Smoke as a Trigger of Calming Behaviour
Beekeepers smoke their hives because “it calms the bees down” and makes inspections easier. Smoke works by masking alarm pheromones and providing a mild anaesthetic effect on the worker bees (amongst other things, depending on your fuel source). This reduces colony aggression and makes for smoother frame management. When done properly (fumes not too hot!), the practice is beneficial for both bees and beekeepers. After all, a bee dies after stinging you- leaving the colony with one less forager, and leaving you with one more irritant.
How does pheromone masking work? Does smoke trigger an evolutionary response that causes honey bees to gorge themselves on honey in preparation for evacuation? Or does it dull their antennal receptors, leading to reduced perception? Does the type of smoke (wood, grass, shells) matter?
Reduced Aggression: Alarm Pheromone Masking
When smoked bees are exposed to alarm pheromone components, they showed little defensive responses (Visscher et al., 1995). Under normal conditions, the alarm pheromone causes older bees to extend their stinger and perhaps attack the target source. Rather than responding to that stimulus, the smoked bees continued gorging themselves on honey or fleeing deeper into the hive.
The authors suggest three possibilities:
- Compounds in the burnt material bind to the bees’ antennae receptors, preventing it from registering the alarm pheromones,
- Smoke has a blanket interference with the bees’ sense of smell (also on the antenna),
- The smoke particles, “soot”, physically sticks to the bees’ antennae, preventing it from registering new odours.
Bees are frequent cleaners- they clean themselves and one another (autogrooming & allogrooming respectively), so the effects of smoke do not linger. Indeed, after ~15 minutes, the bees their alarm pheromone responsiveness returned to normal.
Bees Stay Engorged Two Hours After Smoking
Despite bees recovering their perceptions within 15 minutes, the tendency to gorge themselves on honey lingers for up to two hours (Free, 1968). I assume that this adaptation is in response to the bee’s cautionary approach of “the fire danger may have not passed”.
Free’s paper notes that bees of all ages gorged themselves on honey, with the middle-aged bees (ages 12-21 days) holding the largest honey crop content. This is observation aligns with the bees’ task allocation, for they are the primary wax producers and comb builders inside the hive.
As beekeepers know, the engorged bees are less likely to sting as they are unable to bend their abdomen into the stinging position. Free offers the advice that “a beekeeper is least likely to be stung about ten minutes after a colony is smoked” (Free, 1968). I don’t agree with waiting that long after a colony is smoked, but if you’ve got the time, you’re more than welcome to try it.
Bees Released Less Venom from their Sting After Smoking
If the bee’s stomach was not filled with honey, she can still attempt to sting you after smoking, however her stinger would contain less venom (Gage et al., 2018). The authors tested four variables involved in stinging: abdomen curling, tergite separation (to make way for the lancet), sting extension and venom droplet release. They found the first three factors to be largely unaffected between smoked and unsmoked colonies1. However, the amount of venom released by the stinger was reduced in smoked bees.
In nature, the rate of venom release correlates to the level of agitation. In Gage and colleague’s experiment, electric volts were used to induce a response from the bee, with the higher voltages (and hence more agitation) producing more venom. Their finding suggests that the volume of venom is an indication of the individual bee’s defensiveness, rather than the entire colony.
We know that when bees unsheathe the stinger, they release alarm pheromones to warn their nestmates of the potential danger, so the dulling of the venom release suggests that smoke interferes with the defensive responses of the bees and making them docile.
Composition of the Bee Sting: More Than Venom
Bee venom is 88% water (reviewed in Dotimas & Hider, 1987). At any given time, a worker bee holds 3-4 microlitres of venom in her sac. Alarm pheromones like isopentyl acetate are present, as well as other volatile compounds that aid with its dispersion. Previous studies have shown that venom extracts produced a mixed response to bee aggression; this varied depending on whether the venom sac, venom gland, or pure venom was tested (e.g. Cassier et al., 1994). Pure venom extract triggered some responses from guard bees, but this wasn’t as potent as gland and sac samples.
This suggests that venom is one component released by the worker in anticipation of stinging. The cocktail is completed by the alarm pheromone compounds, which signals defensiveness by the hive, and all are repressed under the presence of smoke.
