Background/Question/Methods Plants vary greatly in flower number and size. While pollinators are well-studied agents of selection on floral traits, herbivores were considered only more recently. Nursery pollinators provide a particularly interesting case, since adults act as pollinators (while they oviposit inside the flowers) but larvae act as herbivores through seed predation. Nursery pollinators may prefer to oviposit in larger flowers, if these provide the greatest quantity or quality of food to their larvae. We tested whether flower size predicts the amount of food and whether it influences larval growth in a plant/nursery pollinator system, the dioecious Silene latifolia (Caryophyllaceae), and its seed predator Hadena bicruris (Noctuidae). We conducted a greenhouse experiment: a) we hand-infested the plants with moth eggs, and b) we used small- and large-flowered selection lines of the plant. Results/Conclusions Large-flowered plants produced significantly larger fruits with more seeds. There was no significant difference among lines in seed provisioning, but as a result of variation in seed number per fruit, large-flowered plants had higher total carbon and nitrogen content per fruit. Larvae developing from eggs deposited on large-flowered plants grew faster than those developing from eggs deposited on small-flowered plants. It is known that plants of this species often abort attacked fruits. We found that larval growth was impaired in aborted fruits, but no difference among selection lines in the probability of fruit abortion. Increased larval growth on large-flowered plants suggests that selection should favor oviposition preference for larger flowers in this plant-seed predator system, which will be tested in the future.