To defend their colonies against intruders, honeybees release chemical signals called alarm pheromones that alert their nestmates to the source of danger and recruits them for defensive tasks. But the resulting aggressive behaviors are costly: Stinging always results in the death of the bee since the stinger detaches from its abdomen. That’s why the process must be tightly regulated at both the individual and colony level. According to new work published in Nature Communications this week, when aggressive bees are exposed to floral odors, they calm down. Bees respond more strongly to signals containing information about food than to alarm pheromones. 

Honeybee aggression is aimed at protecting the nest, which contains the food, the brood, and the queen. Guard bees are responsible for responding to disturbances near the colony. They’re highly responsive to visual cues like movement or dark colors, and they signal threats to soldier bees inside the nest by releasing the sting alarm pheromone – a blend containing at least 40 compounds that we know about. To the right, with its front legs off the ground and antennae pointing forwards, this alerted bee is about to jump on an intruder. 

To better understand olfactory cues, University of Queensland’s Morgane Nouvian and colleagues studied the effects of floral chemicals on the aggressive behavior of honeybees after the release of alarm pheromones. They collected Apis mellifera ligustica bees from four unrelated colonies in Brisbane between April 2013 to October 2014. In their experimental setup (see video below), individual or small groups of bees were confronted with a dark, moving target within a circular arena where various odors can be released. The bees received a quick puff of odor just before facing the target – a rotating dummy made of a syringe barrel, black suede leather, a black feather, and electrical tape. 

The team measured aggressiveness by counting the number of times a guard or soldier bee attempted to sting or bite the dummy. They found that the common floral odor compounds linalool and 2-phenylethanol, as well as the scent of lavender, were able to block the recruitment and aggressive responses triggered by alarm pheromones. 

Furthermore, this calming effect wasn’t because the flower scents were simply masking the alarm pheromone. Instead, the floral odors represent a food reward: These elicit feeding and foraging, while preventing disturbed bees from engaging in defense. These findings, the authors write, offer a possible mechanism for the decision-making process underlying honeybee aggression.

You can see how fast bees react to the sting alarm pheromone in this video. A pair of bees attack the dummy within seconds of their introduction inside the test arena. 

^{Video Credit: Morgane Nouvian/CRCA–QBI
Image in the text: David Vogel/CRCA}