Organic compost belowground and floral diversity aboveground interactively shape natural enemies in urban gardens

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 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.