Variation in predation costs with Chiastocheta egg number on Trollius europaeus: how many seeds to pay for pollination?

1. In obligate plant/seed parasite–pollinator mutualisms, the plant is exclusively pollinated by an insect whose larvae are specific seed predators. Hence, outcomes of the interaction for the plant can vary with the number of eggs laid and the number of seeds eaten per larva. 2. In the work reported here, predation by Chiastocheta larvae on seeds of Trollius europaeus was analysed as a function of the number of eggs laid on the flower. Flowers with an increasing number of eggs were bagged in three populations and seeds were counted after the end of larval predation, in order to assess whether there was competition among larvae. 3. Seed predation on single‐egg flowers was high and variable (mean per population ranging from 15 to 40% of the developed seeds). Seed predation increased weakly with increasing egg load and was lower than gross seed production (always