FRUIT ABORTION IN YUCCA-ELATA AND ITS IMPLICATIONS FOR THE MUTUALISTIC ASSOCIATION WITH YUCCA MOTHS

The yucca moth Tegeticula yuccasella is the sole pollinator of Yucca e lata, and relies on yucca fruits to complete its life cycle. A high pe rcentage of pollinated flowers aborts, killing the T. yuccasella eggs and larvae in them. We examined patterns of fruit production and abort ion in Y. elata, and related these patterns to vegetative characterist ics, moth abundance, and environmental conditions. We studied 38 inflo rescences throughout their flowering period, during one season in sout hern New Mexico, USA. Each night we recorded the number of flowers ope ning, the number of fruit formed, the relative abundance of yucca moth s, and climatic conditions. We monitored 11786 flowers, resulting in 6 99 mature fruit. Large inflorescences produced more, but proportionate ly fewer fruit than small inflorescences. Inflorescences flowering lat e in the season produced proportionately more fruit than inflorescence s flowering early. Only 6.6% (extremes 1.4-15.1%) of flowers produced mature fruits. Hand-pollination of all flowers on inflorescences did n ot significantly increase the proportion of flowers that developed int o mature fruit. Fruit production appeared to be resource-, not pollina tor-limited. Ninety per cent of observed moth-pollinated flowers abort ed (N = 31), resulting in the death of moth eggs and larvae laid in th e flowers opening during a 'window' of consecutive nights, lasting for five nights on average (36% of the flowering period of an inflorescen ce). The timing of the window of fruit production was highly variable and unpredictable: mature fruits were produced from flowers opening at the beginning, in the middle, or at the end of the flowering period o f an inflorescence. The results for Y. elata do not support existing h ypotheses that attempt to explain patterns of selective fruit producti on. High rates of abortion of initiated fruit, and the apparently unpr edictable pattern of fruit production by individual inflorescences, ma y stabilize the mutualistic interaction by preventing yucca moths from hyper-ovipositing flowers that have a high probability of developing into mature fruits. A risk-spreading strategy of oviposition is likely to be more successful for the moth than multiple ovipositions per flo wer.