Queen pheromones

Communication in bees is highly elaborate and has been the subject of numerous studies. There is, of course, the well-known “waggle dance” or “figure-eight dance,” but what is less well known is that this dance serves solely to indicate the location of a food source. While dancing, the bee releases chemical messengers that recruit other foragers and inform them about the type and richness of the source. Communication therefore indeed relies on the exchange of chemical substances known as pheromones.

In addition to the so-called “recruitment pheromone,” there are many others that enable colony cohesion, individual recognition, task allocation, the dissemination of alarms, swarming signals, and the location of food, water and propolis sources, as well as potential nesting sites for swarms. These are chemical substances with highly complex formulas, emitted and perceived by every individual in a colony: the queen, workers, drones, and even brood larvae (BEP: Brood Ester Pheromone). These pheromonal molecules that shape behaviour within the colony can be divided into two categories: releaser pheromones and primer pheromones. Each of these categories can in turn be subdivided into stimulating and inhibiting groups. Releaser pheromones act mainly on behaviour with a rapid effect. They are the best known, and more than a hundred have already been identified. Primer pheromones act more on the physiology of the bee and have longer-term effects. Their main function is to coordinate the physiological and behavioural development of groups of individuals (nurse bees, foragers, cleaners, guards, etc.). According to researchers, the bee’s head alone is the source of around thirty pheromones, only a few of which have been isolated to date. In Apis mellifera, about ten different glands are known, all capable of emitting numerous pheromones and located mainly in the head and abdomen. At present, only a small proportion of their functions and a very limited number of pheromones are known. Pheromone transmission between bees occurs through cuticle-to-cuticle contact, cuticle-to-antenna contact, pollen-to-antenna contact, ingestion, and through the handling of pollen and nectar. Airborne pheromones are perceived via the olfactory sense located in the antennae, which are known to carry numerous sensory organs.

Queen pheromones: Although it has now been scientifically demonstrated that royal pheromones are not the only ones that guide and influence the social life of a colony, they remain the most important, as they play a crucial role in maintaining colony cohesion. These are mainly the queen mandibular pheromones (QMP), composed of several chemical molecules including 9-hydroxy-(E)-2-decenoic acid (9-HDA), methyl p-hydroxybenzoate (HOB), 4-hydroxy-3-methoxyphenylethanol (HVA), and 9-keto-2-decenoic acid (9-ODA). The artificial mixture of these synthesised molecules is well known under the name “Bee Boost®”. The queen’s mandibular and tarsal pheromones have inhibiting effects by blocking ovarian development in workers, preventing the rearing of new queens and delaying drone rearing, as well as stimulating effects by promoting brood care, comb building and foraging behaviours. When, due to a weakened, old or deficient queen, these inhibiting pheromones no longer reach all bees in the hive, this leads to the construction of queen cells (supersedure). The complete absence of these royal substances (loss or death of the queen) triggers the reflex to rear new queens (emergency cells) to ensure colony survival. If queen rearing fails and the royal inhibitory substance remains absent for a prolonged period, some workers develop their ovaries and begin to lay eggs; the colony then becomes drone-laying. These laying workers emit pheromones similar to those of the queen, but their unfertilised eggs give rise only to male bees.

Sex pheromones: Secreted by the queen’s mandibular glands (QMP), 9-keto-2-decenoic acid plays a role during the queen’s nuptial flight by attracting drones. Males also produce an airborne sex pheromone that attracts queens ready to mate toward drone congregation areas. When a virgin queen approaches such an area, another queen pheromone, 9-hydroxy-2-decenoic acid, triggers the chase that leads to mating. The queen emits this pheromone until her spermatheca is fully filled.

Social cohesion pheromones: 9-keto-2-decenoic acid (QMP) is, as noted, a sex hormone, but it also fulfils another essential role by ensuring colony cohesion and simultaneously directing workers to feed, groom, lick and protect the queen. This royal pheromone, taken up by workers, is then distributed throughout the colony, inhibiting ovarian development in workers. The bees’ epidermal glands also produce methyl-4-hydroxybenzoate, a pheromone that likewise appears to benefit colony cohesion.

Aggregation pheromones: The best-known pheromone among beekeepers is that emitted by the Nasonov gland. It contains, among other compounds, 3,7-dimethyl-2,6-octadienal, also known as citral, which gives it its characteristic lemongrass scent. It is used to attract and gather the bees of a swarm when they “call back”. It may also help the queen locate the hive upon returning from mating flights. It is a mixture of geraniol, citral, geranic acid and nerolic acid. The Koschevnikoff glands located near the queen’s sting also contribute to the cohesion of the swarm cluster. This airborne pheromone can be detected by bees over several hundred metres, but only those whose swarming drive is at its peak respond to it.

Alarm pheromones: 2-Heptanone is the principal of four pheromones secreted by the mandibular glands of workers that alert the colony when an intruder approaches the hive or when a bee is attacked. It is also released in response to abrupt movements, strong vibrations and loud sounds near the hive. This ketonic substance may be confused by bees with other ketones found in certain nail varnishes or with benzophenone, used as a fixative in almost all perfumes, which explains the increased risk of stings for people wearing nail polish, perfumes, deodorants or other cosmetics.

Attack pheromones: Isoamyl acetate is produced by a gland bordering the venom sac. When a bee stings, this gland remains attached to the exposed sting and continues to emit the attack signal. The pheromone released, which smells like banana, increases aggressiveness in other bees and incites them to sting. More recently, another attack pheromone, 4-11-eicosen-1-ol, emitted by the stinging apparatus, has also been identified.

Marking pheromones: At the ends of the legs lies the Arnhart gland, which allows bees to mark the landing board and hive entrance. Like workers, queens and drones also possess this gland, which emits a pheromone called epagin. Foragers leave traces of it on visited flowers, which are detected by other foragers and indicate that the flower has just been visited and no nectar remains.

Ethyl oleate (EO): Researchers have shown that this pheromone, produced by foragers (the oldest bees), plays a key role in the maturation and transformation of younger bees: it acts as a chemical inhibitor that delays the onset of foraging. The start of foraging in younger bees depends on this pheromone and represents one of the key mechanisms of self-organisation in response to colony needs. The ethyl oleate emitted by older foragers inhibits the transition of young bees into foragers as follows:

• During a strong nectar flow and fine weather, foragers are outside the hive at work, and young bees remaining in the hive are not exposed to ethyl oleate. They therefore transition more rapidly into foragers, as the colony mobilises its workforce to exploit the nectar flow. This results in a shortage of nurse bees in the hive and encourages the queen to increase egg laying.
• In bad weather, by contrast, foragers are confined within the hive and diffuse ethyl oleate to young bees, which then remain longer in the nurse stage. A large population becomes clustered inside the hive, with a very high proportion of young nurses. This imbalance among bee castes frequently triggers swarming, and very often, as soon as good weather returns, swarming occurs.

As with the pheromones discussed above, there are dozens of others, all of which play a fundamental role and act or interact in a very precise manner on each actor within a honey-bee colony. Scientists estimate that several hundred remain to be discovered, illustrating just how complex the bees’ communication system truly is.

Reference: Chemical communication and social regulation in the honey-bee colony (Apis mellifera L.) – Alban Maisonnasse, M. Feltin, 2017