Insect pheromones


Insect pheromones are neurotransmitters that serve the chemical communication between individuals of an insect species. They thus differ from kairomones, in other words, neurotransmitters that transmit information to non-species organisms. Insects produce pheromones in special glands and release them into the environment. In the pheromone receptors of the sensory cells of the recipient, they produce a nerve stimulus even in very low concentrations, which ultimately leads to a behavioral response. Intraspecific communication of insects via these substances takes place in a variety of ways and serves, among other things, to find sexual partner, to maintain harmony in a colony of socially living insects, to mark territories or to find nest sites and food sources.
In 1959, the German biochemist and Nobel Prize winner Adolf Butenandt identified and synthesized the unsaturated fatty alcohol bombycol, the sex pheromone of the domestic silk moth, as the first known insect pheromone. The sex pheromones of female butterflies are mostly mono- or bis-olefinic fatty acids or their esters, fatty alcohols, their esters or the corresponding aldehydes. Male butterflies use a wide range of chemicals as sex pheromones, for example pyrrolizidine alkaloids, terpenes and aromatic compounds such as benzaldehyde.
Research into the chemical communication of insects is expanding our understanding of how they locate their food sources or places to lay eggs. For example, beekeepers use an artificially produced Nasanov pheromone containing terpenes such as geraniol and citral to attract bees to an unused hive. The agriculture and forestry industries use insect pheromones commercially in pest control using insect traps to prevent egg laying and in practicing the mating disruption. It is expected that insect pheromones can also contribute in this way to the control of insect-borne infectious diseases such as malaria, dengue fever or African trypanosomiasis.

Etymology and classification

Adolf Butenandt and Peter Karlson proposed the term pheromones in 1959 for substances that serve intraspecific communication. The definition of the term pheromone was given in the same year by Karlson and the Swiss zoologist Martin Lüscher. According to this, pheromones are
"Substances released externally by one individual that elicit specific responses in another individual of the same species."
– Peter Karlson, Martin Lüscher, 1959.
The word pheromone consists of the ancient Greek parts of speech φέρειν phérein, überbringen, melden, and ὁρμᾶν hormān, antreiben, erregen. According to Karlson and Lüscher, the goal was to coin an internationally understandable scientific term for a class of substances based on a clear definition. It was to be a short word that could be spoken in many languages. The ending mone served as a suffix, as it occurs in the words hormone, kairomone, and allomone thus emphasizing their relationship. The term pheromone replaced the term ectohormone or homoiohormone, which Albrecht Bethe had already proposed in 1932 with the same definition. Bethe's term was not accepted because, according to Butenandt, the terms ecto and hormone were mutually exclusive. The mechanism of action of a pheromone also does not correspond to that of a hormone absorbed into the circulatory system by another individual and was therefore considered misleading.
The classification of intraspecific pheromones in the group of semiochemicals, in other words, neurotransmitters that serve communication between organisms, is shown in the following diagram:
Karlson further divided them into the sense of smell and the oral-acting insect pheromones according to the mode of reception. In 1963, Edward O. Wilson, who had discovered ant trace pheromones the year before, and William H. Bossert introduced the concepts of releaser and primer pheromones. Releaser pheromones, which are usually perceived olfactorily, cause an instantaneously observable behavioral reaction, whereas primer pheromones, which are often oral, trigger physiological changes in the recipient. Primerpheromones, for example, suppress the formation of ovaries in worker bees.
Often, pheromones are defined according to their behavior-triggering function. In addition to the well-known sex attractants, they act, among other things, as aggregation pheromones, dispersion pheromones, alarm pheromones, tracking pheromones, marker pheromones, brood recognition pheromones, egg-laying pheromones, recruitment pheromones, or as caste recognition agents.
Vincent Dethier divided insect pheromones into six categories according to their general behavior-triggering effects. These include prisoners, which are normally only perceptible at short distances and cause an insect in motion to stop, and locomotor stimulants, which increase the insect's speed or decrease the number of directional changes. Lockstoffe are attractants that trigger an oriented movement toward the odor source, whereas repellents trigger an escape movement away from it. Feeding and oviposition stimulants, respectively, trigger feeding or oviposition. Deterrents, on the other hand, inhibit feeding or oviposition.
Functionally defined insect pheromones often contain mixtures of different components in precisely defined proportions. These so-called pheromone cocktails often contain substances of different categories with near and far orientation functions. For example, the aggregation pheromone cocktail of the German cockroach Blattella germanica contains both substances that act as attractants and substances that act as arrestants.
In part, insect pheromones are named after the site of their biological production. Males of various moth species, such as the banana butterfly, possess so-called androconial organs in the abdomen that release pheromones. These insect pheromones are appropriately called androconial pheromones. The queen bee of the Western honey bee produce the queen bee pheromone in mandibular glands. In English, they are therefore often referred to as queen mandibular gland pheromones.

