Plant diversity has contrasting effects on herbivore and parasitoid abundance in Centaurea jacea flower heads

High biodiversity is known to increase many ecosystem functions, but studies investigating biodiversity effects have more rarely looked at multi‐trophic interactions. We studied a tri‐trophic system composed of Centaurea jacea (brown knapweed), its flower head‐infesting tephritid fruit flies and their hymenopteran parasitoids, in a grassland biodiversity experiment. We aimed to disentangle the importance of direct effects of plant diversity (through changes in apparency and resource availability) from indirect effects (mediated by host plant quality and performance). To do this, we compared insect communities in C. jacea transplants, whose growth was influenced by the surrounding plant communities (and where direct and indirect effects can occur), with potted C. jacea plants, which do not compete with the surrounding plant community (and where only direct effects are possible). Tephritid infestation rate and insect load, mainly of the dominant species Chaetorellia jaceae, decreased with increasing plant species and functional group richness. These effects were not seen in the potted plants and are therefore likely to be mediated by changes in host plant performance and quality. Parasitism rates, mainly of the abundant chalcid wasps Eurytoma compressa and Pteromalus albipennis, increased with plant species or functional group richness in both transplants and potted plants, suggesting that direct effects of plant diversity are most important. The differential effects in transplants and potted plants emphasize the importance of plant‐mediated direct and indirect effects for trophic interactions at the community level. The findings also show how plant–plant interactions critically affect results obtained using transplants. More generally, our results indicate that plant biodiversity affects the abundance of higher trophic levels through a variety of different mechanisms.