NONRANDOM FRUIT RETENTION IN YUCCA-FILAMENTOSA - CONSEQUENCES FOR AN OBLIGATE MUTUALISM

Post-pollination selective flower abortion can provide plants with a m echanism to exact some control over which fruits are matured. In Yucca filamentosa, seed quality and quantity are limited by the oviposition and pollination behaviors of its obligate mutualist, the yucca moth T egeticula yuccasella. The plant produces many more flowers than can ev er become mature fruit and selectively retains those flowers receiving few eggs and large pollen loads. This study addresses additional fact ors that affect patterns of floral abortion. We document intrinsic dif ferences in floral abortion probability in Y. filamentosa based on flo ral position, and abortion based on herbivory by a beetle that coexist s with the yucca moths. We then ask how they affect the plant-moth int eraction. Pollination experiments showed that the probability of flora l retention was positively correlated with late flowering within an in florescence, more distal position within side branches, and number of flowers on a side branch. Pollination of all flowers, early flowers on ly, or late flowers only did not result in different overall fruit set , suggesting that resources for fruit maturation can be allocated in r esponse to fluctuations in pollinator availability. Competition among flowers for limited resources plays an important role in determining w hich flowers become fruits. Flowers attacked by larvae or adults of th e nitidulid beetle Carpophilus melanopterus invariably abscised, regar dless of pollination status. Beetles were estimated to be responsible for 37% of all floral abscission. Beetle damage precedes T. yuccasella visitation and any moth eggs within aborted flowers do not survive. F or this reason, moths should avoid oviposition on beetle-infested flow ers. However, there was no evidence that T. yuccasella avoided oviposi tion within beetle-infested flowers. Intrinsic differences in floral a bortion probability and abortion triggered by the beetles add to the p reviously documented egg- and pollination-driven factors of the yucca moth. Together, these factors create a complex landscape of floral ret ention probabilities that may influence the oviposition behaviors of t he yucca moths.