Social bees are known to avoid inflorescences marked with dead conspecifics or their smell. The avoidance response could be triggered by alarm signals actively given by attacked bees or by substances passively released through injuries as a by-product of the attack. To discriminate between these two options we note that both social and solitary bees are expected to react to nonsignalling cues associated with predation risk, while only social bees are expected to give alarm signals. We simulated risky inflorescences by pinching a landing bee with forceps, and compared the rate at which bees visited these experimental inflorescences and unmanipulated control inflorescences. We conducted the experiment with four species of social bees, Apis mellifera, Apis dorsata, Apis florea and Bombus terrestris and with three species of solitary bees, Eucera sp., Panurgus sp. and Nomia strigata. We found that while the three species of solitary bees responded similarly to control and experimental inflorescences, all four species of social bees strongly rejected inflorescences where we simulated a predation attempt. The finding that only social species avoided landing on dangerous inflorescences strongly suggests that the release of the alarm cue has been selected for its signalling value in social bees. (C) 2012 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.