Semiochemicals are small organic compounds that transmit chemical messages. They are used by insects for intra- and interspecies communication. Insects detect semiochemical directly from the air with olfactory receptors. In most insects, the receptors are located in sensilla hairs on the antennae. The term “semiochemical” has been in use since 1971. It is derived from the Greek word “semeon,” which means “sign” or “signal.” Semiochemicals were already being studied in the 1880s. Although their chemistry was not yet understood at that time, people already knew about using female insects to lure males into traps. Semiochemical research in its present form has been conducted since the 1950s, when the first pheromones were isolated and identified. From the 1950s up until today, more than 3,000 semiochemicals connected to the chemical communication of insects have been identified.
The chemical analysis of semiochemicals should consider both effect and structure. In terms of effect, semiochemicals can be classified as pheromones or allelochemicals based on how they are used and who benefits. In terms of structure, semiochemicals can be divided into twenty-four classes according to functional groups. Familiarization with the practical applications of semiochemicals is also important to the study of their chemistry.
CHEMICAL AND PHYSICAL PROPERTIES OF SEMIOCHEMICALS
The semiochemical database Pherobase contains approximately 3,000 semiochemicals so far. Most of the molecules are small and simple, but some have structures that can be quite complicated. Semiochemicals have molecular weights ranging from 17 to 880 g/mol, but they are usually volatile. The heaviest molecules have the longest carbon chains, but there are fewer than ten semiochemicals in the database with a mass above 550 g/mol. The length of the carbon chains in semiochemicals varies from zero to forty-five carbons. The number of double bonds in semiochemical structures varies from zero to thirteen. Along with double bonds, cis-trans isomerism is a typical feature of semiochemical compounds, although positional and optical isomerism also occur. Based on effect, semiochemicals are divided into two main categories: pheromones and allelochemicals. An examination of semiochemicals must take their functions into account, since the same molecule could act as a pheromone for one insect species and as a kairomone or allomone for another. In nature, a species-specific chemical message could be generated based on an exact molar ratio, a particular form of isomerism, or isomeric mixtures, for example.
PHEROMONES
Pheromones are compounds used by insects for intraspecies communication. The term “pheromone” is derived from the Greek words “pherein” (to carry) and “horman” (toexcite/stimulate). The term was introduced in 1959 by Karlsson and Butenandt and by Karlsson and Lüscher simultaneously. The difference between pheromones and hormones is that hormones are produced in an insect’s endocrine glands. They have an effect on the insect that produces them, whereas pheromones affect other individuals instead. Based on their effect, pheromones can be divided into at least the following categories:
- Aggregation pheromones: compounds that increase the concentration of insects at the pheromone source.
- Alarm pheromones: compounds that stimulate insects’ escape or defense behavior.
- Sex pheromones: compounds that help individuals of the opposite sex to find each other.
- Trail pheromones: among social insects, compounds used by workers to mark the way to a food source, for example.
- Marking pheromones: compounds used by insects to mark the boundaries of a territory. Dragonflies (Miettinen, A., 2006).
The effect of a semiochemical is influenced by its molecular weight. Most of the alarm pheromones, for example, have a molecular weight below 200 g/mol. Once the danger has passed, they evaporate. Trail pheromones, on the other hand, have higher molecular weights. They are not supposed to evaporate immediately.
ALLELOCHEMICALS
The allelochemicals are classified as allomones, kairomones or synomones. Allomones are a class of compounds that benefit the producer, but not the receiver. Allomones are often found in nature as part of a chemical defense, such as toxic insect secretions. Predators also use allomones to lure prey. Kairomones are a class of compounds that are advantageous for the receiver. The term “kairomone” is derived from the Greek word “kairos,” which means “opportunistic” (Nordlund et al. 1981, 18). Kairomones benefit many predators and bugs, for example, by guiding them to prey or potential host insects. Synomones (from the Greek “syn” for “with” or “together”) are compounds that are beneficial to both the receiver and the sender.
SEMIOCHEMICALS AS A POTENTIAL FOR INTEGRATED PEST MANAGEMENT
Semiochemicals (Gk. semeon, a signal) are chemicals that mediate interactions between organisms. Semiochemicals are subdivided into allelochemicals and pheromones depending on whether the interactions are interspecific or intraspecific, respectively. Within both allelochemicals and pheromones it is sometimes useful to refer to chemicals as arrestants, attractants, repellents, deterrents, stimulants or other descriptive terms. These terms can indicate what behavior is involved in the response such as a feeding stimulant or flight arrestant. Pheromones (Gk. phereum, to carry; horman, to excite or stimulate) are released by one member of a species to cause a specific interaction with another member of the same species. Pheromones may be further classified on the basis of the interaction mediated, such as alarm, aggregation or sex pheromone. It is the sex pheromones of insects that are of particular interest to agricultural integrated pest management (IPM) practitioners.
The concept of IPM is based on the recognition that no single approach to pest control offers a universal solution, and that the best crop protection can be provided by a fusion of various tactics and practices based on sound ecological principles. Pheromones are a commonly used component of many insect IPM programs (Dent, 1993 and Anonymous, 1995.) The existence of pheromones has been known for centuries, apparently originating in observations of mass bee stinging in response to a chemical released by the sting of a single bee. The two primary uses of insect pheromones are for detection and monitoring of populations and for mating disruption. These uses take advantage of sex pheromones on which a vast majority of insect pests rely to mediate reproduction.
Male annihilation is trapping carried to a seemingly logical conclusion. Place enough traps, catch enough males, and leave the females of the species without mates. This approach has been used against pink bollworms in an isolated area of Arizona with low numbers of overwintering moths. Any untrapped males simply mate more frequently. Mating disruption does not depend on traps for control, although traps are frequently used to monitor the extent of mating disruption in the population. Failure to trap males is taken as an indication that males are unable to find females which may or may not be true. Thus, trap data must always be related to actual levels of crop infestation.
Present commercial formulations of pheromones for both trap baits and mating disruption mimic the natural chemical blends of females as clearly as possible. Most insect sex pheromones are multicomponent with precise ratios of components which may be expensive to manufacture. Thus, insect sex pheromones and products containing pheromones, are commercially available primarily for insects of economic importance.
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