What was done
Working at the Aspen FACE site in Harshaw, Wisconsin, USA, the authors conducted a two-year study of the individual and combined effects of elevated CO2 (+200 ppm) and O3 (1.5 x ambient) - which experimental setting they describe as providing "the most 'realistic' simulation of future elevated CO2 and O3 atmospheres currently possible - on the performance of an aphid species (Cepegillettea betulaefoliae Granovsky) feeding on paper birch trees (Betula papyrifera Marsh.) in what they call "the first investigation of the long-term effects of elevated CO2 and O3 atmospheres on natural insect herbivore populations."

What was learned
At the individual scale, Awmack et al. report that "elevated CO2 and O3 did not significantly affect [aphid] growth rates, potential fecundity (embryo number) or offspring quality."  At the population scale, on the other hand, they found that "elevated O3 had a strong positive effect," but that "elevated CO2 did not significantly affect aphid populations."

In comparing their results with those of prior related studies, the three scientists report that "the responses of other aphid species to elevated CO2 or O3 are also complex."  In particular, they note that "tree-feeding aphids show few significant responses to elevated CO2 (Docherty et al., 1997), while crop-feeding species may respond positively (Awmack et al., 1997; Bezemer et al., 1998; Hughes and Bazzaz, 2001; Zhang et al., 2001; Stacey and Fellowes, 2002), negatively (Newman et al., 1999) or not at all (Hughes and Bazzaz, 2001), and the same species may show different responses on different host plant species (Awmack et al., 1997; Bezemer et al., 1999)."

What it means
In summarizing their observations, Awmack et al. state that "aphid individual performance did not predict population responses to CO2 and O3," and they conclude that "elevated CO2 and O3 atmospheres are unlikely to affect C. betulaefoliae populations in the presence of natural enemy communities."  These findings and conclusions are similar to a those of a number of other studies we have reviewed on our website [see Insects (Aphids) in our Subject Index], and they suggest that the changing composition of the atmosphere will have little net effect on aphid-plant interactions in a high-CO2 world of the future.

References
Awmack, C.S., Harrington, R. and Leather, S.R.  1997.  Host plant effects on the performance of the aphid Aulacorthum solani (Homoptera: Aphididae) at ambient and elevated CO2.  Global Change Biology 3: 545-549.

Bezemer, T.M., Jones, T.H. and Knight, K.J.  1998.  Long-term effects of elevated CO2 and temperature on populations of the peach potato aphid Myzus persicae and its parasitoid Aphidius matricariae.  Oecologia 116: 128-135.

Bezemer, T.M., Knight, K.J., Newington, J.E. et al.  1999.  How general are aphid responses to elevated atmospheric CO2?  Annals of the Entomological Society of America 92: 724-730.

Docherty, M., Wade, F.A., Hurst, D.K., Whittaker, J.B. and Lea, P.J.  1997.  Responses of tree sap-feeding herbivores to elevated CO2.  Global Change Biology 3: 51-59.

Hughes, L. and Bazzaz, F.A.  2001.  Effects of elevated CO2 on five plant-aphid interactions.  Entomologia Experimentalis et Applicata 99: 87-96.

Newman, J.A., Gibson, D.J., Hickam, E., Lorenz, M., Adams, E., Bybee, L. and Thompson, R.  1999.  Elevated carbon dioxide results in smaller populations of the bird cherry-oat aphid Rhopalosiphum padi.  Ecological Entomology 24: 486-489.

Stacey, D. and Fellowes, M.  2002.  Influence of elevated CO2 on interspecific interactions at higher trophic levels.  Global Change Biology 8: 668-678.

Zhang, J., Liu, J., Wang, G. et al.  2001.  Effect of elevated atmospheric CO2 concentration on Rhopalsiphum padi population under different soil water levels.  Yingyong Shengtai Xuebao 12: 253-256.