Insect pheromones areneurotransmitters that serve the chemical communication between individuals of an insectspecies. They thus differ fromkairomones, in other words, neurotransmitters that transmit information to non-species organisms. Insects produce pheromones in specialglands and release them into the environment. In the pheromone receptors of the sensory cells of the recipient, they produce a nervestimulus 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 acolony ofsocially 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 theunsaturatedfatty alcohol bombycol, the sex pheromone of thedomestic silk moth (Bombyx mori), as the first known insect pheromone. The sex pheromones of femalebutterflies are mostly mono- or bis-olefinicfatty acids or theiresters, fatty alcohols, their esters or the correspondingaldehydes. Male butterflies use a wide range of chemicals as sex pheromones, for examplepyrrolizidine alkaloids,terpenes andaromatic compounds such asbenzaldehyde.
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 producedNasanov pheromone containing terpenes such asgeraniol andcitral to attract bees to an unusedhive. The agriculture and forestry industries use insect pheromones commercially inpest control usinginsect traps to prevent egg laying and in practicing themating disruption. It is expected that insect pheromones can also contribute in this way to the control of insect-borne infectious diseases such asmalaria,dengue fever orAfrican trypanosomiasis.
Adolf Butenandt andPeter Karlson proposed the term pheromones in 1959 for substances that serve intraspecific communication.[2] The definition of the term pheromone was given in the same year by Karlson and the Swiss zoologistMartin 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.[3]
The word pheromone consists of theancient Greek parts of speechφέρειν phérein,überbringen,melden, andὁρμᾶν hormān, antreiben, erregen.[3][4] 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 asuffix, as it occurs in the wordshormone,kairomone, andallomone thus emphasizing their relationship.[5] The term pheromone replaced the term ectohormone or homoiohormone, which Albrecht Bethe had already proposed in 1932 with the same definition.[6] 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 thecirculatory system by another individual and was therefore considered misleading.
The classification of intraspecific pheromones in the group ofsemiochemicals, in other words,neurotransmitters that serve communication between organisms, is shown in the following diagram:[7]
Karlson further divided them into thesense of smell and theoral-acting insect pheromones according to the mode of reception.[8] In 1963,Edward O. Wilson, who had discoveredant trace pheromones the year before, and William H. Bossert introduced the concepts of releaser and primer pheromones.[9][10] Releaser pheromones, which are usually perceived olfactorily, cause an instantaneously observablebehavioral reaction, whereas primer pheromones, which are often oral, trigger physiological changes in the recipient. Primerpheromones, for example, suppress the formation of ovaries inworker bees.
Often, pheromones are defined according to their behavior-triggering function. In addition to the well-known sex attractants, they act, among other things, asaggregation 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.[11] 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 theGerman cockroachBlattella germanica contains both substances that act as attractants and substances that act as arrestants.[12]
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.[13] Thequeen bee of theWestern honey bee produce the queen bee pheromone inmandibular glands. In English, they are therefore often referred to as queen mandibular gland pheromones.[14]
In 1609, the EnglishbeekeeperCharles Butler observed that the sting of abee released a liquid. This liquid attracted other bees, which then began to sting en masse.[15] Butler thus demonstrated for the first time the effect of an alarm pheromone of bees, which was identified asisoamyl acetate in the 1960s.[16] 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, SirJohn Ray suspected thatpeppered 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."
"It developed from a stick-shaped birch moth caterpillar: it was a female, and came out of her chrysalis, which was shut up in my cage: the windows were open in the room or chamber where she was kept, and two male moths flying about were caught by my wife, who by a lucky chance was in the room that night: they were attracted, it seems to me, by the scent of the female, and came in from outside."
– John Ray[17]
The FrenchentomologistJean-Henri Fabre also reported in the mid-19th century on experiments withSaturnia andoak eggar in which females trapped in wire cages attracted hundreds of males within a few days at specific times.[18] In experiments with taggeddomestic 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.[18] 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 byinfrared or other radiation were not confirmed.[18] The organization ofeusociality remained equally inexplicable for a long time. The writer and bee researcherMaurice Maeterlinck speculated about the spirit of the hive, the (team) Spirit of the hive, without being able to determine its essence in more detail.[19] Theories about the attraction by infrared or radiation also remained unexplained for a long time.
