Organic compost belowground and floral diversity aboveground interactively shape beneficial insects in urban gardens

Abstract

Plant diversity aboveground can exert top‐down pressure on herbivores by attracting predatory insects, while organic soil amendments rich in beneficial microbes can limit herbivores from the bottom‐up by enhancing plant defensive chemistry. Aboveground and belowground forces always operate simultaneously to shape herbivore pressure, but understanding how they interact is a longstanding and persistent challenge. Here, we examine how organic composts mediate the effects of plant diversity across trophic levels, using zucchini plants (Cucurbita pepo) as a study system. Over two field seasons, we manipulated vermicompost treatments in 18 experiments in school gardens that varied in surrounding plant and floral resource diversity and measured responses of insect herbivores and their natural enemies. Vermicompost strengthened a positive relationship between flower richness and foliar‐feeding omnivores, suggesting that robust reservoirs of omnivores at flower‐rich sites mounted stronger responses to compost‐treated host plants. Predators increased with flower richness, but were not affected by vermicompost. Net outcomes of vermicompost and plant diversity were neutral for herbivores. Synthesis and applications. Altogether, our results reveal that bottom‐up factors protecting plants are modified by their environmental context, and may more effectively attract natural enemies in landscapes with diverse floral resources. Therefore, we recommend augmentation of biodiversity aboveground (i.e. floral resources) together with biodiversity belowground (organic soil amendments) to strengthen crop protection.