Increased density of honeybee colonies affects foraging bumblebees Kerstin Walther-Hellwig, Gerriet Fokul, Robert Frankl, Ralph Büchler, Klemens Ekschmitt and Volkmar Wolters Apidologie, 38 1 (2007) 124 Published online: 08 February 2007 DOI: 10.1051/apido:200701