Recruitment pheromones

Recruitment can be defined as communication that directs members of a society to some point in space to perform certain tasks. This altruistic type of communication can be explained by kinship theories that predict a close relationship between the recruiter and the recruited individual. In fact, the distribution of recruitment pher-omones is limited to social organisms such as ants, bees, termites, naked mole rats, and social lepidopteran larvae. In these species, recruitment pheromones assemble nestor groupmates for joint efforts in food retrieval, nest construction, nest defense, or colony migration. Due to this multitude of tasks that is mediated by recruitment pheromones, this category cannot be clearly separated from others such as alarm or aggregation pheromones.

In general, the following four different recruitment mechanisms are discriminated.

The first type of chemical recruitment is called 'tandem running'. Here, the returning scout leads a single recruit to a food source or a new nest by means of a pheromone on its body and tactile contact to the recruit. This form of chemical recruitment is generally regarded as primitive and can be found in ants (e.g., Leptothorax sp. or Camponotus sericeus), where cuticular surface phero-mones or pygidial gland secretions evoke path of the leader. More elaborate is the recruitment via odor trails that can guide smaller groups (5-30 individuals, that is, group recruitment) or scores of individuals (i.e., mass recruitment) to the destined site.

Group recruitment is used, for instance, by the ant species Camponotus socius which places chemical signposts around newly discovered food items and a trail from the food source to the nest. Also, eusocial naked mole rats (Heterocephalus glaber) lay foraging trails back to the nest after having discovered a new food source. While tandem running and group recruitment requires a leader that guides the recruited nestmates, mass recruitment is self-organized (i.e., very complex patterns result from surprisingly simple behaviors performed by individuals relying on only local information).

Mass recruitment systems per definition can be induced and regulated by volatile chemicals that orient the recruits to the target site (e.g., food). The number of activated nestmates depends on the amount of recruitment pheromone deposited, which itself is a function of the attractiveness of the food source or the demand for working force. Mass recruitment can be found in many species of ants (e.g., Solenopsis invicta (Figure 8), Atta cephalotes), termites (e.g., Nasutitermes comiger), or tent

Nest Food

Figure 9 Bee workers release the so-called Nasonov pheromone that guides other workers on an airborne chemical trail. The pheromone is used to orient returning foragers back to the nest or to indicate an entrance to their hive, which was lost by a calamity. By raising their abdomens and fanning their wings vigorously, bee workers disperse the Nasonov pheromone, which consists of a number of different terpenoids including (E,E)-farnesol, geraniol, nerolic acid, citral, and geranic acid. Photograph by Joachim Eberhardt.

Figure 9 Bee workers release the so-called Nasonov pheromone that guides other workers on an airborne chemical trail. The pheromone is used to orient returning foragers back to the nest or to indicate an entrance to their hive, which was lost by a calamity. By raising their abdomens and fanning their wings vigorously, bee workers disperse the Nasonov pheromone, which consists of a number of different terpenoids including (E,E)-farnesol, geraniol, nerolic acid, citral, and geranic acid. Photograph by Joachim Eberhardt.

Figure 8 Workers of the fire ant (Solenopsis invicta) that return from a profitable food source deposit a volatile trail pheromone that forms a semi-ellipsoidal active space. Under laboratory conditions, this trail lasts only about 2 min. Photograph: Michael A. Seymour, with permission. Reprinted from Animal Behaviour, Vol. 10, Wilson EO, Chemical communication among workers of the fire ant Solenopsis, pp. 123-147, Copyright (1962), with permission from Elsevier.

caterpillars (e.g., Malacosoma americanum). Even though at the species level trail pheromones do not differ chemically as much as sex pheromones, they tend to be species specific and are usually peculiar to genera or to species groups within genera. The longevity of an odor trail depends strongly on the circumstances. The permanence of foraging trails, for example, is affected by the profitability of a given food source, which in turn is reflected by the fade-out time of the pheromones used for trail marking. Solenopsis fire ants whose insects prey are rapidly depleted lay ephemeral trails that may fade within minutes (Figure 8), whereas termites (Schedorhinotermes lamanianus), for instance, that feed on longer-lasting food sources such as dead vegetation use persistent and nonvolatile pheromones.

Odor trails can either be laid continuously, thereby allowing a highly accurate chemo-orientation of the recruit, or be deposited with large gaps in between, which may function as odor beacons. Such discontinuous trails with spots every 2-3 m are laid, for example, by stingless bees of the genus Trigona that return from a rich food source. This kind of recruitment system is believed to facilitate a three-dimensional chemo-orientation of the succeeding workers in the understorey of tropical forests, which is the preferred habitat of these bees.

Terrestrial recruitment pheromones are laid on a solid substrate. Pheromone plumes, however, can also be released in air or water where they depend on the movement of the medium to generate the trail. Aerial trails are known from a number of flying insects from the order of Orthoptera, Homoptera, Coleoptera, Mecoptera, Lepidoptera, Diptera, and Hymenoptera, where they often function as sex attractant or aggregation pheromone. An example of an aerial trail that functionally resembles terrestrial trails as described above is the Nasonov pher-omone released by honeybees (Figure 9). These airborne trails consist of (E,E)-farnesol plus six other monoterpenes and are typically released when a new nest entrance which was lost by a calamity is discovered, at low odorant food sources (e.g., sugar syrup or water), or if workers detect a queen that was separated from a swarm in flight.

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