Non-Random Fruit Retention in Yucca filamentosa: Consequences for an Obligate Mutualism

Abstract

Post-pollination selective flower abortion can provide plants with a mechanism 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 Tegeticula yuccasella. The plant produces many more flowers than can ever become mature fruit and selectively retains those flowers receiving few eggs and large pollen loads. This study addresses additional factors that affect patterns of floral abortion. We document intrinsic differences in floral abortion probability in Y. filamentosa based on floral position, and abortion based on herbivory by a beetle that coexists with the yucca moths. We then ask how they affect the plant-moth interaction. Pollination experiments showed that the probability of floral retention was positively correlated with late flowering within an inflorescence, more distal position within side branches, and number of flowers on a side branch. Pollination of all flowers, early flowers only, or late flowers only did not result in different overall fruit set, suggesting that resources for fruit maturation can be allocated in response to fluctuations in pollinator availability. Competition among flowers for limited resources plays an important role in determining which flowers become fruits. Flowers attacked by larvae or adults of the nitidulid beetle Carpophilus melanopterus invariably abscised, regardless 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. For this reason, moths should avoid oviposition on beetle-infested flowers. However, there was no evidence that T. yuccasella avoided oviposition within beetle-infested flowers. Intrinsic differences in floral abortion probability and abortion triggered by the beetles add to the previously documented egg- and pollination-driven factors of the yucca moth. Together, these factors create a complex landscape of floral retention probabilities that may influence the oviposition behaviors of the yucca moths.