Functions and Effects

Apart from their benefits to origin-related or receiving organisms, infochemicals are also often divided by their function in the interactions between organisms. With regard to the category of pheromones, these infochemicals in most cases function by influencing other members of the same species, not the individual that produced them. Because the functions of infochemicals in general are not mutually exclusive but depend on the ecological context, various, partly overlapping, functional classes of pheromones can be found in literature (see e. g. [Ago92, JM93, RL68, Sho76,

Wya03]). A non-exhaustive list of functional pheromone classes includes:

• Sex pheromones: The most thoroughly documented cases of long-range chemical communication are those of sex substances used in signaling a mating partner. For example, female moths release chemical stimuli into the air to signal their availability, and thereby attract males over long distances.

• Alarm pheromones: Used primarily by social animals to warn other mem- bers in the colony of impending danger or any threatening situation. The behavior of most animals upon reception of an alarm signal is basically the same. They initially orient osmotactically to the source at low pheromone concentration and at high concentration go into frenzied activity, occasionally attacking the pheromone source.

• Aggregation pheromones: Used for causing other members of the same species to aggregate in a particular area, e. g. a food source or a suitable habi- tat. The aggregations may be dense, e. g. thousands of bark beetles arriving at a designated host tree, or not dense, e. g. a single male arriving at the vicinity of a female that is signaling her readiness to mate (in terms of sex pheromones). The behavioral response of an insect stimulated by the aggregation pheromone is movement toward the pheromone source and/or cessation of locomotion, at least temporarily, once the insect has arrived at the source.

• Dispersal or spacing pheromones: Used to increase the spacing between conspecifics, and thus reduce intraspecific competition. They may be used to prevent overcrowding of resources such as food, mates, egg-laying sites, and refugia.

• Home range pheromones: Used to mark an area within which an organism normally confines its activities. When this area or territory is defended against other organisms, the pheromones are called territorial marking pheromones. • Trail pheromones: Used by many insects, especially by ants, for orientation

to food sources or new nest sites. Wilson [Wil71] described the odor trail system as the most elaborate of all known forms of communication in social insects.

• Surface pheromones: Used in social insect colonies and allow recognition of nest mates, kin, or even members of different castes. This broad class also includes recognition pheromones, releasers of grooming behavior and secretions that stimulate food exchange.

The perception of a pheromone may result in an immediate behavioral response (releaser pheromones) or a complex set of physiological responses that are sim- ply set in motion by the initial perception (primer pheromones) [WB63]. Primer pheromones trigger physiological changes in the recipient, which then equip the organism with a new behavioral repertory. Very often sex pheromones function as primer pheromones for instance. The effect of primer pheromones is generally long-term, without an obvious immediate response. Releaser pheromones in con- trast cause an immediate and reversible change in behavior mediated directly by the central nervous system. All above mentioned classes may function as releaser pheromones.

With regard to the category of allelochemicals, a differentiation between releaser and primer allelochemicals is also possible, although not all allelochemicals are fur- ther subdivided. Ruther et al. [RMS02] have proposed a classification of kairomones according to the function for the benefiting organism, i. e. the receiver. This clas- sification comprises four main classes, whereas the first three classes attract the receiving organism and the fourth class repels it:

• Foraging kairomones: Used by the benefiting organism in the context of food location, e. g. volatiles used by herbivores, parasitoids, parasites, or fun- givores to locate hosts or host plants.

• Sexual kairomones: Used by the benefiting organism for sexual purposes, e. g. plant volatiles used to locate feeding mates or enhancing response toward pheromones.

• Aggregation kairomones: Used by both sexes of the benefiting organism to form aggregations, e. g. for optimal exploitation of food resources, mate finding, or as defense reaction.

• Enemy-avoidance kairomones: Used by the benefiting organism to recog- nize the presence of natural enemies, e. g. predator-borne volatiles inducing escape reaction in potential prey.

Classified according to the effect on the benefiting organism, all classes can be regarded as releaser kairomones. However, sexual as well as enemy-avoidance kairo- mones may also be considered as primer kairomones [RMS02]. Sexual kairomones may induce physiological reactions in the context of sexual behavior, e. g. plant volatiles inducing pheromone production and release. Enemy-avoidance kairomones may induce physiological reactions that reduce the negative impact of a natural enemy, e. g. predator-borne chemicals causing the development of defensive morpho- logical structures.

4.1 Principles 93

According to [RMS02], the classification of kairomones can be transferred on al- lomones as well. Very often the term allomone is used in the context of typical defense chemicals which hence are enemy-avoidance allomones. Scents emitted by predacious organisms to lure their prey, e. g. in the aggressive chemical mimicry shown by bola spiders mentioned above, may be classified as foraging allomones. Those volatiles emitted by orchids mimicking the sex pheromones of their pollina- tors may be interpreted as sexual allomones, since these plants do not reward their pollinators and the responder does not benefit from this interaction. A differentia- tion between primer and releaser allomones is principally possible, too.

In the case of synomones, the transfer of the kairomone classification is difficult. In many cases, a synomone will have different ecological functions for the emitter and for the receiver. Hence, the criterion for the classification can not be defined unambiguously [RMS02].