Bottom-up effects of reduced fertilization on natural enemies and biocontrol efficacy

In agroecosystems, arthropod communities may be influenced by bottom-up forces induced by environmental variations (e.g., fertilization) through the modification of plant traits. The way bottom-up forces affect the second trophic level is well documented, but how these effects cascade to the third trophic level is less understood. We aimed to understand: 1) how bottom-up effects vary between natural enemies with contrasted ecology, i.e., parasitoids and predators; and 2) how the diet regime of the predators affected the intensity of bottom-up effects. We set-up a lab experiment measuring the effects of reduced fertilization (from standard rates to no fertilization) on tri-trophic systems in tomato. The selected herbivores are frequent pests on tomato systems (Macrosiphum euphorbiae, Bemisia tabaci and Tuta absoluta). The respective parasitoids were Aphidius ervi and Encarsia formosa, and the respective predators Adalia bipunctata (carnivorous), Macrolophus pygmaeus (omnivorous), Dicyphus errans (omnivorous) and Chrysoperla carnea (carnivorous). We recorded plant growth and leaf carbon and nitrogen content, herbivore fecundity as well as various parasitoid/predator traits including development, longevity, reproduction and biocontrol efficacy. We found evidence of diluted bottom-up forces through trophic levels depending on the herbivore types, with variable but overall marginally positive effects of reducing fertilization from high to intermediate levels on host quality and biocontrol efficacy. Parasitoids were overall less affected than predators. This work offers perspectives in the framework of Integrated Pest Management where reduced fertilization may help better control pest populations without significant impacts on plant growth and thus yield.