The effect of petal-size manipulation on pollen removal, seed set, and insect-visitor behavior in Campanula americana

The effect of petal-size variation was investigated in tall bellflower, Campanula americana, on pollen removal rates, seed set, and pollinator behavior. Pollen removal, seed set, and the behavior of two bee visitors (Bomhus and Halictus) were assessed in flowers that had 0%, 50%, or 100% of their petal lobes removed. Pollen removal rates did not differ significantly among the three treatment groups, probably due to the presence of an inetfective ("ugly") pollinator (Halictus), which quickly removed pollen from completely reduced flowers. Seed set was significantly positively correlated with petal size. Bumblebees discriminated against reduced flowers, thereby lowering pollen movement to reduced flowers. Halictid bees rarely contacted stigmatic surfaces and thus pollen delivery to reduced flowers was limited. The presence of an ugly pollinator probably obscured the effect of petal reduction on pollen receipt, thus masking the impact of attractive structures on male fitness. Given the positive relationship between seed set and petal size, we caution against ascribing expenditure on attractive structures exclusively to male tunction. In plants with many ovules, expenditure on attractive structures may affect seed set.