Influence of number of flowers and number of previous and simultaneous foragers on bumblebees’ local foraging decisions

The number of both flowers and co-foragers may affect bumblebees’ local foraging decisions and the effort they devote to each flower during foraging. We studied bumblebees’ local foraging decisions by using an experimental set-up in which bumblebees ( Bombus terrestris ) foraged for a single species ( Salvia farinacea ) in patches containing S. farinacea and Tagetes bonanza in varying densities (25, 50 and 75 % Salvia ). We did not restrict the number of co-foragers, which allowed us also to assess the influence of simultaneous and previous foragers on foraging decisions. Bumblebees could adjust the effort per inflorescence and flower to the number of available inflorescences and flowers in the patch. They increased between-patch and between-plant movements as the number of previous foragers increased, and this effect was stronger in the patches with more Salvia , probably due to the synergic effects of number of flowers and number of previous foragers. Simultaneous foragers, on the contrary, favoured within-plant movements. The probability of leaving a patch was positively related to inflorescence exploitation time and to the number of previous foragers. The movements between patches followed Salvia abundance and were modified by the experience gained in the observation period. This experiment shed light on the influence of foragers’ abundances on local foraging behaviour, and the interactions and synergies between flower abundance and foragers’ abundances at a local scale, which are still much unknown.