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

1 Altered atmospheric composition, associated with climate change, can modify herbivore population dynamics through CO2 and/or O3-mediated changes in plant quality. 2 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. 3 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. 4 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. 5 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. 6 Herbivore population sizes may not readily be predicted from growth rates of individual organisms under atmospheric conditions associated with global climate change.