Effects of nectar content on the number of bumblebee approaches and the length of visitation sequences in Echium vulgare (Boraginaceae)

The effect of nectar reward on a bumblebee's decision to stay on or to leav e an individual plant of Echium vulgare was studied in the field. Nectar st anding crop was varied by covering plants for different periods. A stochast ic model is developed which describes the bumblebee's "decision strategy" a s function of the duration (in seconds) of the current flower visit (y). Th e duration of the previous flower visit and the number of previous flower v isits on the same plant did not influence the bumblebee's departure behavio ur. Bumblebees on E. vulgare use a probabilistic decision rule. The chance (p) of staying on the plant can be described by a hyperbolic function of the ti me spent in the current flower (y): p = 0.97 - (0.11/(0.04 + y)). Simulatio ns were carried out in which the distribution of nectar over the flowers wi thin a plant was varied. High variation in nectar content shortened visitat ion sequences. In the field we covered plants to increase nectar standing crop per flower. This resulted in longer visitation sequences, but no effect of nectar cont ent on the number of approaches was Found. This is attributed to the fact t hat the first few bumblebees which approach a plant empty most flowers with in a relatively short period with respect to the total observation time.