A comparative study of floral traits and pollination biology of milkweed, Asclepias, species in Jamaica and Pennsylvania
Milkweed species in the genus Asclepias, are represented throughout the Caribbean and eastern North America, with particular species found in each geographic region. This comparative study was designed to examine the floral traits of two milkweed species common on the Manchester Plateau of Jamaica, Asclepias nivea and Asclepias currasavica, and two species common in the Lehigh Valley of Pennsylvania, Asclepias syriaca, and Asclepias incarnata. Both the Manchester Plateau and Lehigh Valley are characterized by limestone bedrock and karst geology. Milkweed populations were evaluated during June and July for plant density and the percent of stems bearing flowers. Traits of individual plants surveyed include counts of the number of inflorescences, the number of inflorescences bearing open flowers, the number of open and closed flowers per inflorescence, the size of individual flowers, and the number of follicles per stem. Insect visits to plants and flowers were monitored for five minute periods, with the number of visits counted, and the taxon of visitors identified.
In our surveys, populations of the two Jamaican milkweed species were found to have a large percentage of stems without flowers and seed-bearing follicles while the Pennsylvania milkweed species produced a large proportion of flowering stems with a greater number of follicles per stem than Jamaican plants. For Jamaican plants, the number of inflorescences per stem and the flower number per inflorescence, was significantly smaller than the same traits on Pennsylvania plants. Finally, for Jamaican plants, the number of visitors to flowers was small, with most visits made by individuals of several butterfly species. The Pennsylvania plants were visited in by flies, bumblebees, and some butterflies. We would indicate that we did not verify that visitors were effective pollinators, although milkweed pollinia required for pollination, were found on the legs of individual visitors to Pennsylvania milkweeds. These results suggest that there are distinct differences in floral biology and insect visits between the two Jamaican and Pennsylvania milkweeds. While we have measured annual reproductive success of Pennsylvania milkweeds, we must acknowledge that the tropical milkweeds may experience reproductive success at another time of year not documented in our study.