Plant taxonomic and functional diversity promotes interactions at higher trophic levels, but the contribution of functional diversity effects to multitrophic interactions and ecosystem functioning remains unclear. We investigated this relationship in a factorial field experiment comparing the effect of contrasting plant communities on parasitism rates in five herbivore species. We used a mechanistic trait-matching approach between plant and parasitoids to determine the amount of nectar available and accessible to parasitoids. This trait-matching approach best explained the rates of parasitism of each herbivorous species, confirming the predominant role of mass-ratio effects. We found evidence for an effect of functional diversity only in analyses considering the ability of plant communities to support the parasitism of all herbivores simultaneously. Multi-species parasitism was maximal at intermediate levels of functional diversity. Plant specific richness had a negligible influence relative to functional metrics. Plant communities providing large amounts of accessible nectar and with intermediate levels of functional diversity were found to be the most likely to enhance the conservation biological control of diverse crop herbivores.