Plant phenolics mediated bottom‐up effects of elevated CO2 on Acyrthosiphon pisum and its parasitoid Aphidius avenae

Elevated concentrations of atmospheric CO₂ can alter plant secondary metabolites, which play important roles in the interactions among plants, herbivorous insects and natural enemies. However, few studies have examined the cascading effects of host plant secondary metabolites on tri‐trophic interactions under elevated CO₂ (eCO₂). In this study, we determined the effects of eCO₂ on the growth and foliar phenolics of Medicago truncatula and the cascading effects on two color genotypes of Acyrthosiphon pisum (pink vs. green) and their parasitoid Aphidius avenae in the field open‐top chambers. Our results showed that eCO₂ increased photosynthetic rate, nodule number, yield and the total phenolic content of M. truncatula. eCO₂ had contrasting effects on two genotypes of A. pisum; the green genotype demonstrated increased population abundance, fecundity, growth and feeding efficiency, while the pink genotype showed decreased fitness and these were closely associated with the foliar genstein content. Furthermore, eCO₂ decreased the parasitic rate of A. avenae independent of aphid genotypes. eCO₂ prolonged the emergence time and reduced the emergence rate and percentage of females when associated with the green genotype, but little difference, except for increased percentage of females, was observed in A. avenae under eCO₂ when associated with the pink genotype, indicating that parasitoids can perceive and discriminate the qualities of aphid hosts. We concluded that eCO₂ altered plant phenolics and thus the performance of aphids and parasitoids. Our results indicate that plant phenolics vary by different abiotic and biotic stimuli and could potentially deliver the cascading effects of eCO₂ to the higher trophic levels. Our results also suggest that the green genotype is expected to perform better in future eCO₂ because of decreased plant resistance after its infestation and decreased parasitic rate.