We investigated interactions between five species of phloem-feeding aphids
(Homoptera: Aphididae) and their host plants at elevated CO2; Acyrthosiphon
pisum (Harris) on Vicia faba L., Aphis nerii Boyer de Fonscolombe on Ascle
pias syriaca L., Aphis oenotherae Oestlund on Oenothera biennis L., Aulacor
thum solani (Kaltenbach) on Nicotiana sylvestris Speg. & Comes and Myzus pe
rsicae (Sulzer) on Solanum dulcamara L. Host plants grown at elevated CO2 g
enerally had greater biomass, leaf area and C:N ratios than those grown at
ambient CO2, while plants with aphids had lower biomass and leaf area than
those without aphids.
The responses of aphid populations to elevated CO2 were species-specific wi
th one species increasing (M. persicae), one decreasing (A. pisum), and the
other three being unaffected. CO2 treatment did not affect the proportion
of alate individuals produced. In general, aphid abundance was not signific
antly related to foliar nitrogen concentration.
We performed separate analyses to test whether either aphid presence or aph
id abundance modified the response of host plants to elevated CO2. In terms
of aphid presence, only three of the potential 15 interactions (five aphid
species x three plant traits) were significant; A. solani slightly modifie
d the response of the plant biomass to elevated CO2 and M. persicae affecte
d the response of leaf area and allocation. In terms of aphid abundance, on
ly two of the potential 15 interactions were significant with A. nerii modi
fying the plant response to CO2 in terms of total leaf area and allocation.
We conclude that, in contrast to other insect groups such as leaf chewers,
populations of most phloem-feeders may not be negatively affected by increa
sed CO2 concentrations in the future. The reasons for this difference inclu
de the possibility that aphids may be able to compensate for changes in hos
t plant quality by altering feeding behaviour or by synthesizing amino acid
s. In addition, there is little evidence that aphid herbivory, even at high
levels, will substantially modify the response of plants to elevated CO2.