At the beginning of the 20th century,Ernest Starling discoveredhormones as the first biological neurotransmitters.[20] In 1932, theneurophysiologistAlbrecht Bethe, who at that time headed the Institute ofAnimal Physiology at theGoethe University Frankfurt, published an article on an expanded hormone concept in which he distinguished between endohormones and ectohormones.[6] 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 thelactation hormone, which is released by afetus to its mother and causes the growth of themammary gland and subsequently thesecretion of milk.[6] He also proposed this concept for chemical communication among insects.
"In bees, for example, the workers (i.e. not the mothers) 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 (although it has not been proven) that ectohormones of the salivary gland secretions play the main role in this redifferentiation."– Albrecht Bethe[6]
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 ofallelochemicals.[6]
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 (Bombyx mori). It is a butterfly originally native to China, belonging to thefamily of theBombycidae, which is used forsericulture and its breeding and keeping was well known. It was only after almost 20 years of work that he finally succeeded inextracting and purifying a substance from more than 500,000 insects, which Butenandt later namedBombykol.
Byelemental analysis, Butenandt determined thechemical formula of the substance to be C16H30O.Infrared spectroscopic studies indicated the presence ofconjugated double bonds. Using methods common at the time, such as catalytichydrogenation,melting point determination, and oxidative degradation bypotassium permanganate, Butenandt showed that the substance sought was anunsaturatedfatty alcohol, (10E,12Z)-10,12-Hexadecadien-1-ol.[21]
Butenandt then synthesized bombycol fromvernolic acid [(12R,13S)-Epoxy-9-cis-octadecenoic acid] in several steps via diol formation, its cleavage to thealdehyde,double bond isomerization, andWittig reaction. He synthesized the four possiblestereoisomers and tested them for theirbiological activity.[22] 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[23]
Towards the end of the 1950s,Edward O. Wilson defined substances that trigger the alarm and burrowing behavior of ants as chemical releaser.[24] In 1961, the BritishbiochemistRobert Kenneth Callow identified another pheromone, also known as the queen bee pheromone, with the compound(E)-9-oxo-dec-2-enoic acid, or9-ODA for short.[25] The effect of this pheromone was obviously of a different nature than that of the alarm pheromones, since it had a long-term effect on thephysiology of the recipients.
In 1963, Wilson, who the year before had already discovered the trace pheromones of ants, and William H. Bossert introduced the term releaser and primer pheromones for this purpose, to distinguish the behavior-controlling effect of, for example, sex attractants from the pheromones that interfere with theendocrine system of the receiver.[9][10]
As a result of the enormously refined extraction andanalytical chemistry over the years, chemists and biologists identified numerous other pheromones. For the detection of the second component of the pheromone cocktail ofBombyx mori, the bombycal [(10Z,12E)-hexadecadienal], an extract of 460 glands from which 15nanograms of the aldehyde were isolated was already sufficient in 1978.[26]
In addition to studying the function and reception of pheromones and their chemical identification, scientists extensively investigated thebiochemistry of pheromone production. In 1984, Ashok Raina and Jerome Klun discovered that the production of the female sex attractant of theowlet mothsHelicoverpa zea is controlled by hormonal substances called pheromone biosynthesis-activating neuropeptides (PBANs) in the brains of female moths.[27] Other modern research focuses on the study of insect pheromone reception by means of the sense of smell and taste, genetic factors, and evolutionary biology issues, such as thecoevolution of female sex pheromone production and reception in the male.[28]
Combating disease vectors such asAnopheles is another focus of research. According toWorld Health Organization estimates, the number of malaria infections in 2012 was about 207 million with 627,000 deaths.[29]Culex mosquitoes transmit the causative agent offilariasis orWest Nile virus. Traps equipped with oviposition pheromones offer one way to contain these populations. To optimize these, the scent-binding proteins in the antennae of females, which play a critical role in detecting oviposition sites, are being intensively studied.[30]
The pheromones in insects are often the secondary products of fatty acids, such as saturated and unsaturated hydrocarbons, fatty alcohols, esters and aldehydes, but also isoprenoids and other compounds. Pheromones are often not pure substances, but so-called pheromone cocktails consisting of various components. Often, only one specific enantiomer of a compound triggers a behavioral reaction, while the other enantiomer triggers no reaction or a different reaction.
Sometimes the biosynthesis of the pheromone occurs only when the biochemical precursors in the form of certain alkaloids have been ingested from food plants. In this case, the sex pheromone simultaneously signals the presence of food sources.[31]
Due to the potential commercial application in crop protection, the intensity of the study of pheromones increased greatly after Butenandt's discovery, leading to the development of highly sensitive analytical methods[32] and the widespread application of chemo-, regio-, and stereoselective syntheses in organic chemistry.
