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Semiochemicals Semiochemicals are small organic compounds that transmit chemical messages. They are used by insects for intra- and interspecies communication. Insects detect semiochemicals 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. Current research on semiochemicals involves continued molecular mapping, synthesis and studies of biosynthesis. Another research area that has gained importance over the years has been an effort to understand the neurophysiological sensory functions of insects and how hormonal regulation in insects affects pheromone biosynthesis and release. The practical goal of semiochemical research is to develop means and methods of exterminating and controlling insect pests. Semiochemical research is placed in Pasteur's Quadrant of the Stokes model. It is based on pure research in chemistry, but the final goal is still to develop solutions for practical problems through applied research. In Finland, semiochemical research is focused on protecting plants and forests. Means of insect extermination and control are being developed. Research related to forest and plant protection is being carried out at the universities of Joensuu, Helsinki and other places. Agrifood Research Finland is also studying semiochemicals, with an emphasis on plant protection. The chemical analysis of semiochemicals should consider both effect and structure (see Figure 1). 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. 2.1.1 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. Figure 1. Concept map of semiochemical classification and properties Semiochemical classification based on effect 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" (to excite/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 (Figure 2): 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. Figure 2.2. 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. Semiochemical classification based on structure Semiochemicals are a diverse category in terms of their chemical structure. In Pherobase, they are divided into twenty-four subcategories based on functional groups. Table 1. Classification of semiochemicals according to chemical structure, from the largest category to the smallest. Number Functional group Category Esters 1. Carboxylic esters 2. Acetate esters 3. Cyclic esters 430 340 75 - Basic structure - Substituents - Characteristic features - Carbon chain (C2–C41) - Double bonds - Isomerism - Ring structures (cyclic esters) - Methyl and propyl groups O OR R (see section 22.214.171.124) 4. Hydrocarbons 580 alkaani alkane (see section 126.96.36.199) 5. Ketones alkeeni alkene alkyyni alkyne 400 O R R Alcohols 6. Primary alcohols 7. Secondary alcohols 8. Tertiary alcohols 210 150 30 9. Amines 300 10. Aldehydes 260 OH H OH R H OH R R R H R R prim. Primary sek. Secondary tert. Tertiary R R R N N N H H H R R R prim. sek. tert. Primary Secondary Tertiary O R H - Carbon chain (C2–C45) - Degree of saturation - Isomerism - Ring structures and aromatic compounds - Methyl and propyl groups - Carbon chain (C3–C31) - Double bonds - Isomerism - Ring structures and aromatic compounds - Methyl and propyl groups - Some molecules have nitrogen in the structure - Often serve as part of a chemical defense - Carbon chain (C3–C30) - Double bonds - Isomerism - Ring structures - Methyl and propyl groups - Carbon chain (C0–C32) - Double bonds - Isomerism - Ring structures - Methyl groups - Carbon chain (C1–C28) - Double bonds - Isomerism - Methyl groups Table 1, continued 11. Carboxylic acids OH R 12. Epoxides - Carbon chain (C0–C40) - Double bonds - Isomerism - Methyl groups - Many act as defensive chemicals - Carbon chain (C8–C23) - Double bonds - Isomerism - Ring structures - Methyl groups - Carbon chain (C6–C13) - Chloro, nitro and methoxy groups - Methyl groups O 210 O 100 R R OH 13. Phenols 55 e.g. 14. Spiroacetals 50 O R 15. Diols 40 R O OH HO e.g. 16. Quinones - Carbon chain (C8–C13) - Volatile - Isomerism 40 O O - Carbon chain (C2–C19) - Isomerism - Methyl groups - Carbon chain (C6–C15) - Methyl, ethyl, propyl, vinyl and methoxy groups e.g. 17. Dioxy 30 R 18. Sulfur compounds 19. Ethers - Carbon chain (C6–C10) - Isomerism O O R 30 - Structure includes –S– bond 20 - Structure includes oxygen bridge –O– - - O 20. Furans 20 21. Polyhydroxy 20 22. Pyrans 15 - Structure includes several –OH groups e.g. - Category includes sugars - O e.g. 23. Triols OH 5 - HO OH e.g. 24. Oximes Total 5 >3200 HO N R - According to Table 1, the structural subcategories with the largest number of semiochemicals are esters, hydrocarbons, ketones, alcohols, amines, aldehydes and carboxylic acids. The structural categories of semiochemicals cannot be placed within particular categories of behavioral effects because they can be found in all of the effect categories. Esters Esters are carboxylic acid derivatives with a –COOR functional group. They are the largest category of semiochemicals in the database, and are widely distributed in nature. Esters have a pleasant odor and taste. They are characterized by having a high boiling point, relative to molecular size, due to the oxygen atoms in their structure. Semiochemicals include three types of esters: carboxylic esters, acetate esters and cyclic esters. The category also includes nitrogen-containing compounds such as ammonium acetate (C2H5NO2). Stereoisomerism occurs among the semiochemical esters. The most common substituents in the category are methyl and propyl groups. Hydrocarbons Hydrocarbons are the second-largest category of semiochemicals: there are approximately 580 of them (see Table 1). Of the hydrocarbons with short carbon chains (C2–C13), most are alkenes. With longer carbon chains (C14–C45), the occurrence of alkanes in the category increases. The category also includes cyclic hydrocarbons with ring structures and aromatic compounds, but only one alkyne. Isomerism in this category occurs frequently, usually positional isomerism in the alkanes and optical or cis-trans isomerism in the alkenes. The basic structure of this category is a simple, straight carbon chain (C2–C45). The chemical diversity of the category arises from differences in degrees of saturation, isomerism, cyclic structure and aromaticity. The most frequently occurring side chains in this category are methyl and propyl groups. Semiochemical applications Semiochemicals are an important area of research in which chemistry and biology expertise converge. An important application of semiochemicals is insect pest control on cultivated land and in stored products. For example, insect pests are controlled by using traps that contain semiochemicals. Semiochemicals can be used to monitor the size of insect pest populations with survey traps, or to lure them into large traps where they are exterminated. Environmental concentrations of semiochemicals can also be increased so as to interfere with insect communication in an attempt to impede reproduction, for example.