Sting theory: how guard bees fight so fiercely when attacked

When exposed to a pheromone, serotonin levels rise in the insects’ brains

January 30, 2018 10:17 pm | Updated 10:17 pm IST - Chennai

Scientist Souvik Mandal in protective gear during the experiment with bees.

Scientist Souvik Mandal in protective gear during the experiment with bees.

Researchers have unravelled the neuro-molecular mechanism of defence by honeybees when exposed to the sting alarm pheromone that they release in the face of a threat.

The team of researchers led by Prof. Martin Giurfa from the Universite de Toulouse in Toulouse, France has found that smelling isoamyl acetate, the main component in the alarm pheromone, increases the level of serotonin and dopamine in their brains, which, in turn, increases the stinging behaviour in bees and thus repels a threat. By itself, the alarm pheromone does not behave as a stimulus, but increases the likelihood of bees guarding the hive to repel a threat by stinging.

Bees collected from four hives that were involved in defending the colony were used in the study. When exposed to isoamyl acetate in the lab, guard bees collected from two colonies exhibited greater proclivity to sting than bees from the other two colonies.

First indication

While the level of dopamine did not show significant difference across brain regions in bees from the four colonies, the level of serotonin was significantly elevated in the central brain (where olfactory processing takes place) in bees that exhibited greater proclivity to sting. This was the first indication of serotonin’s role in the defensive behaviour.

The link between serotonin and responsiveness to threat was verified in individual bees that showed defensive and passive behaviour by measuring the amount of serotonin and dopamine in the brain. Both serotonin and dopamine were significantly elevated in the central brain following exposure to isoamyl acetate. Serotonin level was higher even in the optic lobes and sub-oesophageal zone of bees that showed greater defensive behaviour.

“This established the relationship between higher serotonin level following exposure to isoamyl acetate and stinging behaviour,” says Dr. Souvik Mandal, a PhD scholar from Bengaluru’s Indian Institute of Science (IISc). He was a visiting scientist at the Universite de Toulouse to perform behavioural experiments on the bees and one of the authors of a paper published in Proceedings of the Royal Society B .

“We confirmed that stinging behaviour of bees by topically introducing in the thorax different concentrations of serotonin and dopamine. Compared with controls, the aggressive behaviour was greatly enhanced when highest concentration of serotonin and dopamine was topically applied,” says Dr. Mandal.

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