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A semiochemical (semeon means a signal in Greek) is a generic term used for a chemical substance or mixture that carries a message. These chemicals act as messengers within or between species. It is usually used in the field of chemical ecology to encompass pheromones, allomones, kairomones, attractants and repellents.
Probably all insects use semiochemicals; natural chemicals released by an organism that affect the behaviors of other individuals. Pheromones are intraspecific signals that aid in finding mates, food and habitat resources, warning of enemies, and avoiding competition. Interspecific signals known as allomones and kairomones have similar functions. The goals of using semiochemicals in pest management are
- to monitor pest populations to determine if control is warranted and
- to alter the behavior of the pest or its enemies to the detriment of the pest. In general, the advantages of using semiochemicals are
- they have adverse effects only on target pests,
- they are relatively nontoxic and required in low amounts,
- they are nonpersistent and environmentally safe
- they appear difficult for insects to develop resistance against. Monitoring of pest populations with pheromones is often integrated in management programs.
A pheromone (from Greek phero "to bear" + hormone from Greek - "impetus") is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology. Their use among insects has been particularly well documented. In addition, some vertebrates and plants communicate by using pheromones.
The term "pheromone" was introduced by Peter Karlson and Martin Lüscher in 1959, based on the Greek word pherein (to transport) and hormone (to stimulate). They are also sometimes classified as ecto-hormones. German Biochemist Adolf Butenandt characterized the first such chemical, Bombykol (a chemically well-characterized pheromone released by the female silkworm to attract mates).
An allomone is any chemical substance produced and released by an individual of one species that affects the behaviour of a member of another species to the benefit of the originator but not the receiver. Production of allomones is a common form of defence, such as by plant species against insect herbivores or prey species against predators. Sometimes species produce the sex pheromones of the organisms they exploit as prey or pollinators (such as bolas spiders  and some orchids ). "Allomone" was proposed by Brown, Eisner, and Whittaker to denote those substances which convey an advantage upon the emitter.
A kairomone is a semiochemical, emitted by an organism, which mediates interspecific interactions in a way that benefits an individual of another species which receives it, without benefitting the emitter. Two main ecological cues are provided by kairomones; they generally either indicate a food source for the receiver, or give warning of the presence of a predator. Often a pheromone may be utilized as a kairomone by a predator or parasitoid to locate the emitting organism 
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