Does this Mean Stings from Smoked bees will not Recruit Nestmates?
When the bees are smoked, they no longer perceive olfactory communications, but will still attempt to sting you if they’re physically threatened (whether by electric shocks in the lab or your hive tool). However, should the smoked bee defend herself, does this mean her sting will not recruit other nestmates to attack you, owing to the lower concentrations of venom and/or alarm pheromones?
My own observations support this, having been stung from smoked and unsmoked hives. In a smoked hive, the stings are haphazard and far apart – maybe one sting every few minutes, if at all. In the unsmoked hives, they follow in tandem – one sting led to a few more several seconds later… had I stayed longer, they would have been dozens more!
Bear in mind this is an N=1 observation, and the bees were inspected in the late summer, where the quantity of workers is at its highest. Perhaps you’ve had a different experience?
Does the Source of Smoke Matter?
For a backyard beekeeper, I don’t think the type of smoke matters. Not all combustible material is created equal. For example, pine needles smoulder instead of burn, producing a thicker, cooler smoke. If you don’t have access to pine needles– cane sugar mulch from the gardening store is a good substitute. Hot smoke would singe the wings and other membranous surfaces on the bees, damaging them in the long term. Also, too-hot or too-much smoking may have the opposite effect and make bees more aggressive.
That being said, different plants produce different compounds when burnt, and this can influence the bees’ response. For example, Gage and colleagues compared hessian (burlap) fibers against spent hop pellets (i.e., the remnant of the hop plants that were used in brewing), and found both plant types to reduce venom droplet release (Gage et al., 2018).
However, hops reduced venom at a greater rate than burlap at the highest level of electric shocks (“agitation”). The authors assumed that this was due to the plant itself, as the lupulin glands in the hop have a variety of secondary metabolites that are used to “reduce the excitability of the striated muscles and motor nerve endings, diminish irritability of the nervous system, and induce narcosis” (Gage et al., 2018 citing Van Cleemput et al. 2009).
Before you start comparing different types of plants, bear in mind that it’s difficult to control the rate and thickness of smoke release in a smoker – maybe the effects the authors observed were due to the density of smoke exposure or heat of the fumes, rather than the plant composition itself.
Fuel alternatives: Mushrooms, Hairs & Feathers
In areas where dry plant material is not readily available (e.g. the tropics) non-plant smoker fuel alternatives were investigated by Wood for their calming properties (Wood, 1982). He draws on previous accounts of European beekeeper that used fungi (giant puffball mushroom, Lycoperdon giganteum), and native North Americans to relax bees prior to inspections.
Wood’s studies were conducted in Africa, where he noted that burnt African puffball mushrooms smelt like “burning human hair or chicken feathers”2, and proceeded to test the latter two compounds for their calming effects on bees. He found these compounds also calmed colonies and did not impact their lifespan relative to unsmoked bees.
The proposed mechanism behind these effects is attributed to the protein keratin and its sulfur-containing amino acids (e.g. cysteine). The formation of hydrogen sulfide during combustion produces a narcotic effect on bees; another gas, hydrogen cyanide, is also given credit. Before you scream “danger!”, remember:
- The dose makes the poison: lower dose, lower (chance of) poison.
- Wood tested the lifespan of smoked bees compared to control and found no difference. I’ve pasted his results below.
I wonder which chemicals are responsible for sedating bees in my smoker fuel of dried lucerne…
Are there Age & Caste-Specific Responses to Smoke?
I couldn’t find much in the literature that described whether the age of bee dictates her smoke response, so I’m going to hypothesize: Yes, there is an age-specific response to the beekeeper’s smoker.
- The newly-emerged worker bees have not completed development, and their cuticle is still soft – the heat from smoke, or the smoke particles themselves, may adhere to their cuticle and have minor impacts on their development.
- However, this is too insignificant to impact the colony – oversmoking or fire will give you bigger problems than poorly-developed bees.
- Nurse bees would have a generic response of going deeper inside the hives.
- Could this be an adaptive response to shield brood from the heat with their bodies? Questions for Plato.