Insect pheromones are produced by a variety ofexocrine glands consisting primarily of modifiedepidermal cells at various sites on the insect body. For example, the abdominal glands of the female silkmoth release traces of the (E,E)-isomer ofalcohol as well as the analogous (E,Z)-aldehyde bombycal, in addition to the sex pheromone bombycol.[33][34] Suitable surface geometries in the vicinity of the glands, such as grooved pore plates, may favor effective evaporation of a leaked pheromone.[35] Honey bees possess 15 glands with which they produce and release a number of different substances, thus maintaining a complexcommunication system based on pheromones.[36][37] Males of various butterfly species possess so-called androconial organs in the abdomen with which they can disseminate pheromones, while other moths release them viascent shed or scent bristles on their forewings or the end of the abdomen. The scent bristles and scent scales serve to increase surface area and facilitate the evaporation of insect pheromones.[35]
Rather than evolving a completely unique set ofenzymes for pheromone biosynthesis, insects often modify normal metabolites into pheromones with high regio-, chemo-, (E/Z)-, diastereo-, or enantioselectivity and in well-defined ratios.[38] Biosynthesis of insect pheromones occurs eitherde novo following the scheme offatty acid synthesis by successive addition ofmalonyl-CoA to an initialacetyl or by uptake ofprecursors from the diet. Many butterflies use the biosynthetic possibility of producing a specific mixture ofderivatives of simple fatty acids. The evolution of the enzyme Δ-11-desaturase combined with chain-shortening reactions allows them to produce a variety of unsaturatedacetates, aldehydes, and alcohols in various combinations.[39]
Dehydrogenation of thecarbon chain andreduction of the acid function to the alcohol occurs, if necessary, through special enzyme systems. Further steps can beoxidation to the aldehyde oracetylation to theacetate.[40] InBombyx mori, the biosynthesis is activated diurnally by pheromones through aneurohormone, the so-called pheromone biosynthesis-activating neuropeptide (PBAN).[40] The pheromone biosynthesis-activating neuropeptide (PBAN) is a neurohormone.
The hormonal mechanisms of pheromone production vary considerably from species to species.[41]Juvenile hormones, for example, control pheromone production in the owl butterflyMythimna unipuncta. These are produced in mostly pairedcorpora allata located behind the brain and released into thehemolymph. There they bind to certain transport proteins. If the corpora allata are removed, the females do not produce pheromones. Juvenile hormones, however, interfere more indirectly with thecircadian release of PBAN.[42]
Male butterflies of theDanainae family sometimes wound caterpillars that have ingested alkaloids frommilk weeds with tiny claws on their feet to ingest the exuding fluid, a behavior described as kleptopharmacophagy. The moths use the ingested alkaloids for defense against predators and to produce sex pheromones.[43]
Malefire-coloured beetles of the speciesNeopyrochroa flabellata and also various other beetle species use theterpenoidcantharidin as a sex pheromone and aphrodisiac pheromone, respectively. Thisisoprenoid is ingested byNeopyrochroa flabellata with food and transferred to females and subsequently to the brood during the mating act.[44] Females test the content of a gland on the head of the male before mating. The cantharidin acts as a feeding toxin and renders the eggs unpalatable to predators; females therefore prefer males with a high cantharidin content.[47]
Moths such asUtetheisa ornatrix andTirumala limniace ingestpyrrolizidine alkaloids from food plants such ascrotalaria,heliotropium, orAchillea ageratum in thecaterpillar, which the adult male converts by oxidation into pheromones such ashydroxydanaidal. As in the fire beetle, the alkaloids, which are potent feeding poisons and act against predators such asspiders,ants, ornet-winged insects, are transferred to females and eggs.[45] Adultmonarch butterflies ingestplant secondary metabolism to increase their pheromonal attractiveness.[46] Sometimes biosynthesis of the pheromone occurs only when biochemical precursors in the form of certainalkaloids have been ingested from food plants. In this case, the sex pheromone simultaneously signals the presence of food sources.[47]
The uptake of pheromone precursors from plants is also known for certain species oforchid bees andpeacock flies. Male bees collect a mixture ofterpenoids fromorchids and use them as an aggregation pheromone to formlek mating. Sometimes the plant constituents control the development of the pheromone glands of male butterflies.[48]
Karl Ziegler andGünther Otto Schenck succeeded in synthesizing cantharidin as early as 1941.[49][50] The preparation of pheromones requires the use of highlychemo-,regio- andstereoselective syntheses. In the 1970s,asymmetric synthesis using theSAMP method succeeded in producing various pheromones enantiomerically pure.[51] Furthermore, chemists used asymmetricepoxidation, asymmetricdihydroxylation,biocatalysis,olefin metathesis, and many other stereoselective reactions to synthesize pheromones.[52] TheWittig reaction is suitable for thechemical synthesis of pheromones with (Z)-olefinicdouble bonds.[53]
