The insect pheromones
V. D. IVANOV
Saint Petersburg state University
THE LANGUAGE OF SMELLS
The first information about the presence of insects, particular smells is able from a distance to attract individuals of the opposite sex, appeared almost a century ago. In the experiments of the great French entomologist Henri Fabre (1823-1915) showed that males of Pawlikowski Saturnia pyri fly to the females from a distance of several kilometers. If the smell of females to pass any subject, the subject attracts males. On the perception of pheromones in insects is the antenna located on the head, without them, the males unable to find females by smell.
In our country J. D. Kirshenblat  for chemical communications suggested the name "telegony", but in the scientific community was accepted the term "pheromones", which translates from Greek as carriers of excitement. As the authors of the term P. Karlson and M. Luscher (1959), pheromones are substances produced and released into the environment by living organisms and causing a specific response (behavior or characteristic characteristic process development) in their perceiving individuals of the same species.
Odorous substances synthesized in the body in small doses, which complicates their chemical analysis. First of pheromones and subjected to chemical analysis - bombykol, the sexual attractant of the female silkworm Bombyx mori. Success came to A. Butenandt, G. Gekker, and D. the Strain in 1961 after nearly 20 years of research. The experiments were carried millions of cocoons and hundreds of thousands of females, however, the yield of the substance was measured in milligrams. It turned out that from a chemical point of view bombykol represents 10,12-TRANS, CIS-hexadecan-1-ol (Fig. 1, a-C).
Modern methods of gas chromatography, mass spectroscopy and NMR spectroscopy allow the analysis of even nanograms of material obtained from one individual. Progress in the field of analytical chemistry, physiology, senses and ethology (science of behavior) has allowed to investigate odorous signals from thousands of species. Chemical signaling is widespread in nature and found not only the vast majority of animals, but also in algae and lower fungi . The study of pheromone communication is one of the main directions in the young field of science - chemical ecology. The main object of chemists and ecologists, exploring the pheromones serve insect. Block communication between males and females of the pest and thereby the violation of reproduction include a part in the promising integrated methods of plant protection. The pheromone research opens the way to control the behavior of insects. Finally, pheromones are used by animals for recognition of species and serve to interspecific isolation. The study of pheromone and other types of communication allows you to better understand evolutionary processes.
A VARIETY OF PHEROMONES
There are two main types of pheromones: relizer that triggers a specific behavioral reaction, and primers that change the physiological state of an individual and modify its development. Relizer usually represented by high-volatile substances that is spreading through the air, while primers are often transmitted by contact. Among relizerov, which is better understood primer, there are several subtypes. These are the attractants - calling individuals substances (sex pheromones and aggregation pheromones), repellents repellent, arrestant - stopping, stimulants - causing activity (e.g., alarm pheromones), deterrence - inhibitory reaction. Some stimulants (aphrodisiacs) are substances of sexual arousal, causing the mating behavior. Aggregation pheromones stimulate the accumulation of insects, trace pheromones designed for marking of territory and guides the movement of individuals.
Pheromones usually are not one substance, but a mixture of primary, dominant mass component with small additives (minor components). One substance can have several different functions. A typical example - "the king's substance" of honeybees (Fig. 1), secreted from the glands of the upper jaw for the Queen bee. This pheromone attracts both males, and worker bees to the uterus, acting as releaser. Primernoe the effect is to suppress the development of ovaries in worker bees (all females) and blocking the reflex that defines the construction of uterine cells. Licking pussy and transmitting pheromone with food, the worker bees in the nest to spread information about the condition of the bee family. Males react to "Royal stuff" only during their short mating flight. In addition to the main component part of the "Royal stuff" also includes other substances that are separately inactive but will greatly enhance the effect of the main component in the mixture.
|Fig. 1. Some insect pheromones: a - the silkworm (a - full and b - condensed structural formulas - a brief chemical formula); g - Gypsy moth, d - housefly, e - ants of the genus Acanthomyops (the alarm pheromone) and bazalnah bees of the genus Trigona (trace pheromones), Zh - pine Sawfly, R bark beetle (aggregation pheromone), and is the main component of the "Royal stuff" in honey bees. Green represents carbon atoms, blue - oxygen yellow - hydrogen double bond is highlighted in red|
The chemical structure of pheromones is much more varied compared to their functional types. For relizerov characterized by high volatility and the relative ease of synthesis in the body: insect sometimes forced to allocate the pheromone for a long time, waiting for a reaction partner. Pheromones are often used for species recognition and in connection with this function should be specific to each species.
