Variation in Swallowtail/Plant Interactions: Host Selection and the Shapes of Survivorship Curves

Seasonal and yearly variation in host selection and survivorship were studied in a bivoltine population of Papilio zelicaon (Lepidoptera) on Angelica lucida (Umbelliferae) in coastal Washington, USA during spring and summer, 1985 and 1986. The A. lucida population varied seasonally in size structure and availability of flowers. Angelica lucida plants in larger size classes were more likely to receive eggs than plants in smaller size classes in three of the four P. zelicaon generations studied. Flowering A. lucida plants were available for oviposition only in the summer generation each year. In 1985 P. zelicaon attack was randomly distributed between flowering and non-flowering individuals, but in 1986 non-flowering plants had a higher probability of attack. This difference between years did not result from differences in the sizes of flowering plants. Regardless of season, significantly more eggs were laid on the undersides rather than the uppersides of leaves; on flowering plants females laid more eggs on floral than vegetative parts. Larvae on leaves spent a majority of their time on the uppersides of leaves in all generations; larvae on flowering plants spent a majority of their time on floral parts. Survivorship of P. zelicaon eggs and larvae was significantly higher in the summer than in the spring generations in both years. Comparison of these results with other studies of Papilio spp. suggests that most populations experience a fairly constant probability of death during their larval instars. Few other studies, however, have analyzed death of eggs relative to larvae. Overall, the shapes of survivorship curves in this and other Papilio species vary only moderately within a small subset of potential shapes. The results for this single population include the range of shapes of survivorship curves reported for this genus worldwide, including both temperate and tropical habitats.