Small pollen grain volumes and sizes dominate the diet composition of three South American subtropical stingless bees

Abstract

The pollen preferences of three Meliponini species (Geotrigona argentina, Scaptotrigona jujuyensis and Tetragonisca fiebrigi) were studied in the Chaco forest of South America. Surprisingly, a large number of pollen types with small volumes and size were found in pollen masses stored in the 18 nests studied. Small pollen grains are commonly found in small flowers with short pistils, a flower morphology adapted to the pollination by these small- to medium-sized bees. The prevalence of grains with small volumes may be due to the preference of Meliponini bees for small flowers with short pistils, supporting a previously suggested hypothesis of diffuse co-evolution between mass-flowering trees and these tiny bees in Neotropical habitats. However, the opportunistic foraging habit of these generalist bees would suggest that the large number of grains with small size is due to their greater availability in the Chaco forest nearby the nests. This study also provides measurements and descriptions of pollen cytoplasmic shapes as well as a new classification of pollen volumes based from measurements of the five pollen size categories using non-acetolyzed grains.