The Right Chemistry: Honey Bees, Bananas, and a Solved Mystery

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

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Swiss entomologist François Huber was blind, but that didn’t stop him from studying honey bees and publishing his groundbreaking discoveries in 1792. He needed help, of course, and it was provided by 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 unfolds after a bee sting and that pain is not the only problem the victim has to deal with. Other bees quickly appear with the intention of joining the attack. Huber knew that when a bee stings, its stinger embeds itself in the skin and separates from the bee’s body as he struggles to retract it. The result is the death of the brave insect that has sacrificed its life to alert other bees that their home needs protection. But how do you attract support troops? Huber wondered. How do they get the message that there was a potential threat to the colony? Was a clue found in the stingers? He instructed his assistant to remove the bees’ stingers and place them near a hive.

Sure enough, a swarm emerged and headed for the stingers. Did the removed stingers release some kind of odor that was picked up by the other bees? Huber was unable to continue as there was no way at the time to determine which specific chemical might be responsible for raising the alarm. Identification of the alarm pheromone would have to wait until 1962, when researchers at the Canadian Department of Agriculture noted that the stingers left behind by bees had a sweet odor reminiscent of the smell of bananas. That was intriguing enough for a follow-up experiment. The stingers were removed, macerated, and the resulting solution was subjected to gas chromatography (GC) analysis.

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GC is an instrumental technique that separates a mixture of gases into individual components as they are pushed by an inert gas through a column filled with a solid material to which the components are bound to varying degrees. The time it takes for a compound to emerge from the column, known as the retention time, is specific to that compound and is recorded as a peak on a moving graph paper. Each peak represents a different compound, so a gas chromatograph can determine the number of compounds in a mixture. The invention of this technique is generally attributed to a 1952 report by British scientists AT James and AJP Martin, although German chemist Erika Cremer described the possibility of building such an instrument in a paper submitted to the German journal Science of Nature. in 1944. The article was accepted but not published because the magazine’s printing press was destroyed during an Allied bomber attack. Cremer’s article was finally published in 1976 as a landmark article, but by this time she had been given credit to James and Martin.

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When the Canadian researchers analyzed their chromatogram of the bee sting extract, they found a significant spike. Having noticed that the stingers released the aroma of bananas, they suspected that this spike was due to isoamyl acetate, a compound known to be a major component of banana fragrance. Interestingly, isoamyl acetate had been reported to smell like bananas before it was detected in bananas.

In the mid-19th century, chemists identified several families of molecules and learned how to use chemical reactions to synthesize new 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 candy, beverages, and ice cream. Specifically, the reaction of isoamyl alcohol with acetic acid produced isoamyl acetate with its strong banana fragrance. This imitation scent delighted audiences at New York City’s Crystal Palace Exposition in 1853, a hundred years before it was discovered to occur naturally in bananas!

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It was not difficult to verify that the chromatographic peak in the bee sting 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. In fact, it was identical to the suspected compound in the bee’s stinger. Then came confirmation by treating cotton balls with synthetic isoamyl acetate and placing them in front of a hive. The bees became alert and agitated, making it clear that isoamyl acetate was the honey bee’s alarm pheromone.

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

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When the bees’ thorax, from where drugs can be easily 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 causes belligerence, but instead stimulates serotonin upregulation which, in turn, signals individual bees to attack and try to repel a threat. Congratulations to the researchers. One suspects that it must take a bit of manual dexterity to carry out an anatomical dissection of a bee’s brain.

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

Even though bananas contain isoamyl acetate, there is no evidence that eating a banana near a hive increases the risk of being stung. There is no need for bees.

[email protected]

Joe Schwarcz is director of the Office of Science and Society at McGill University (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3-4 p.m.

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