Regulation of Mutualism between Yuccas and Yucca Moths: Is Oviposition Behavior Responsive to Selective Abscission of Flowers?

Abstract

In the obligate pollination/seed predation mutualism between yuccas and yucca moths, yuccas may exert considerable control over the interaction by abscising a high proportion of visited flowers. Because egg mortality in abscised flowers is 100%, yucca moths should avoid ovipositing in, or decrease the number of eggs laid in flowers more likely to abscise. The focus of out study was to demonstrate experimentally (1) the extent to which abscission is non-random with respect to the number and kinds yucca moth ovipositions, and (2) the extent to which yucca moths modify their oviposition behavior in response to the probable fate of a flower as determined by the number and kinds of previous ovipositions in a flower. To address these problems we studied the interaction between Yucca kanabensis and its two species of pollinating yucca moths, one each from the deep and shallow groups in the Tegeticula yuccasella species complex. We demonstrate that the presence of deep ovipositions increased abscission rates of flowers with shallow ovipositions, the presence of shallow ovipositions had no effect on abscission of flowers with deep ovipositions, and the probability of abscission increased with the number of deep ovipositions but was independent of the number of shallow ovipositions. These results indicate that regulation of mutualism through selective abscission of fruit works for deeps but not for shallows. We predicted that yucca moths should avoid flowers with deep ovipositions and seek out flowers with shallow ovipositions. We found, however, that shallows showed only a weak avoidance of flowers with deep ovipositions, and that deeps did not avoid ovipositing in flowers that already contained deep eggs. We consider three hypotheses that could explain the weak fit between out predictions and observations.