Determination of Flower Constancy in Bombus atratus Franklin and Bombus bellicosus Smith (Hymenoptera: Apidae) through Palynological Analysis of Nectar and Corbicular Pollen Loads.

The flower constancy (the visit to a single plant species during a foraging trip) in pollinator insects is a theme widely discussed in behavioral ecology and has an important implication in the evolution of angiosperms. This behavior was studied in the bumblebees Bombus atratus Franklin and Bombus bellicosus Smith through palynological analysis of the nectar and pollen loads of individuals captured while foraging in a restricted area. In both species, there were more individuals with constant flights than with non-constant ones, although in the nectar loads of B. atratus there were no significant differences between individuals with each flight types. It was verified that the nectar loads of the individuals that made either constant or non-constant flights did not differ in the number of pollen grains they contained. Considering this measurement as an estimate for flight duration, the results would indicate that the probability of changing between plant species during nectar collection is independent of the foraging trip duration. In both species, most individuals who collected nectar and/or pollen from more than one plant species visited just two plant species. In these cases, the pollen of one plant species was predominant. In the bumblebees in which it was possible to analyze nectar and pollen loads, the botanical origin of both resources was the same or they shared the principal species (with the exception of two individuals), showing that bumblebees do not often use a botanical source in an exclusive way to collect nectar and another to collect pollen.