The Cost of Floral Attractants in Achillea ptarmica (Asteraceae): Evidence from a Ray Removal Experiment

The present study of Achillea ptarmica, a perennial in which the basic attraction units are inflorescences (heads) rather than single flowers, examines whether the production of attracting organs (rays) entails a cost in terms of fruit and flower production. To test for such tradeoffs, I subjected plants to ray removal and compared the reproductive success of these plants with the reproductive success of unmanipulated controls. Hand‐pollinated plants on which all rays were removed at the bud stage had a significantly higher percent fruit set than hand‐pollinated plants in the control group, suggesting that the investment in attractive structures is made at the expense of fruit maturation. Ray removal did not significantly affect the number of heads initiated. Given these observations and the lack of tradeoffs between fruit set and other yield components, I hypothesize that the optimal ray size is a compromise between pollinator‐mediated selection for larger displays and tradeoffs with fruit production.