Little is known about the effects of increasing CO₂ and tropospheric O₃, associated with global climate change, on herbivore population dynamics. To better understand the effects of these greenhouse gases on herbivore populations, we assessed changes in pea aphid, Acyrthosiphon pisum, populations exhibiting a pink / green genetic polymorphism on broad bean plants, Vicia faba, grown under elevated CO₂ and/or O₃. Here we show that global atmospheric changes significantly alter herbivore gene frequencies. This change in herbivore populations was a result of strong genotype x CO₂ interactions, as green morphs were strongly influenced by CO₂ concentrations, unlike pink morphs. These two genotypes also showed marked differences in phenotypic plasticity (i.e. wing induction), under varying atmospheric conditions. Thus, as many herbivore natural enemies show host colour preferences, changing atmospheric conditions may substantially alter community-level dynamics.