History

First discoveries

In 1609, the English beekeeper Charles Butler observed that the sting of a bee released a liquid. This liquid attracted other bees, which then began to sting en masse. Butler thus demonstrated for the first time the effect of an alarm pheromone of bees, which was identified as isoamyl acetate in the 1960s. Butler's observation was the first to show the effect of an alarm pheromone of bees, which was identified as isoamyl acetate in the 1960s.
As early as 1690, Sir John Ray suspected that peppered moths attracted male conspecifics by means of a scent:
"It emerged out of a stick-shaped geometer caterpillar: it was a female and came out from its chrysalis shut up in my cage: the windows were open in the room or closet where it was kept, and two male moths flying round were caught by my wife who by a lucky chance were into the room in the night: they were attracted, as it seems to me, by the scent of the female and came in from outside."
– John Ray

The French entomologist Jean-Henri Fabre also reported in the mid-19th century on experiments with Saturnia and oak eggar in which females trapped in wire cages attracted hundreds of males within a few days at specific times. In experiments with tagged domestic silk moth, 40% of males from a distance of four kilometers and 26% of males from eleven kilometers still found their way to a trapped female. The morphology of the moths was also reported in the mid-19th century.
In many insect species, researchers long puzzled over the mechanism of mating: visual or acoustic stimuli could not explain Fabre's experiments, nor how moths found females ready to mate with great certainty. Theories of attraction by infrared or other radiation were not confirmed. The organization of eusociality remained equally inexplicable for a long time. The writer and bee researcher Maurice Maeterlinck speculated about the spirit of the hive, the Spirit of the hive, without being able to determine its essence in more detail. Theories about the attraction by infrared or radiation also remained unexplained for a long time.

Definitions of Bethe

At the beginning of the 20th century, Ernest Starling discovered hormones as the first biological neurotransmitters. In 1932, the neurophysiologist Albrecht Bethe, who at that time headed the Institute of Animal Physiology at the Goethe University Frankfurt, published an article on an expanded hormone concept in which he distinguished between endohormones and ectohormones. According to this, endohormones act in the producing organism itself and correspond to the classical hormone definition. In contrast, the organism releases ectohormones externally and transfers them to other individuals. As an example, Bethe cited the effect of the lactation hormone, which is released by a fetus to its mother and causes the growth of the mammary gland and subsequently the secretion of milk. He also proposed this concept for chemical communication among insects.
"In bees, for example, the workers are able to raise a sexually capable queen from an egg or a young larva by special food and transfer of secretions of their salivary glands. There can hardly be any doubt that ectohormones of the salivary gland secretions play the main role in this redifferentiation."
– Albrecht Bethe
Bethe further divided the ectohormones into homoiohormones, which – according to today's definition of a pheromone – act on individuals of the same species, and alloiohormones, which act on individuals of a different species. He thus coined the precursor term of allelochemicals.

Works from Butenandt

Adolf Butenandt also suspected that communication among insects was based on neurotransmitters, and in the 1940s he began a project to identify the sexual attractant of the domestic silk moth. It is a butterfly originally native to China, belonging to the family of the Bombycidae, which is used for sericulture and its breeding and keeping was well known. It was only after almost 20 years of work that he finally succeeded in extracting and purifying a substance from more than 500,000 insects, which Butenandt later named Bombykol.
By elemental analysis, Butenandt determined the chemical formula of the substance to be C16H30O. Infrared spectroscopic studies indicated the presence of conjugated double bonds. Using methods common at the time, such as catalytic hydrogenation, melting point determination, and oxidative degradation by potassium permanganate, Butenandt showed that the substance sought was an unsaturated fatty alcohol, -10,12-Hexadecadien-1-ol.
Butenandt then synthesized bombycol from vernolic acid in several steps via diol formation, its cleavage to the aldehyde, double bond isomerization, and Wittig reaction. He synthesized the four possible stereoisomers and tested them for their biological activity. Only one isomer showed the same activity as the extract. Butenandt thus provided evidence that communication among insects takes place on a substance-by-substance basis.
"By extraction and condensation experiments, however, it has been convincingly shown that a material principle must be present which is secreted by the female butterflies from scent organs of the last abdominal segments and perceived by the males with their antennae."
– Adolf Butenandt