Hawkmoth Pollination of Aquilegia chrysantha (Ranunculaceae) in Southern Arizona

Abstract

ABASTRACT.-Aquilegia chrysantha occupies mesic habitats in mountainous regions of the southwestern United States and northern Mexico. The faintly scented, yellow flowers are protandrous and the spurs secrete large quantities of nectar (29% total sugars). Seed set for plants under cages was 31% compared with 55% for open-pollinated flowers. The most common pollinator of the species in the Chiricahua Mountains was the crepuscular hawkmoths Eumorpha achemon (Sphingidae), although two Sphinx species also foraged effectively for nectar. The mean tongue length (4.4 cm) of Eumorpha is relatively short compared with the length (6.7 cm) of the floral spurs of A. chrysantha. This suggests that longer-tongued hawkmoths such as the Sphinx (5.4 cm) may be more important pollen vectors for A. chrysantha than is indicated by their low frequency of visitation in this study. Diurnal visitation to A. chrysantha flowers at a very low rate by large bees and hummingbirds, or recent evolution as a moth-visited taxon, may account for the failure of this hawkmoth-pollinated species to have developed a strictly crepuscular or nocturnal anthesis. Southwestern North America is a region where hawkmoths (Sphingidae) are abundant and many plant species have flowers adapted for visitation by these insects (Gregory, 1963, 1964; Miller, 1981, and included references). Also in this area there are a number of closely related Aquilegia species with pale-colored flowers and unusually long floral spurs. Grant (1952) suggested that members of this group (the A. caerulea James complex) are probably pollinated by hawkmoths. Subsequent observations have shown that A. caerulea is indeed adapted for pollination by hawkmoths, with bumblebees and hummingbirds also visiting its flowers (Miller, 1978, 1981). However, A. micrantha Eastwood, a species that Grant placed in the A. caerulea species group, may be better adapted for pollination by nectar-foraging bumblebee queens than for pollination by hawkmoths (Miller and Willard, 1983). Aquilegia chrysantha Gray is another member of the A. caerulea species group considered to be adapted for hawkmoth pollination (Knuth, 1908; Grant, 1963), although corroborating observations in the field are few (Grant, 1952, 1976; Ritter, cited in Lott, 1979). The species occurs in mesic habitats at elevations of 910-3350 m throughout much of Arizona, New Mexico, Sonora, western Chihuahua, and into Coahuila and Texas where it intergrades with A. longissima Gray ex. Watson (Kearney and Peebles, 1960; Lott, 1979). This paper describes the floral characteristics of A. chrysantha and reports the pollinator activity on several populations of this species in southern Arizona, thus contributing to our understanding of the role pollinators have played in the floral evolution of the columbines.