• Witches’ brooms on Berberis vulgaris are induced by a systemically infecting rust fungus, Puccinia arrhenatheri. These witches’ brooms bear yellow discolored leaves on which the fungus exposes its gametes in a sugary nectar. During the spermatial stage of the fungus the infected leaves emit a strong, flowery scent.
• An exclusion-experiment was used to evaluate whether fungal reproductive success, defined by the ability of the fungus to produce aeciospores, depended on gamete transfer by insects. To determine whether insects were attracted to the infected leaves, and if so, why, visitation to infected and uninfected leaves was quantified and volatiles produced by leaves, infected leaves and flowers were analyzed.
• The production of aeciospores was significantly higher on witches’ brooms with insect visitation. Visitation rates were higher and visits were longer on witches’ brooms than on uninfected branches. A wide diversity of visitors, mainly Diptera and Hymenoptera, was observed. The volatiles emitted by infected leaves were composed of sweet floral fragrances and insect pheromones.
• Our results suggest that sexual reproduction of the pathogen requires out-crossing by insects and that infected leaves attract insects by floral mimicry (bright yellow color and the production of sugary nectar and volatiles).