Species diversity at lower trophic levels generally improves ecosystem functioning. However, the impact of greater predator diversity on herbivore regulation is uncertain because predator species both compete with and prey on each other. In a large-scale field experiment we examined the relationship between predator species diversity and the suppression of two herbivores, green peach and cabbage aphids, on collard plants. We show that, for both aphid species, the strength of herbivore suppression increased with higher predator biodiversity. Greater resource exploitation by predators in diverse communities generally led to improved predator survivorship and reproduction. Herbivore population size was negatively correlated with plant biomass, providing evidence that greater aphid suppression leads to improved plant growth. Our study suggests a harmonious relationship between predator conservation and herbivore control, and a relatively weak role for predator interference, within this community.