The Right Chemistry: Honey bees, bananas and a mystery solved


Often after a bee stings, others quickly appear to join the attack. How are the support troops attracted?

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Swiss entomologist François Huber was blind but that did not prevent him from studying honey bees and publishing his pioneering findings in 1792. He did need some help of course, and that came from his wife Marie and his faithful servant François Burnens who would become his “ eyes.”

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Huber was familiar with the scenario that often plays out after a bee sting and that pain is not the only problem the victim has to contend with. Other bees quickly appear with an attempt to join the attack. Huber knew that when a bee stings, its stinger gets imbedded in the skin and is torn from the bee’s body as it struggles to retract it. The outcome is the death of the gallant insect that has sacrificed its life to alert other bees that their home is in need of protection. But how are the support troops attracted, Huber wondered? How do they get the message that there was a potential threat to the colony? Was a clue to be found in the stingers? He instructed his assistant to excise stingers from bees and place them near a hive.

Sure enough, a swarm emerged and headed for the stingers. Did the excised stingers release some sort of odor that was sensed by the other bees? Huber could proceed no further since at the time, there was no way to determine what specific chemical may be responsible for raising the alarm. The identification of the alarm pheromone would have to wait until 1962, when researchers at Canada’s Department of Agriculture noted that the stingers left behind by bees had a sweet scent reminiscent of the odor of bananas! That was intriguing enough for a follow-up experiment. Stingers were extracted, macerated, and the resulting solution subjected to analysis by gas chromatography (GC).

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GC is an instrumental technique that separates a mixture of gases into individual components as they are being pushed by an inert gas through a column packed with a solid material to which the components bind to different extents. The time it takes for a compound to emerge from the column, known as the retention time, is specific for that compound and is recorded as a peak on a moving chart paper. Each peak represents a different compound, so that a gas chromatograph can determine the number of compounds in a mixture. The invention of this technique is generally credited to a 1952 report by British scientists AT James and AJP Martin, although German chemist Erika Cremer had actually described the possibility of constructing such an instrument in a paper submitted to the German journal Science of Nature in 1944. The paper was accepted but not published because the journal’s printing press was destroyed during a raid by Allied bombers. Cremer’s article was eventually published in 1976 as a historical item, but by that time credit had been given to James and Martin.

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When the Canadian researchers analyzed their chromatogram of the extract of the bee stingers, they found one major peak. Having already noted that the stingers released the scent of bananas, they suspected this peak to be due to isoamyl acetate, a compound that was known to be a major component of banana fragrance. Interestingly, isoamyl acetate had been described as having a banana smell before it had ever been detected in bananas!

By the mid-1800s, chemists had identified various families of molecules and learned to make use of chemical reactions to synthesize novel compounds. For example, when alcohols were reacted with carboxylic acids, they formed “esters” that often had fruity aromas and found application as artificial flavors for candies, beverages and ice cream. Specifically, reacting isoamyl alcohol with acetic acid produced isoamyl acetate with its potent banana fragrance. This imitation essence delighted the public at the New York City Crystal Palace exhibition in 1853, a hundred years before it was found to occur naturally in bananas!

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It wasn’t hard to verify that the chromatographic peak in the bee stinger extract was isoamyl acetate. All that was required was the introduction of an authentic sample of this compound into the instrument to determine its retention time. Indeed, it was identical with that of the suspected compound in the bee stinger. Next came confirmation by treating cotton balls with synthetic isoamyl acetate and placing them in front of a beehive. The bees were alerted and became agitated, leaving no doubt that isoamyl acetate was the alarm pheromone of the honey bee.

But how did isoamyl acetate actually triggered aggression in bees? That was answered by a group of French and Australian researchers who managed to untangle the neural mechanism involved. Bees were exposed to an air stream containing isoamyl acetate, were anaesthetized, and then quickly frozen in liquid nitrogen to prevent further biochemical activity. They were then dissected and their brain fluids subjected to gas chromatography with the results being compared to bees that had not been subjected to isoamyl acetate. The treated bees had higher levels of the neurotransmitter serotonin in their brain. Could this be the compound that prompts the aggressive behaviour?

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When bees’ thorax, from where drugs can be readily absorbed, was treated with serotonin, the bees became hostile, and when a serotonin antagonist was similarly applied, they calmed down. The implication is that isoamyl acetate is not the direct trigger that provokes belligerence but rather stimulates the upregulation of serotonin that in turn signals individual bees to attack and attempt to repel a threat. Kudos to the researchers. One suspects it must take a fair bit of manual dexterity to carry out an anatomical dissection of a bee brain.

Today, synthetic isoamyl acetate is commonly used to impart banana flavor to foods and has to be identified on labels as “artificial flavouring” even though it occurs naturally in bananas. In the future, we may have to rely on it even more given that the Cavendish banana, the one we find in our stores, is threatened by Panama disease, a fungal infection. A wildly popular song, “Yes! We have No Bananas,” released in 1923, may once again become trendy. It was said to have been inspired by Panama disease that at the time was wiping out the “Gros Michel” banana, destined to be replaced by the Cavendish.

In spite of bananas containing isoamyl acetate, there is no evidence that eating a banana around a beehive increases the risk of being stung. No need to bee-ware.

joe.schwarcz@mcgill.ca

Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 pm

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