Semiochemical substances

Organic, volatile compounds that elicit reactions in individuals through external action are generally referred to as “semiochemical substances”. They are secreted by exocrine glands and act outside the individual; they are unrelated to hormones, which are secreted by endocrine glands and act internally.

They constitute a form of chemical language that compensates for limited sound detection, low tactile sensitivity, and the deficient vision of many insects, which often cannot see beyond a few metres.

Semiochemical substances are divided into three types:

• Allomones, directed at another species but beneficial to the emitting species;
• Kairomones, directed at another species and beneficial to the receiving species;
• Pheromones, directed at the emitting species itself.

There may be informational duality: the same substance can function both as a pheromone and as a kairomone.

For example, the substance emitted by a bee larva to trigger cell capping is secreted to attract wax workers that will construct the operculum; it therefore acts as a pheromone. At the same time, it signals to varroa mites the imminent closure of the cell and the favourable moment for the parasite to enter. For varroa, the same substance functions as a kairomone.

Pheromones

Previously referred to as ectohormones and exohormones before the Second World War, and later as parahormones (1952), the term “pheromone” was proposed several times and officially adopted in 1960 by P. Karlson. It derives from the Greek “pherein” (“to carry”, “to transmit”). Pheromones are means of information conveyed by odours; they are volatile substances produced and released by one or more individuals that induce physiological reactions in one or more other individuals, even if they belong to a different species. Pheromones have been systematically studied for more than fifty years; elucidation of their molecular structure and synthesis became routine by the late 1960s, not only in honey bees but especially in numerous insect pests of crops (particularly in the Americas), with the aim of implementing integrated pest management. Integrated control includes the use of sexual pheromone dispensers designed to disrupt mating (sexual confusion) and thereby limit population growth.

There are several types of pheromones:

• Sex pheromones
• Aggregation pheromones
• Alarm and attack pheromones
• Trail-marking pheromones (aerial)
• Marking pheromones (ground)
• Additional types are anticipated based on ongoing studies.

Sex pheromones

Secreted by the mandibular glands and present in the queen substance, 9-keto-2-decenoic acid plays a role during the queen’s nuptial flight by attracting drones; its action occurs in conjunction with another reducing acid, 9-hydroxy-2-decenoic acid. Males also produce an aerial sex pheromone that attracts queens ready to mate to drone congregation areas. When a virgin queen approaches such an area, her pheromones trigger the pursuit leading to mating.

Aggregation pheromone

9-keto-2-decenoic acid also plays an important role in maintaining colony cohesion: it induces workers to feed, groom, and lick the queen. The queen substance thus licked is subsequently distributed to all workers, inhibiting their ovaries. Epidermal glands produce methyl-4-hydroxybenzoate, which appears to contribute to colony cohesion. The Nasonov gland is used to recall and guide swarms back and may also help queens locate the hive during mating flights; it releases a mixture of geraniol, citral, and geranic and nerolic acids. The Koschevnikov glands located near the queen’s sting may also contribute to maintaining cluster cohesion.

Alarm and attack pheromones

The alarm pheromone 2-heptanone (CH₃CO(CH₂)₄CH₃), secreted by the mandibular glands of workers, alerts the colony when an intruder approaches the hive or when a bee is attacked. Aggression may follow if abrupt movements occur nearby. This ketonic substance can be confused by bees with other ketones, such as acetone from certain nail polishes or benzophenone used as a fixative in many cosmetics, explaining the increased risk of stings in such cases. Alarm pheromones used by social insects elicit an immediate but short-lived alert response. The attack pheromone isoamyl acetate ((CH₃)₂CHCH₂CH₂OCOCH₃) is a volatile substance produced by cells lining the venom sac. After a sting, the detached sting continues to emit the attack signal. It is advisable to move away from the hive quickly, as the signal is powerful. Behavioural changes induced by the pheromone can be amplified by abrupt movements, body odours, or sounds near the hive. Recently, another pheromone, 4-11-eicosen-1-ol, emitted by the stinging apparatus, has been identified and is also considered an alarm pheromone.

Trail-marking pheromones

The Nasonov gland, located beneath the posterior part of the sixth tergite (the penultimate one), releases several constituents simultaneously: geraniol and citral, as well as geranic and nerolic acids. This mixture serves to mark the route to a food source. Bees lay an aerial trail of volatile substances from the food source to the hive. Foragers can intercept this trail by zigzag flights, detect it, and follow it back to the source. It is sometimes stated that bees use it to mark the foraging site on the ground, but the position of the gland does not allow ground marking.

Marking pheromone

At the extremities of the legs, the Arnhart gland allows ground marking of trails at the hive entrance. Queens and drones also possess this gland, which emits the trail-marking pheromone known as Epagine ETA. The mandibular glands of workers secrete a pheromone similar to that of the queen, 10-hydroxy-2-decenoic acid; however, when workers become laying workers, their mandibles emit the same pheromone as the queen, 9-keto-decenoic acid. According to some researchers, the bee’s head is the source of around thirty pheromones, of which only a few have been isolated. In Apis mellifera, eleven different glands are recognised (mainly located in the head and abdomen), and only part of their functions are currently known.

Bee foraging is linked to a pheromone: ethyl oleate

This pheromone regulates the foraging behaviour of young bees. It is emitted by foragers and inhibits the transition of young bees into foragers.

Therefore:

• During strong nectar flows (and good weather), foragers are out in the field; young bees remaining in the hive are not exposed to ethyl oleate and thus develop more rapidly into foragers. This corroborates the observation that a colony can mobilise its workforce to exploit a good nectar flow.
• In poor weather, foragers are confined to the hive and release ethyl oleate to young bees; these remain longer in the nurse stage. This likely contributes to swarming (more royal jelly present in the hive) and supports the observation that bad weather increases swarming tendency.

Emission and reception of a pheromone

Emission and reception of a pheromone may occur via three different pathways:

• aerial diffusion;
• persistence on the ground or direct contact between individuals;
• exchange of food.

Without it being possible to determine with certainty which pathway is actually used in a given case.

How are pheromones perceived?

The bee’s sense of smell is located primarily on the antennae, which bear a very large number of sensory organs. Olfactory sensilla are found mainly on the eight distal segments of the twelve-segmented antenna.