THERMAL RELATIONSHIPS BETWEEN CALENDULA ARVENSIS INFLORESCENCES AND USIA AURATA BOMBYLIID FLIES

Abstract

The effect of temperature on the relationship between the Mediterranean marigold, Calendula arvensis (Asteraceae), and one of its most assiduous flower visitors in central Spain, the bombyliid fly Usia aurata, was studied for a five-year period. Calendula arvensis starts flowering when pollinators are still scarce. Mean temperature of the inflorescence surface was positively related to solar radiation and was 2°C higher than the surrounding air. Inflorescences proved to be heliotropic, traversing a 19° mean arc from east to west every day, and maintaining excess temperature during the period of maximum sunshine. Calendula arvensis showed high fruit production by self pollination. Neither fruit production nor fruit mass was limited by pollinators, but pollen germination was heavily limited by temperature. The optimum range for germination in the laboratory was 25–40°C. Under natural conditions, the temperature on C. arvensis heads surpassed the minimum threshold for optimum pollen germination only during the period of maximum sunlight (midday). Usia aurata adults only foraged on C. arvensis heads, they only moved to feed, and they flew from head to head when sunshine was intense. In the laboratory, the temperature at which insects were able to fly ranged from 22° to 31°C for females, and between 21° and 31°C for males. No efficient physiological ability for thermoregulation was found either in females or in males, although females were able to warm themselves slightly. Nevertheless, heat captured passively on C. arvensis inflorescences was sufficient to reach the flying temperature range. Fly behavior on inflorescences also depended on their surface temperature: the higher is the temperature on heads, the longer the time spent feeding, and the shorter the period spent basking. The role of solar warming of heads in plant reproductive fitness was related to stimulation of pollen germination rather than to pollinator attraction. The optimal temperature range for pollen germination overlapped with optimal temperature range for insect flying. U. aurata therefore acted as a pollinator of C. arvensis without playing a decisive role in plant reproductive fitness.