Genetically modified tobacco plants can also produce sex pheromones. The fatty alcohols obtained from them by extraction are subsequentlyacetylated to obtain the respective target sex pheromones. This semisynthetic route of production produces insect pheromones in relatively large quantities and with high purity.[54]
Chemical communication between living beings by means of pheromones follows the same principles as technicaldata transmission. Atransmitter, for example thegland of a female insect, emits the signal in the form of a chemicalsubstance. Both the chemical structure of themolecules and their quantity ratio determine theinformation content and serve as amodel of communication for the species. The physical properties of the substances, such asvapor pressure, determine the function of their molecules as short- or long-range information transmitters.[55]
The insect pheromone is transmitted by direct contact or via a medium such as water or air. From thereceiver, for example thepheromone receptors in theantenna of a male insect, the substance is received and triggers a behavioral response. The term antenna was first used to refer to the antennae of insects and subsequently in engineering.[56] Insect pheromones have a highly species-specific effect, meaning that they elicit the desired behavioral response only inbiological specificity but not in individuals of other species. For example, although the chemical compounds that act as sex pheromones in butterflies may be the same in different species, the composition of the pheromone cocktail is different in all species.[55] In addition, pheromone cocktails often contain substances that act as behavioral inhibitors for other species, such as significantly reducing the rate of approach of males of alien species to an attracting female.[56] The pheromone cocktail is a highly effective means of inhibiting the behavior of males of other species.
The pheromones are usually produced as a liquid and are either transmitted by direct contact or released into the environment as a liquid orvapor. They can be either heavy orlight volatile. Diffusivity significantly affects the function of the pheromone.[57] Alarm pheromones are often highly volatile to spread quickly bydiffusion. Therefore, they are often short-chain substances with relatively high vapor pressure and low complexity.[40] There is no high requirement for species-specificcoding effect as in sex pheromones. Sex pheromones have a higher complexity than most alarm pheromones, but a lower molar mass than marker pheromones, which permanently indicate an area.[58]
In the case of flying insects - such as butterflies - the pheromone as a molecule must not be too large, otherwise thevapor pressure and volatility are too low. Thus, over 200 identified sex pheromones of butterfly species are mono- and bis-olefinicfatty aldehydes, fatty alcohols and their acetates with chains of 10 to 18 carbon atoms.[40]
Depending on the function, there are different emission and reception scenarios. Ants, for example, emit alarm pheromones intermittently or continuously in the usually windless environment of the anthill. Trace pheromones are emitted by an ant as a moving source. Silkmoth sex pheromones are emitted in discrete scent threads in an air stream.[57]
Male monarch butterflies do not emit volatile pheromones, but pheromone-ladennanoparticles called pheromone transfer particles, which they use to transfer arrestants or aphrodisiac pheromones to females. The pheromone transfer particles position the males on their brush hairs and scatter them during courtship flight. The nanoparticles adhere to the antennae of the females, which are equipped with pheromone receptors, where they slowly release the pheromones, resulting in a long-lasting stimulus for the female.[59]
Females of theArctiinaePyrrharctia isabella emit anaerosol consisting exclusively of sex pheromone droplets. The amount of pheromone released in this process is much greater than in other known female moths. The apparent wastefulness of the sex pheromone is explained by the short amount of time an adult has to find a reproductive partner due to the short Arctic spring.[60]
Recipients usually perceive pheromones in an environment characterized by the presence of many other chemicals. To ensure specific perception, the pheromone chemical must either be so complex that it does not occur more than once in nature, or the correct ratio of several individual components must trigger the stimulus. However, it has been shown that only in exceptional cases does a single substance convey the message. Often, amixture of substances must be present in very precise proportions that, in addition to the chemical structure of the individual pheromones, determine the informational content of the pheromone cocktail.[40]
The chemical structure of pheromones is directly related to their signaling function andsignaling environment. Pheromones released into the air often have acarbon chain of 5 to 20 atoms and amolar mass of about 80 to 300 g-mol-1. With a carbon chain of less than five carbon atoms, the number of possible isomers is small and targeted species-specific coding is difficult.[58] With longer carbon chains, the number of possible isomers increases rapidly.
