Target and side effects of Ascophyllum nodosum in a plant-herbivore-parasitoid system

Abstract The beneficial effects of biostimulants derived from the alga Ascophyllum nodosum have been documented for a wide variety of plant species. However, crops are part of complex agroecosystems, where different trophic levels interact and affect crop development. The present study aimed to assess the potential impact of A. nodosum in a tritrophic system consisting of the faba bean crop ( Vicia faba , L.) , the herbivore Aphis fabae , and its parasitoid Aphidius matricariae , in a laboratory setup. The experiment, conducted on whole plants in confined cages, included six treatments combining the presence or absence of the algae extract, herbivores, and parasitoids to assess the effects of their interactions. The target effect of the algae extract on plant growth and development was evaluated by measuring plant height, leaf number, leaf area, and dry weight. Possible side effects of the extract were assessed using aphid population development and parasitoid mummy counts and weights. Under the tested conditions, the application of A. nodosum extract did not affect any of the variables used as indicators of V. faba growth and development, i.e. no measurable impact on the plant under normal conditions or in the presence of a biotic stressor. The extract did not influence the population development of the herbivore A. fabae . Furthermore, its application did not impair the aphid-suppression efficacy of A. matricariae , nor did it alter individual mummy weight or, as preliminary observations indicate, mummy abundance. This result suggests that the algae extract and the parasitoid biological control agent evaluated can be used in combination without antagonistic interactions.