Allomone

Allomones are interspecific chemical signals that play a significant role in ecological interactions among organisms. They are a class of semiochemicals produced by individuals of one species that affect the behaviour or physiology of members of another species in ways that benefit the emitter but do not benefit—or may even disadvantage—the receiver. The term derives from the Ancient Greek meaning “other” and was introduced by Brown and Eisner in 1968 to describe substances that confer an advantage upon the organism releasing them. Allomones form one category of allelochemicals, alongside kairomones and synomones, which together constitute the major chemical mediators of communication across species boundaries.
In nature, allomones occur widely among plants, insects, and other animals. Their ecological functions range from defence against predators to prey acquisition and competitive interference. The evolutionary dynamics surrounding allomones often reflect arms races in which recipients evolve countermeasures, sometimes transforming the allomone into a kairomone or adopting it for their own defensive or communicative purposes.

Concepts and Classification

Allomones represent a subclass of allelochemicals, which include:

• Allomones: Benefit the emitter, disadvantage or do not benefit the receiver.
• Kairomones: Benefit the receiver, often to the emitter’s detriment.
• Synomones: Benefit both emitter and receiver, representing mutualistic chemical interactions.

Brown and Eisner’s initial definition did not explicitly exclude cases in which both organisms benefited, leading some early classifications to treat mutualistic interactions involving chemical signals as a form of allomonal communication. Subsequent refinements established synomones as a separate category to avoid ambiguity.
Allomones are used in a diverse range of ecological contexts:

• defence against herbivores and predators,
• suppression or inhibition of competitors,
• prey immobilisation or deception,
• interruption of interspecific communication.

These functions underscore the strategic importance of chemical signalling in environments where visual or acoustic cues may be limited.

Ecological Functions of Allomones

Defence Against Herbivory and Predation

Plants commonly produce allomones as part of their defence arsenal. These chemicals deter, repel, or inhibit insect herbivores by affecting behaviour, development, or survival. Major categories include:

• Antibiotics: Chemicals that disrupt growth, digestion, or longevity in herbivores.
• Antixenotics: Deterrents that alter normal host-selection behaviour, including repellents, suppressants, or locomotory excitants.

Such defences contribute to plant fitness by reducing tissue loss and lowering the reproductive success of herbivores. In turn, insects have evolved strategies to neutralise or adapt to these compounds. Some species develop positive behavioural responses, transforming allomones into kairomones, whereas others modify or appropriate them, integrating these compounds into their own communication or defence.

Prey Capture

Allomones may function offensively, enabling predators to subdue or manipulate prey. A well-documented example involves the larvae of the berothid lacewing Lomamyia latipennis, which prey on termites:

• The first instar larva approaches a termite and waves its abdomen near the termite’s head.
• Within minutes, the termite becomes immobile and soon paralysed.
• Contact is not necessary; the chemical acts through the air.
• Subsequent instars feed similarly and may subdue multiple termites at once.

The specificity of the allomone is notable: it affects termites but does not disturb other insects nearby.

Competitive Interference

Chemical signalling also mediates interactions among competing species. Some bark beetles release allomones that interfere with the pheromone systems of other beetle species occupying the same host tree. For example:

• A compound emitted by G. sulcatus can disrupt the pheromonal communication of G. retusus, limiting competition for Pinus taeda.
• In California Ips pini, individuals possess two receptor types—one for their own species’ pheromones and another for those of competitors such as I. paraconfusus. This dual sensitivity ensures that interspecific signals do not overpower intraspecific cues.

These mechanisms illustrate chemical competition, enabling emitters to reduce competition for resources such as food and space.

Predator Avoidance

Some arthropods have adopted sophisticated chemical mimicry strategies. Beetles, cockroaches, and other solitary insects may emit chemicals identical to the alarm pheromones of ants. When released:

• Worker ants detect the alarm signal and retreat to their nest or adopt defensive behaviours.
• The emitter uses the confusion to escape predation before recruitment of additional ants can occur.

This form of deceptive allomonal signalling demonstrates the adaptive utility of exploiting another species’ communication channels.

Plant and Insect Producers of Allomones

Plant Producers

Several plant species produce allomones as part of their natural defensive repertoire. For example, species of Desmodium (tick-trefoils) emit chemicals that deter herbivores or disrupt their feeding patterns.
Plants release diverse compounds including:

• volatile organic chemicals that repel insects,
• toxins that reduce digestibility,
• growth inhibitors that impair herbivore development.

Such strategies are integral to plant–herbivore coevolution and contribute to biodiversity by shaping ecological niches.

Insect Producers

In addition to lacewings and beetles, numerous insect taxa utilise allomones. Their purposes vary but often involve:

• subduing prey,
• communicating territorial boundaries,
• inhibiting reproductive or feeding behaviour of competitors.

These examples highlight the broad ecological relevance of allomonal communication in arthropod communities.

Evolutionary Dynamics

The interaction between allomone producers and receivers often follows an evolutionary arms race. Receivers may evolve:

• resistance mechanisms or metabolic detoxification pathways,
• behavioural changes that neutralise the signal,
• adaptations turning the allomone into a cue for locating hosts or prey.

Producers, in turn, may evolve more potent or specialised chemicals. This dynamic interplay contributes to ecological diversification and the development of complex multispecies interactions.

Broader Significance

Allomones are vital components of interspecific communication networks. They influence ecological balance by mediating predator–prey relationships, competition, and plant–herbivore interactions. Their study spans chemistry, ecology, evolutionary biology, and behavioural science, providing insight into how organisms manipulate their surroundings and respond to the challenges of survival.