Cascading effects of N input on tritrophic (plant-aphid-parasitoid) interactions.

Because N is frequently the most limiting mineral macronutrient for plants in terrestrial ecosystems, modulating N input may have ecological consequences through trophic levels. Thus, in agro-ecosystems, the success of natural enemies may depend not only from their herbivorous hosts but also from the host plant whose qualities may be modulated by N input. We manipulated foliar N concentrations by providing to plants three different nitrogen rates (control, optimal, and excessive). We examined how the altered host-plant nutritional quality influenced the performances of two aphid species, the generalist green peach aphid, and the specialist cabbage aphid, , and their common parasitoid . Both N inputs led to increased N concentrations in the plants but induced contrasted concentrations within aphid bodies depending on the species. Compared to the control, plant biomass increased when receiving the optimal N treatment but decreased under the excessive treatment. Performances of improved under the optimal treatment compared to the control and excessive treatments whereas parameters declined following the excessive N treatment. In no-choice trials, emergence rates of developing in were higher on both optimum and excessive N treatments, whereas they remained stable whatever the treatment when developing in . Size of emerging females was positively affected by the treatment only when it developed in on the excessive N treatment. This work showed that contrary to an optimal N treatment, when N was delivered in excess, plant suitability was reduced and consequently affected negatively aphid parameters. Surprisingly, these negative effects resulted in no or positive consequences on parasitoid parameters, suggesting a buffered effect at the third trophic level. Host N content, host suitability, and dietary specialization appear to be major factors explaining the functioning of our studied system.