The most successfully developed of the study of sex pheromones, which identifies components of pheromone mixtures of hundreds of species, mostly of agricultural and forest pests. Most of deciphered to date belong to the pheromone alcohols and their acetates, and aldehydes and ketones (see Fig. 1). It would seem that such a modest set of substances provides limited opportunities to vary the smells, however, there are special mechanisms to maintain species specificity of sex pheromone blends. Insects make extensive use of the phenomenon of chemical isomerism: structural and optical. In addition to the isomerism for the correct recognition signal among many natural scents are used the proportions of the individual isomers and the ratio of the main and minor components in pheromone blends.
A comparative study of sex pheromones of Lepidoptera have allowed to reveal significant evolutionary change. It is well known butterflies belong to the higher Lepidoptera - Ditrysia; they have a carbon chain molecules are usually not branched and contain from 12 to 18 atoms. In addition, moth and scoops used unsaturated hydrocarbons and their epoxides. So, widely known dispalys, or 7,8-CIS-epoxy-2-methyloctadecane, sexual attractant of the Gypsy moth Porthetria dispar (Fig. 1, d). Glands that produce pheromones have higher Lepidoptera, located between 8th and 9th segments at end of abdomen often bear tufts of hairs that increase the surface for evaporation of heavy long-chain pheromones produced in microscopic quantities. These hairs can bulge when the female raises the abdomen, urging the male.
Primitive Lepidoptera (Monotrysia) the chemistry of pheromones are more diverse. Among them, evolutionary advanced groups are long-chain pheromones are similar to those ditrysia Lepidoptera. But the mole-baby (Nepticuloidea) and birch leaf-mining moth (Eriocranioidea), standing at the base of the evolutionary tree of butterflies have different short-chain pheromones with the number of carbon atoms less than 10. Almost the same short-chain alcohols and ketones are the caddisflies (Fig. 2), which related Lepidoptera, but are at a lower stage of evolution. The caddisflies are particularly popular-based compounds 7, 8 and 9 carbon atoms. The site of pheromone synthesis from Trichoptera and primitive Lepidoptera is also the most special is a sternal gland, located at the bottom on the border of the 4th and 5th segments of the abdomen. Another common feature is tens and hundreds times the amount of the synthesized pheromone. The similarity of the communication systems of both units is no accident and reflects the status that was inherent in the common ancestor of Trichoptera and Lepidoptera.
In other orders of insects sex pheromones studied incompletely for discussion of evolutionary change. However, their chemical structure most belong to the same classes of substances that Lepidoptera: alcohols, ethers, ketones, fatty acids (see Fig. 1). Molecules of pheromones usually polar, they have low boiling point and therefore high-volatile.
SEX PHEROMONES AND AGGREGATION SUBSTANCE
Place of synthesis and release of pheromones is not necessarily associated with specific glands, like the caddisflies and Lepidoptera. Often the components of the pheromone mixture produced in the fat cells of the body, filling the spaces between internal organs, then dissolve into the hemolymph (blood of insects) and is displayed through the integuments, as in sawflies. Finally, cockroaches-cockroaches Blatella germanica aphrodisiac 3,11-dimethyl-nonacosane-2-it is contained in a wax cover on the surface of the body of the female. This substance causes the male cockroach antennae touched the female, sexual arousal and characteristic posture of raised wings prior to mating. In most cases, however, evaporating the pheromones diffuse in the air and act at considerable distances.
For the response of males is sufficient to have very small amounts of pheromone in the air. For example, the maximum effective concentration the discovery of bombykol in the air is only 10-12 µg/ml, and for the excitation sensitive receptor cells on the antenna, only a single molecule. In sensitive bodies, hemoretseptorna sencillo - when this limit is reached the sensitivity threshold. Receptors in insects, thus, can be considered separate molecules. In one sensilla, having the form of hairs or bumps on the surface of the tendril, sometimes hiding different receptor neurons, each tuned for a specific type of chemical. Until recently it was thought that there are neurons-specialists reacting to only one type of molecules, generalists, designed to accommodate a wide range of substances due to the loss of sensitivity. Recent studies have identified the specialization of those neurons that have previously attributed to the generalists: it turned out that while the selectivity of the receptor cells can mistakenly react to the related molecule, but with lesser intensity.
