Regulation of mutualism between yuccas and yucca moths: population level processes

This study addresses five population level processes associated with regulation of the pollination/seed predation mutualism between Yucca kanabensis and two species of pollinating yucca moths, one each from the "deep" and "shallow" groups of species within the Tegeticula yuccasella species complex. 1) The adult yucca moth population is apparently relatively constant from year to year, as shown by the inverse relationship between the proportion of rosettes flowering and the proportion of flowers receiving oviposition/pollination visits from yucca moths. 2) The relative success of yucca moths as measured by the proportion of visited flowers retained as fruit, is an inverse function of the ratio of moths to flowers, as measured by visitation. 3) The interactions between yucca moths and yuccas is different for deeps and shallows. Not only was there differential retention of flowers visited by shallows relative to flowers visited by deeps; but the differential appears to be frequency dependent. 4) Within seasons, fruiting and therefore the success of yucca moths may be concentrated into a period of just a few days. This non-random pattern of fruiting differed from both the pattern of flowering and the pattern of visitation, and was a function of higher than average retention of visited flowers. However, no set of factors correlated consistently with these population level "windows" of fruit retention. 5) The interaction between yucca moths and yuccas shifts with the density of yucca moths: the number of eggs laid per flower increases with visitation, and the amount of pollen carried by yucca moths decreases with moth density. I discuss these results in the context of hypotheses for the regulation of the yucca/yucca moth mutualism, and hypotheses for how these population patterns should affect the life histories and behavior of yucca moths.