REPRODUCTIVE BIOLOGY OF APIACEAE. I. FLORAL VISITORS TO THASPIUM AND ZIZIA AND THEIR IMPORTANCE IN POLLINATION

Based on the ready availability of nectar and pollen, and on the large numbers and great diversity of insect visitors, species in the Apiaceae have been labeled promiscuous. The distinction between floral visitors and effective pollinators, however, is extremely important and is rarely discerned. Floral visitation was documented for plants of nine populations in a comparative study of three species of the closely related apioid genera, Thaspium Nutt. and Zizia Koch. A pollinator importance index was calculated for each floral visitor using visitor abundance, pollen load composition and foraging behavior as its basic components. Results showed that, despite a high diversity of insect visitors, generally 1–4 species accounted for a minimum of about 74% of the pollinations in all populations. This specialization in pollination appears in part to be the result of an oligolectic relationship between Andrena ziziae (Hymenoptera; Andrenidae) and plants of the taxa studied, but solitary bees of Andrenidae, Colletidae and Halictidae in general were efficient and important pollinators. This study emphasizes that visitation records, when considered alone, effectively disguise specialization in the pollination system.