After the interaction of pheromone molecules with receptor proteins in sensitive cells-neurons is a chain of reactions. Receptor protein interacts with so-called G-protein, and that, in turn, activates phospholipase. In the cell is allocated Inositol-3-phosphate and opens calcium channels; coming into the neuron, calcium ions activate protein kinase kalnishevsky, which with the help of phosphorylated proteins activates mediators of sodium and potassium ion channels. Channels open, ion flow is formed, changing the charge of the cell membrane, there is a receptor potential. Extending through the body of the neuron, the receptor potential is much more intense response in the form of the action potential propagating along a nerve fiber initiated. The set of potentials of the excited nerve cell generates a potential of the antenna, which in time simply can be written in the form of electroantennogram (see Fig. 2). The nerves, the excitation of receptors of antennas is transmitted in the "brain" of an insect - number ganglion. There deutocerebrum (second segmental ganglia of the brain) are higher sections of the olfactory analyzer, where it is processed all the incoming olfactory (olfactory) information.
|Fig. 2. Results analysis of sex pheromone blend of female caddis gas chromatography with simultaneous electroantennogram. Mapping of electrical activity in the antenna of the male (lower curve) and output substances from the chromatograph (upper curve) shows the reaction of the antenna on heptane-2-he (a) heptane-2-ol (b ), nonan-2-he (in) and nonan-2-ol (g). These same components found in the pheromone blends of many other species. The reaction of the antenna on other substances, represented by peaks on the chromatogram in the right part of the figure is missing. Color coding of atoms as in Fig. 1|
The scent of males moving upwind towards the source of pheromone signal, usually the insect does zigzags, assessing the direction of the female. Insect focuses on the wind and the increasing concentration of pheromone in the air. Earlier it was believed that the amount of pheromone decreases monotonically in the direction from the source. Only recently was revealed the complex structure of sex pheromone of jets occurring in the air under the action of turbulence. Complex vortices pheromone jets complicate the search and the discovery of the source of the pheromone, making the male spending a lot of time in search of females. A particular problem is the detection of species in high numbers, as many calling females causes overlapping sex pheromone cloud as a result of a sharp decline in the concentration gradients of pheromones. Probably, in such cases, the insects can use minor components of the pheromone as guidance of search and recognition of the female.
Some types of insects have a very complex system of interaction between the sexes, including different types of incentives. For example, some butterfly-moth (Arctiidae), the male approaches the female from turning odorous glands, focusing on the smell of her pheromone. Sensing chemical signals of a male, the female responds by beating of wings and sound signals. Have Bosinney moth Galleria mellonella males and females to exchange ultrasonic signals with clicks. Hearing the sound of the female, the male secretes a pheromone for accurate aiming. But some butterflies temprado (Hepialidae), the female is attracted to flying in the air males, then both insects fall to the ground, and then the male finds the female by her smell. Very complex communication system identified some species of caddisflies, where, in addition to multicomponent pheromones are possessed by both sexes, there are a variety of vibration signals.
A special type of pheromones - substances aggregation. When swarming, and other forms of aggregation together individuals of the same sex, in obedience to chemical and other stimuli: visual, acoustic, etc aggregation (swarming) of caddisflies in the air, the leading role belongs to the males, which fly females. The richness of pheromone mixes swarming of caddisflies-hydropsyche males that have up to four active components in the pheromone blend significantly ahead of females with only one olfactory-active component. Pheromones females there are fewer than males, and sometimes non-existent.
The bark beetles are particularly widely used aggregation behavior. After the successful attack on the wood to mass settlement that can weaken the plant host and to allow the offspring to develop Stalowa under reduced pressure. Otherwise, a huge number of eye-catching resin close up the tunnels and kill the larvae. The beetles typically use a mixture of several substances-synergists, mutually reinforcing their action. They are all related odorous substances and resins belong to the group of terpenes and terpenoids (see Fig. 1, z). Shown that certain species of bark beetles pheromones are formed in the intestine from the resin of coniferous eaten by bacteria. So, coli Bacillus cereus in the gut of beetles Ips paraconfusus converted a-pinene in verbenol. Primitive bark beetles first attack tree fly females, and then find them males. The higher representatives of this group, on the contrary, the nest lay males and females expect. In both cases, initial colonization of wood occurs in one of the sexes is a characteristic of aggregation.
SUBSTANCES OF ANXIETY AND PROPAGANDA
In many insects, forming a family or cluster (termites, aphids, bedbugs, ants, wasps, bees), there are special chemicals that are designed to alert group members of danger. The alarm pheromones released into the air the insects, who first noticed the danger. After exceeding the threshold concentration of other species that received pheromone signal, in turn, secrete the same substances. Eventually the cluster dissipated or insects have a resistance to the aggressor.
In ants there are several sources of alarm pheromones. Gafurova glands in the abdomen usually synthesized hydrocarbons and methylketone, while the maxillary (mandibular) glands secrete citral, geranial (see Fig. 1, e), and other terpenoids. Citral along with ketones nonan-2-one and heptane-2-one is included with the alarm pheromones of bees of the genus Trigona. Surprisingly, the last two of the listed substances are the sexual pheromones of certain caddisflies. Red forest ants of the genus Formica have pheromone alarm - formic acid, which also serves as a weapon. Other ants in the composition of the pheromones of anxiety may include such substances as Dean, dodecane (carpenter ants of the genus Camponotus), dimethyl, acetates of aliphatic alcohols.
