Allelochemicals

Definition

Biologically active compounds produced by one organism that influence the physiology, behavior, or population dynamics of another organism. In arthropod and plant contexts, these secondary metabolites mediate interspecific chemical communication, including defense, attraction, deterrence, or physiological disruption. Allelochemicals are classified by function rather than structure: allomones benefit the emitter (e.g., plant toxins that deter herbivores), kairomones benefit the receiver (e.g., volatile plant cues exploited by parasitoid wasps to locate hosts), and synomones benefit both parties (e.g., floral scents attracting pollinators).

Etymology

From Greek allēl- (of one another, mutual) + chemical, coined in the context of allelopathy research

Example

Brassicaceous plants release isothiocyanate allelochemicals when damaged by caterpillars; these compounds directly deter feeding by Pieris butterflies while simultaneously attracting specialist parasitoid wasps (Cotesia species) that use the volatile signal as a kairomone to locate hosts.

Related Terms

• Allelopathy
• secondary metabolites
• allomone
• Kairomone
• synomone
• Semiochemical
• phytochemical
• induced plant defense
• trophic cascade

Usage Notes

Distinguished from pheromones, which mediate intraspecific communication. The term allelochemical emphasizes the interspecific, often interkingdom, nature of the interaction. In entomology, usage overlaps heavily with semiochemical, though semiochemical is broader and includes pheromones. Specialists may specify functional subclass (allomone, kairomone, synomone) when the ecological outcome is known; 'allelochemical' alone carries no implication of benefit or cost to either party.