we assessed whether: (i) elevated CO₂ and/or O₃ concentrations alter aphid growth and development and (ii) individual aphid growth rates predict aphid population densities. We showed that growth and development of individual green and pink aphids were not influenced by CO₂ and/or O₃ concentrations when reared as individual or mixed genotypes. Individual growth rates, however, did not predict population densities. Reared as a single genotype, green pea aphid populations decreased in response to elevated CO₂ concentrations, but not in response to elevated CO₂ + O₃ concentrations. Pink pea aphid populations reared as a single genotype were unaffected by augmented CO₂ or O₃. Populations of mixed genotypes, however, were reduced under elevated CO₂ concentrations, irrespective of O₃ concentrations. Herbivore population sizes may not readily be predicted from growth rates of individual organisms under atmospheric conditions associated with global climate change.