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 excl usively pollinated by an insect whose larvae are specific seed predators. H ence, 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 o rder to assess whether there was competition among larvae. 3. Seed predation on single-egg flowers was high and variable (mean per pop ulation ranging from 15 to 40% of the developed seeds). Seed predation incr eased weakly with increasing egg load and was lower than gross seed product ion (always <85%) whatever the number of eggs laid. This corresponds to a s trong decrease in seed consumption per larva with increasing egg load, i.e. severe larval competition for resources. 4. The results suggest that both interference among Chiastocheta larvae and carpel dehiscence may protect T, europaeus seeds from total predation. Est imates of seed predation based on egg load observed in 20 natural populatio ns in the French Alps typically ranged from 30 to 60%. The interaction was always beneficial for the plant and there was no risk of total seed destruc tion by Chiastocheta larvae, favouring stability of the mutualism.