Background
The authors note that "pheromones are utilized by insects for several purposes, including alarm signaling (Kislow and Edwards, 1972; Blatt et al., 1998; Hunt et al., 2003)," which in the case of phloem-feeding aphids (Chaitophorus stevensis) induces high-density groups of them on exposed leaves of trembling aspen (Populus tremuloides Michx.) trees to disperse and move to areas of lower predation risk.

What was done
The study was conducted at the Aspen FACE site near Rhinelander, Wisconsin, USA, where there were four atmospheric trace gas treatments: control (367 ppm CO2, 38 ppb O3), elevated CO2 (537 ppm), elevated O3 (51 ppb), and elevated CO2 and O3 (537 ppm CO2, 51 ppb O3).  Within each treatment, several aspen leaves containing a single aphid colony of 25 ± 2 individuals were treated in one of two different ways: (1) an aphid was prodded lightly on the thorax so as to not produce a visible pheromone droplet, or (2) an aphid was prodded more heavily on the thorax and induced to emit a visible pheromone droplet, following the technique of Mondor and Roitberg (2003).  Subsequently, in the words of the authors, "aphids exhibiting any dispersal reactions in response to pheromone emission as well as those exhibiting the most extreme dispersal response, walking down the petiole and off the leaf, were recorded over 5 min."

What was learned
Mondor et al. found that the aphids they studied "have diminished escape responses in enriched carbon dioxide environments, while those in enriched ozone have augmented escape responses, to alarm pheromone."  In fact, they report that "0% of adults dispersed from the leaf under elevated CO2, while 100% dispersed under elevated O3."

What it means
The effects of elevated CO2 and elevated O3 on aphid response to pheromone alarm signaling appear to be diametrically opposed to each other, with elevated O3 (which is detrimental to vegetation) helping aphids to escape predation and therefore live to do further harm to the leaves they infest, but with elevated CO2 (which is beneficial to vegetation) making it more difficult for aphids to escape predation and thereby providing yet an additional benefit to plant foliage.  Within this context, therefore, ozone is doubly bad for plants, while carbon dioxide is doubly good.  In addition, Mondor et al. note that this phenomenon may be of even broader scope than what is revealed by their specific study, noting that other reports suggest that "parasitoids and predators are more abundant and/or efficacious under elevated CO2 levels (Stiling et al., 1999; Percy et al., 2002), but are negatively affected by elevated O3 (Gate et al., 1995; Percy et al., 2002)."

References
Blatt, S.E., Borden, J.H., Pierce, H.D. et al.  1998.  Alarm pheromone system of the western conifer seed bug, Leptoglossus occidentalis.  Journal of Chemical Ecology 24: 1013-1031.

Gate, I.M., McNeill, S. and Ashmore, M.R.  1995.  Effects of air pollution on the searching behaviour of an insect parasitoid.  Water, Air and Soil Pollution 85: 1425-1430.

Hunt, G.J., Wood, K.V. Guzman-Novoa, E. et al.  2003.  Discovery of 3-methyl-2-buten-1-yl acetate, a new alarm component in the sting apparatus of Africanized honeybees.  Journal of Chemical Ecology 29: 453-463.

Kislow, C.J. and Edwards, L.J.  1972.  Repellent odour in aphids.  Nature 235: 108-109.

Mondor, E.B. and Roitberg, B.D.  2003.  Age-dependent fitness costs of alarm signaling in aphids.  Canadian Journal of Zoology 81: 757-762.

Percy, K.E., Awmack, C.S., Lindroth, R.L. et al.  2002.  Altered performance of forest pests under atmospheres enriched by CO2 and O3.  Nature 420: 403-407.

Stilling, P., Rossi, A.M., Hungate, B. et al.  1999.  Decreased leaf-miner abundance in elevated CO2: reduced leaf quality and increased parasitoid attack.  Ecological Applications 9: 240-244.