- Middle aged bees would be the ones gorging themselves on honey. They are the comb builders, and by filling their honey stomach, they are preparing to evacuate and start rebuilding at a new location.
- Foragers would have a similar response to the middle-aged bees, going deeper within frames and consuming honey.
- I have noticed foragers becoming disoriented and releasing an irritated buzz in times of excess smoke exposure. Counterintuitively, they will also attack the beekeeper – perhaps as a desperate last stand against the irritant… how they discern between smoke from a smoker and smoke from a forest fire is beyond me.
Smoking drones and the queen leads to irritated bees. Unlike workers, they do not have a role to play in hive defence. Drones will fly off the frames, whereas the queen will become agitated and run faster. Repeated exposure to this stressor will impact her health and egg laying, perhaps even making the colony more aggressive on account of increased defensiveness. Fortunately, you’re not deliberately smoking your queen, right?
If you’re wondering “I am not the gentlest beekeeper, how come my hive continues to function normally for years on end?” the answer is because they’re a superorganism – a strong hive may have tens of thousands of bees, and the wounding of a few hundred is not a big deal. Worker bees live for 42 days in the spring and summer, and hundreds emerge from their cell daily. At the colony-level, smoking has no impact, and if anything, should be encouraged as it makes beekeeping more manageable.
No Impacts on Bee Health?
The table above from Wood showed no impact of smoke on the honey bee’s lifespan. Perhaps prolonged smoker use will impact the bee’s healthspan instead of her lifespan? Consider that if your smoke is too hot (to the point where flames are puffed out of the smoker), you can singe her wings and incapacitating her. This singed worker will not be able to fly or forage effectively, slightly burdening the colony3. Remember, gradually place unburnt material atop the burning one to achieve cooler, thicker smoke.
Stingless Bees are Unaffected by Smoke from Multiple Sources.
In the stingless tibua bees of Brazil Melipona compressipes, smoke during inspections did not impact their brood or forager activity (Martins et al., 2020). Smoke isn’t often used in stingless beekeeping, because as the name implies, they don’t sting (instead they bite, which I’m told isn’t that painful…).
Unsurprisingly, the authors found more “aggressive” bees in unsmoked hives. All tested materials (100% sawdust, 50:50 sawdust: eucalyptus leaves & sawdust + dry bark of Amburana cearensis [a native timber tree]) showed no impact on bee health or brood populations, with the smoke from 100% sawdust being most effective in reducing the number of bees adhering to the beekeeper’s gloves and headbands (the “aggression”). This suggests that smoking stingless beehives is safe for use during colony inspections .
Smoker Use and Beekeeper Health
If you’re a beekeeper worried about the health effects from smoker exposure, be not afraid. When the lung function of 34 beekeepers (with an average age of 51 years) was compared against non-beekeepers, they found no difference in respiratory symptoms like coughing, phlegm production, breathing difficulties and pulmonary function tests (Polati et al., 2001).
Despite an average 18 years of beekeeping, the (non-cigarette smoking) beekeeping group had better health outcomes than the control. The authors suggest that it is due to the nature of beekeeping, and duration of exposure – we’re outdoors and the lifestyle is more active: carrying boxes, extracting frames. I would also add that the occasional venom from the bee sting is good for the body, and I think it’s why earlier studies have shown that U.S. beekeepers had a lower cancer mortality compared to the average population (McDonald et al., 1979).
My dear reader smoking will calm your bees and make inspections a whole less stressful. After all, beekeeping is one of the few activities where smoking is good for you.
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- There was more abdominal curling at low concentrations, probably due to the “noxious effects” of smoke when it’s first puffed… you know, that faint chemical odour that first appears before the plant-like smoke smell. Regardless, this an example of a statistically significant outcome. “Statistically significant” is not the same as “real-world significant”… Many such cases. ↩︎
- Other natural products have been tested and showed similar calming effects, dried avocado leaves, coffee husks and castor beans (Lomele et al., 2010). My takeaway: you can use your garden/kitchen waste as smoker fuel instead of sending them to the compost. ↩︎
- I wager that if a nurse or middle-aged bee’s wings were impacted, they would not graduate into foragers, and spend the rest of their lives as guards or nectar receivers ↩︎