Background/Question/Methods Complex interactions within communities shape their structure and dynamics. In studies of multi-trophic interactions, the presence of small, invisible microorganisms associated with plants and the presence of the fourth aboveground trophic level have often been neglected. However, to understand the processes, which act within multi-trophic systems in a more realistic framework, incorporating these neglected parts is crucial. Here, we ask how and if the presence of a fungal endosymbiont which alters plant quality by producing herbivore-toxic substances trickles up the food chain and affects the performance and host choice behaviour of aphid secondary parasitoids. In a laboratory experiment, we offered secondary parasitoids simultaneously hosts from endophyte-free and endophyte-infected environment.

Results/Conclusions Especially older and more experienced females were able to discriminate against hosts from the endophyte-infected environment. Secondary parasitoids emerging from the endophyte-infected environment showed a reduced lifespan. This indicates that aphid secondary parasitoids can perceive the disadvantage for their developing offspring in parasitoids from the endophyte environment and can learn to discriminate against it. This discrimination ability may shift the success of primary parasitoids to endophyte-infected plants, which co-occur with endophyte-free plants in natural grasslands. Control of aphids by primary parasitoids would then be increased in the endophyte environment.