CHEATING IN AN OBLIGATE MUTUALISM - HOW OFTEN DO YUCCA MOTHS BENEFIT YUCCAS

We observed bouts of oviposition / pollination / pollen collection behaviour by the females of two species of yucca moths associated with Yucca kanabensis. Some aspects of the bouts were highly stereotyped: bouts always began with oviposition and never with pollination; two pollination events never followed each other without an intervening oviposition; and, pollen collection was almost always followed by flight to another inflorescence. However, other aspects of the bouts were highly variable: the number of ovipositions per bout varied from 1 to more than 20, and the number of pollination attempts varied from 0 to 12. Although yucca moths are essential for the sexual reproduction of yuccas, yucca moths did not transfer pollen in 50-80% of their bouts, either because moths did not carry pollen or because they failed to attempt to transfer pollen. Bouts without attempted pollination were characterized by fewer ovipositions than bouts with attempted pollination, but the absence of pollination attempts was not an artifact of short bouts. Whether a moth carried pollen did not affect the likelihood of her failing to attempt to pollinate. Moths responded to the presence of previous ovipositions in flowers by ovipositing less frequently, and by pollinating less frequently or even failing to pollinate at all. However, moths also failed to attempt to pollinate in fresh, previously unvisited flowers. Moths that visited more than one flower usually attempted pollination in some flowers but not others. Moths not carrying pollen often failed to collect pollen even when it was available. Bouts without pollen transfer must decrease the potential net seed production for flowers, because there is an increase in the number of seeds likely to be eaten by yucca moth larvae without an increase in seed set. We interpret these results in light of several hypotheses for why moths should fail to attempt to transfer pollen or fail to carry pollen.