Mutualism with Pollinating Seed Parasites Amid Co‐Pollinators: Constraints on Specialization

Abstract

The genus Greya is closely related to the yucca moths, and Greya species interact with their host plants in many of the same ways as yucca moths. Females both pollinate and oviposit in the flowers of their host. Unlike yucca moths, however, pollinating Greya species share flowers with co—pollinators that do not oviposit on the host. We studied the interaction between Greya politella (Walsingham) (Lepidoptera: Prodoxidae) and Lithophragma parviflorum (Hook.) Torr. & Gray (Saxifragaceae) to evaluate the effect on seed output of a pollinating seed parasite against a background of co—pollinators. Flowers were visited and pollinated mostly by bombyliid flies, solitary bees, and G. politella. Bombyliid flies alone composed 68—88% of the 5522 visits recorded over 2 yr. Although both male and female G. politella visited the flowers and probed for nectar, pollination by this species occurred only as females oviposited through the corolla tube, thereby passively transferring to the stigma pollen adhering to the abdomen. Visitation to flowers by all pollinators averaged 0.3—1.9 visits/h during daylight hours. Consequently, most flowers were visited multiple times during the several days that stigmas were receptive, and 77% of the flowers tagged during the 4 yr had some developing seeds. Pollination did not depend upon visitation by G. politella. Flowers receiving G. politella eggs had the same probability of producing some seed and the same mean number of developing seeds as flowers visited only by other insects. Most flowers received the eggs of only one G. politella female, and the larvae ate 15—27% of the developing seeds. The final number of mature seeds remaining in attacked flowers did not differ from unattacked flowers except at one site in 1 yr, in which the values for attacked plants were marginally lower. Other sources of variation affecting seed output masked the effects of seed consumption by Greya. Overall, G. politella females have the potential to be mutualistic with L. parviflorum: they are effective pollinators, generally visit most plants and about half the flowers in the populations, and impose a fairly small cost on seed output. Nonetheless, the abundant and effective co—pollinators, which do not eat the developing seeds, swamp Greya's mutualistic effects. Under the current conditions at Granite Point, the relationship between G. politella and L. parviflorum may be mostly commensalistic. The evolution of specialization to G. politella as an exclusive pollinator would seem to be possible only in L. parviflorum populations in which effective co—pollinators were either rare or unpredictable. That is, the potential for the evolution of specificity in this mutualism appears to depend upon the community context in which the interaction takes place rather than upon the simple outcome of the pairwise interaction between Greya and Lithophragma.