Honeybees sting when you introduce the enemy to a mixture of isoamyl acetate, isoamylamine and solivita with poison. Characteristic odor, something reminiscent of the scent of banana, encourages other bees to the enemy and causes aggression, prompting sting marked with pheromones target. The attack happens in that case, if a bee is crushed, which is well known to beekeepers. In the mandibular glands of honeybees contains other pheromone alarm - heptane-2-he (Fig. 2, a).
The alarm pheromone of aphid - TRANS-b-farnesene causes the escape of insects from the plants during the attack of predators. Krasnolobov (family: Pyrrhocoridae) is similar to the role of hexene-2-al. Termites have a rich set of alarm pheromones from the group of terpenes, such as terpinolene and a-pinene.
The high concentration of substances anxiety, some insects do not cause reactions of aggression, and flight. The similarity of the sexual pheromone of certain species of insects, and alarm pheromones of invasive species suggests that insects can use simulators pheromones alarm to protect. So using pheromone mimicry, the insect can pretend to be not what it really is. Such false pheromone signals are called "substances of propaganda."
Pheromone mimetism widely shown by some species of social parasites for penetration into colonies of hosts and laying their eggs, such as bees cuckoos. An amazing example represent some species of spiders, which synthesize sex pheromones of butterflies to attract victims.
Complex adaptations for living in ant colonies and termites are insects myrmecophily and territorily. These roommates ants, beetles-ground beetles Paussini, there are special exudative group of glands on the body and antennas. Ants constantly lick the allocation of these glands. They feed their tenants in the same way as other members of the colony. If such a beetle to put next to an anthill, the ants dragged her inside, grabbing for short & very thick antenna. Special exudate develop on the body and many other commensals, such as beetles Rove beetles and flies of the family Phoridae. All these insects use "propaganda substances" to calm and attract aggressive owners of the nest. In addition, for living in a colony of social insects on the body of the commensal must contain certain chemical compounds that make up the "passport" member of this colony. The composition of these compounds varies greatly and varies even among colonies of the same species. While the smell of the insect determines which of the colony (his or someone else's) is met by the individual and in accordance with this builds its future behavior. Such chemical passports serve as a pass into the slot. The monotony of the chemical labels is achieved by trophallaxis - the exchange of food between colony members.
Many of the substances used by insects as odorous labels that serve to attract and orientation on the ground. The bumblebees as such trace amounts of pheromones can make the allocation of mandibular glands, such as TRANS-2,3-dihydrofarnesol and its acetate, which are used for marking territory and attract both males, and females. The bees of the genus Trigona for this purpose, the isomers of citral (neral and geranial) and heptane-2-ol (see Fig. 2, b ). Among ants and termites for marking territory and roads as signs are used organic acids, alcohols, terpenes. Many kinds of ants on the run, draw the end of the abdomen on the surface and leave odorous marks oriented in a certain way and slowly evaporating for several hours. The destruction of land marked road leads to disorientation of ants, which, however, is able to restore its trails. Trace pheromones have not been studied, their composition is complex, and biological effects is ambiguous.
PROBLEMS AND PROSPECTS
The study of insect pheromones has just begun. There is still no data on the composition of the pheromones of many groups is poorly understood interaction between individual incentives in creating a coherent communication system and the biosynthesis of pheromones. These studies open the way to the knowledge of the subtle mechanisms of the evolution of pheromones and related to the evolution of species.
Now, pheromone traps are used for control of pests such as bark beetles and leafrollers. Attempts to use the method of pheromone saturation, when the gardens are scattered huge amount of bait (about "pheromone goals") have not yet yielded great results, however this method is promising for selective suppression of specific harmful and does not cause damage to wildlife. Another opportunity to save their homes and gardens from pests - mass distraction insects in pheromone traps for disposal. Muscalur, for example, is part of the many traps for the flies. This method, however, does not provide the complete extermination of the pest, but only helps to maintain numbers at an acceptable level.
The origin of pheromones is still a mystery. Why do different species use a relatively limited set of signal substances? How to interact with different types of incentives to create a coherent message that is passed from one insect to another? What is the leader in the evolution of a pair of signal-receptor - changes of biosynthesis, leading to the emergence of new chemical compounds that don't serve any purpose or change in the receptors that receive the ability to perceive more lack of communication matter? These questions await detailed study.
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Vladimir D. Ivanov, candidate of biological Sciences, associate Professor Department of entomology Institute of biology and soil faculty, St. Petersburg state University and the Department of Zoology at the Forestry Academy. Research interests: evolution and systematics of insects of the order Trichoptera, the evolution of flight and communication of insects. The author of over 60 publications, co-author of two monographs.