Bees can use sophisticated signals to warn their nestmates about the level of danger from predators attacking foragers or the nest, according to a new study. Biologists at UC San Diego and in China found that an Asian species of honey bee can produce different types of vibrational "stop signals" when attacked by giant Asian hornets.

These signals have different effects depending upon type of danger and the context. A bee delivers a stop signal by giving another bee a brief, vibrational pulse, usually through a head-butt.

"Surprisingly, this signal encodes the level of danger in its vibrational frequency, its pitch, and the danger context through the duration of each pulse," said James Nieh, a professor of biology at UC San Diego who headed the research team., which was also led by Ken Tan, a professor at Xishuangbanna Tropical Botanical Garden, Chinese Academy of Science.

The scientists report their discovery, which they say is the most sophisticated form of alarm signaling found in a social insect, in a paper published this week in the open-access journal PLOS Biology.

Six years ago, Nieh discovered that foragers of the European honey bee, Apis mellifera, when attacked at a food source, will return to the nest and deliver stop signals to nestmates recruiting for the dangerous food source. These signals were known to inhibit recruitment, the famous waggle dance of the honey bee, but researchers did not know what triggered stop signals.

"Stop signals are usually delivered by a sender butting her head into a recipient. Understanding that these signals can be triggered by danger and reduce recruitment for dangerous food therefore made sense," explained Nieh.