Impact of the Third Trophic Level in an Obligate Mutualism: Do Yucca Plants Benefit from Parasitoids of Yucca Moths?

In obligate pollination mutualism, selection should favor increased exploitation by either party at the expense of the other. One potential pathway to increase such exploitation at the expense of the mutualist partner is through indirect mutualism across trophic levels, where the natural enemy of a pollinator could reduce pollination costs to the plant. We tested this hypothesis, asking whether a specialized parasitoid wasp that kills yucca moth larvae before they have completed seed feeding will increase the production of intact yucca seeds. In the two years of study in a population of Yucca treculeana Carrie`re (Agavaceae) in west Texas, per capita seed consumption by parasitized larvae of both a pollinator and a nonpollinator yucca moth were significantly reduced when compared to that of nonparasitized conspecific larvae. In one year, fruits that contained at least some parasitized larvae produced significantly more intact seeds than ones with only nonparasitized larvae, whereas the opposite effect was found in the second year. The reason for the latter effect is unclear but is linked with differences in ovule numbers per fruit. The results indicate that natural enemies can benefit Y. treculeana indirectly by reducing its cost of pollination.