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 leave an individual plant of Echium vulgare was studied in the field. Nectar standing crop was varied by covering plants for different periods. A stochastic model is developed which describes the bumblebee's "decision strategy" as function of the duration (in seconds) of the current flower visit (y). The duration of the previous flower visit and the number of previous flower visits on the same plant did not influence the bumblebee's departure behaviour. 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 time spent in the current flower (y): p = 0.97 - (0.11/(0.04 + y)). Simulations were carried out in which the distribution of nectar over the flowers within a plant was varied. High variation in nectar content shortened visitation 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 content on the number of approaches was Found. This is attributed to the fact that the first few bumblebees which approach a plant empty most flowers within a relatively short period with respect to the total observation time.