A new study that explores the effect of smoke on honey bee (Apis mellifera) behavior finds that smoke reduces the instance of bees releasing a venom droplet in their signaling of danger to other bees, which researchers speculate may thereby reduce the amount of alarm pheromone released. (Photo credit: Wikimedia)

By Meredith Swett Walker

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

Meredith Swett Walker

In research published in August in the Journal of Insect Science, Stephanie Gage, Ph.D., with colleagues at the U.S. Department of Agriculture’s Carl Hayden Bee Research Center and at BetaTec Hop Products, presents a scientific evaluation of smoke on the honey bee’s defensive behavior. The researchers focused on the “sting extension response” and 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 (Apis mellifera) hive contains valuable treasure—sweet honey and protein packed larvae—bees must mount a coordinated defense to protect the hive from the many predators that would love to plunder it. A small number of worker bees serve as “guard bees” that patrol the entrance to the hive and watch for intruders. 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, and it releases an alarm pheromone, or a chemical signal, 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 electric shocks. “We were looking for a repeatable way to ‘pester’ an individual bee that would be consistent among bees,” she says. Electric shocks have been used in learning experiments with bees and can be precisely controlled. The shocks used in this experiment were relatively mild, and bees were released unharmed (though probably annoyed).

The researchers chilled the bees to immobilize them; then, using tiny Velcro straps, each bee was secured to a brass plate that could be used to deliver shocks. 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 stronger) was applied, and behavior was observed.

Surprisingly, smoke had no major effects on the sting extension response or the movements leading up to it (abdomen curling and tergite separation), but it did have an unexpected effect on another behavior—venom droplet release. In response to stronger shocks, some of the bees not only extended their stingers but also 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, while 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.” In addition, when the strongest 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 is an escalation of the bee’s defense response, above and beyond the sting extension, and that it may serve to amplify the alarm pheromone signal. The venom contains small amounts of the alarm pheromone, and the droplet itself might serve to increase the surface area releasing the pheromone during the sting extension.

If smoke reduces the probability of venom release, it should reduce the overall defensiveness of the colony and the likelihood that a beekeeper gets stung. The researchers suggest that the ability of hop smoke to reduce 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 it may calm bees, as well.

Gage and colleagues caution that, outside of the lab, honey bees rarely receive electric shocks. The researchers would like to do an evaluation of smoke effects in the field, where bees are exposed to more natural threats and also 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.

Meredith Swett Walker is a former avian endocrinologist and wannabe entomologist. She now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. Find a sampling of her work at www.magpiescicomm.com.