Bottom-up effects of apple cultivars on parasitoids via aphid hosts

Variability of intraspecific host plant quality for phytophagous insects may have consequences on the structure and functioning of associated food webs. The quality of host plants can affect aphids fitness, influencing their life history traits and altering the nutritional resources available to higher trophic levels, potentially affecting the development of solitary parasitoids. Here, we assessed the potential bottom-up effects of intraspecific variability among three cultivars (Gala, Ariane, and Greensleeves) of the domesticated apple tree (Malus domestica) with putative resistance towards the rosy apple aphid (Dysaphis plantaginea) on the aphid's performance, and its cascading effects on the parasitoid Ephedrus cerasicola. We measured aphids pre-reproductive period, lipid, and water contents, and recorded their feeding behavior using the electropenetrography technique. Parasitoid developmental duration, sex ratio, hind tibia size, and female egg load were measured and used to evaluate E. cerasicola performance according to the cultivar on which their aphid hosts had been reared. Only the development time of parasitoids was found to be longer on Ariane and Green Sleeves cultivars than on the Gala cultivar. Aphid-feeding behavior variables related to phloem consumption were negatively impacted on apple tree cultivars on which the development time of parasitoids had been reduced. We discuss ways in which cultivar quality can be an important component of tritrophic interactions: the resistant Ariane and Green Sleeves cultivars negatively impacted the aphids but appeared to have limited bottom-up effects on the parasitoids.