Nectar foraging by stingless bees in Costa Rica: botanical and climatological influences on sugar concentration of nectar collected by Melipona Jacobus C. Biesmeijer, Martin J.A.P. Smeets, Julliette A.P. Richter and Marinus J. SommeijerApidologie, 30 1 (1999) 43-55DOI: https://doi.org/10.1051/apido:19990106