Increased density of honeybee colonies affects foraging bumblebees Kerstin Walther-Hellwig, Gerriet Fokul, Robert Frankl, Ralph Büchler, Klemens Ekschmitt and Volkmar Wolters Apidologie, 37 5 (2006) 517-532 Published online: 12 September 2006 DOI: 10.1051/apido:2006035