Angiosperm diversity stems, in part, from the complex ecological interactions in which they participate. This diversity is expressed through morphological and chemical traits that are vital in mediating the behavior of both mutualists and antagonists. Using extensive sampling of individual plants within a large population of Castilleja sessiliflora, we compared the relative importance of morphological and floral volatile traits in the attraction of mutualistic and antagonistic insects. We used structural equation modelling and multiple linear regressions to identify those plant traits that best predicted visitation by pollinators (fruit set) and seed predators (fruit predation). (Z)-β-ocimene was positively correlated with fruit set, while the related and more abundant volatile, (E)-β-ocimene, along with the number of stems per plant, were negatively correlated with fruit set. Loss of fruits to seed predators was positively correlated with corolla length and seed predator visitation was positively correlated with β-myrcene emission. Traits that predicted pollination were distinct from those that predicted seed predation. Floral volatile traits were generally stronger predictors of both pollinator and seed predator activity than were morphological traits. Indirect and direct measures of seed predator activity provided complementary corroboration of the importance of β-myrcene in influencing seed predation.