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The mysteries of smoke and bees

Smoke has long been the beekeeper's secret weapon to help to avoid getting stung. Ancient Egyptian art dating back more than 2,500 years ago depicts beekeepers blowing smoke into hives. Despite this practice's age and human's enduring fascination with honey bees, we still haven't figured out exactly why smoke soothes bees.

In research published in August of 2018 in the Journal of Insect Science, Stephanie Gage, Ph.D., with her colleagues at the U.S. Department of Agriculture's Carl Hayden Bee Research Center and BetaTec Hop Products, presented a scientific evaluation of smoke on the honey bee's defensive behavior.

These researchers focused on the "sting extension response." They evaluated the effects of two different types of smoke: burlap, which is commonly used by beekeepers and spent hop pellets—a recycled material made from hop flowers after they have been used to make beer.

Because a honey bee hive contains honey and protein-packed larvae—bees must mount a coordinated defense to protect the hive from predators. A small number of worker bees serve as "guard bees" that patrol the entrance and watch for potential threats to the hive. If a threat is detected, the guard will raise her abdomen and extend her stinger into the air. This behavior is called the sting extension response. The movement releases an alarm pheromone. This chemical signal goes out to the rest of the colony, mobilizing other workers to prepare to attack an intruder. If the intruder provokes the bees further, stinging commences.

To provoke the bees in the experimental setting, Gage and colleagues used electrical shocks. "We were looking for a repeatable way to 'pester' an individual bee that would be consistent among bees," she says. in the previous experiments, electrical shocks have been used in learning experiments with bees and can be precisely controlled. In this experiment, the shocks were relatively mild, and bees were released unharmed, though most likely annoyed.

The researchers chilled the bees to immobilize them; then, each bee was secured to a brass plate that could be used to deliver shocks. Using tiny Velcro straps, they positioned the bees on their backs so that they could move their abdomens freely and demonstrate a sting extension response.

The bee on the brass plate was placed in a chamber and exposed to burlap smoke, hops smoke, or no smoke; then, a single electric shock (ranging from very mild to slightly more intense) was applied, and behavior was observed.

To the researchers' surprise, the smoke had no significant effects on the sting extension response or the movements leading to it (abdomen curling and tergite separation). Still, it did have an unexpected impact on another behavior—venom droplet release. In response to stronger shocks, some of the bees extended their stingers and released a droplet of venom from the stinger tip.

"The release of the venom droplet took us by surprise. We didn't start out looking to record it. It became clear upon our first day of testing that some sting extensions released a venom droplet. In contrast, others did not," says Gage. The researchers write that the venom droplet "was more likely to be released with greater perturbation, and the probability of its release was reduced with smoke." When the most substantial shocks were applied (8 volts), only hop smoke reduced the likelihood that bees released venom.

Venom droplet release has been described before, but there has been little discussion of this behavior in the scientific literature. Gage and coauthors hypothesize that venom release escalates the bee's defense response, above and beyond the sting extension. It may serve to amplify the alarm pheromone signal. The venom contains small amounts of the alarm pheromone, and the droplet itself might help to increase the surface area, releasing the pheromone during the sting extension.

If smoke reduces the probability of venom release, it should reduce the colony's overall defensiveness and the likelihood that a beekeeper gets stung. The researchers suggest that hop smoke's ability to minimize venom droplet release in even the most perturbed bees may be due to the presence of lupulin in hops. Lupulin is known to have sedative effects on the nervous system so that it may calm bees.

Gage and colleagues caution that, outside of the lab, honey bees rarely receive electric shocks. The researchers would like to evaluate smoke effects in the field, where bees are exposed to more natural threats and receive social cues from hive members. But that study will take considerable ingenuity. "We are still imagining the parameters of that experiment, but we will get there," says Gage.

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