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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 2.1.3.1)
4. Hydrocarbons
580
alkaani
alkane
(see section 2.1.3.2)
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.