An international team of scientists watched the bees as they guarded the entrance to their hive. The researchers described how the bees shook their abdomens when a hornet approached, a signal that triggered the hornet to retreat. Researchers already knew of this “characteristic shaking signal”, in which all the guards bees simultaneously vibrate their abdomens from side-to-side for a few seconds when a hornet approaches the colony. In the wild, this produces a spectacular “Mexican wave” of vibrating bees. This study, carried out on a small bee hive, revealed the hornets (Vespa velutina) responded directly to the bees’ shaking signal.  Warned wasps would retreat from the colony and try to catch bees in flight instead. To find this out, the researchers tethered live hornets to lengths of wire and held them at a variety of distances from the hive entrance. The closer the tethered hornet was held to the hive, the more intensely the bee guards shook their bodies.  To confirm that the bees were specifically “talking to” the hornets with this signal, the team carried out the same tethering experiment with a harmless butterfly species