Trait-mediated interactions and lifetime fitness of the invasive plant Centaurea solstitialis.

Plants interact with numerous enemies and mutualists simultaneously and sequentially. Such multispecies interactions can give rise to trait-mediated indirect effects that are likely to be common in nature but which are also inherently difficult to predict. Understanding multispecies interactions is also important in the use of biological control agents to control invasive plants because modern approaches to biocontrol rely on releasing multiple agents for each target weed. Centaurea solstitialis is one of the most problematic invasive weeds in California, USA, and the weevil Eustenopus villosus is its dominant biological control agent. We conducted a field experiment to quantitatively assess the direct effect of the recently approved biocontrol pathogen Puccinia jaceae f.s. solstitialis on plant performance and any indirect effects that might arise by altering the plant's interactions with Eustenopus or its pollinators (principally the nonnative Apis mellifera). We documented both synergy and interference between the two biocontrol agents depending on the life stage of the weevil. Puccinia infection increased the impact of bud-feeding by the adult weevils but reduced the impact of seed-feeding by larval weevils. Neither infection nor Eustenopus attack had an effect on pollinator visitation. The net effect was that attack by both the pathogen and the weevil did not reduce plant fitness relative to plants attacked only by the weevil. Because the consequence to the plant of interacting with one species may depend on the presence or absence of other interacting species, a careful consideration of multispecies interactions may be necessary for the selection of biocontrol agents that act in a complementary manner to reduce plant fitness. Likewise, relatively tractable weed-biocontrol systems allow us to examine multispecies interactions that can be difficult to study experimentally in native systems that are composed of numerous species with well-established populations.