Periplanon B, the sex pheromone of theAmerican cockroach, is an example of a complex single substance to which males respond at extremely low levels of 10−5nanograms.[61][62]
The sex pheromone cocktail emitted by a female insect spreads downwind. In the recipient male, the molecules strike the antennae, where reception of the pheromones takes place by means of olfactory cells on the olfactory hairs orsensilla. The antennae adsorb about 30% of the pheromone molecules contained in an airstream.[56] The remaining molecules hit the outerexoskeleton where they are enzymatically degraded.
The pheromone molecules first reach thecuticle of the olfactory hairs and diffuse through pores into a pore cone and from there intotubules. From there, the molecules diffuse further to thedendritic membrane.[40] This membrane has receptors that, when a pheromone is received, cause a change in electrical resistance via the opening ofion channels, creating anelectrical resistance that results in aperception.[40] Even a single pheromone molecule can trigger a nerve impulse.[59] However, the recognition of a specific pheromone cocktail requires a certain level of excitation of differentcell types of varyingspecificity.[56] It is assumed that the characteristic excitations received from the different receptors in thecentral nervous system are modulated there into an excitation pattern. If this excitation pattern, which depends on the quantitative ratio of the received pheromone molecules, matches the coding of an innatebehavioral pattern, this leads to the triggering of a corresponding behavioral response, such as the headwind approach to a pheromone source.[55]
According to their effect, two classes of pheromones can be distinguished, the primer and the releaser pheromones. Under certain conditions, certain pheromones act as both releaser and primer pheromones.[63]
Releaser pheromones have a brief, immediate behavior-controlling effect. The first pheromone discovered, bombycol, is an example. Releaser pheromones typically include aggregation pheromones, dispersal pheromones, alarm pheromones, tracking pheromones, and marking pheromones, among others, in addition to the well-known sex pheromones.[63]
Aggregation pheromones are produced by both sexes and are used for the sex-nonspecific attraction of individuals of the same species. These are known, for example, in thebark beetle and other beetle species,bipeds,hemiptera andgrasshoppers. Insects use aggregation pheromones for defense against predators, inmate choice, and to overcomehost plant resistance to mass attack. A group of individuals at a site is referred to as anaggregation, regardless of sex.[64] Aggregation pheromones, in addition to sex attractants, play a significant role in the development of pheromone traps for selective pest control.[65]
Studies usingelectroantennogram techniques showed that aggregation pheromones elicited relatively high receptor potentials in a sex-nonspecific manner, whereas sex pheromones elicited high receptor potentials in only one sex. Aggregation pheromones may therefore be evolutionary precursors of sex pheromones.[35]
Sex pheromones signal the female animal's readiness tomating. Male animals also emit pheromones; they contain information about sex andgenotype. Many insects release sex pheromones; some butterfly species still perceive the pheromone at a distance of 10 kilometers. The sensory cell response in the male silkmoth begins at a concentration of about 1000 molecules per cubic centimeter of air.[56] The scent signal of a female, as soon as a certain concentration threshold is exceeded, initially triggers an oriented headwind flight in the silkmoth male.[56] In other species such as thecodling moth, on the other hand, the male tests thestereochemical purity of the attractant molecule. As soon as a small admixture of another stereoisomer is present in the pheromone cocktail, the approach to the source stops.[56] In this case, the other stereoisomer acts as arepellent.[40] In addition to the main components, some species release so-called short-range components in small amounts, which influence the behavioral response.[56] In addition to the main components, some species release so-called short-range components in small amounts.
Fouraging honey bees spread the scent of (Z)-11-eicosen-1-ol.European bee wolves are guided by this scent to prey on honey bees. Male bee wolves use this component, and thus the existing sensory preference of females for bee scent, as part of their sex pheromone cocktail to attract them.[66]
-_%2526_(S)-Oleane_Structural_Formula_V.1.svg)
Aphrodisiac pheromones stimulate the readiness to mate. Thespiroacetalolean, for example, is the aphrodisiac pheromone of theolive fruit fly (Bactrocera oleae). Only the (R)enantiomer is effective on males; the (S) enantiomer is ineffective on them. The female produces theracemate, responds to (R)- and (S)-olean, and also stimulates herself with it.[67]
So-called anti-aphrodisiacs have exactly the opposite effect.Bedbugnymphs protect themselves with such a pheromone, which has a specific mixing ratio of the aldehydes(E)-2-hexenal,(E)-2-octenal and4-oxo-(E)-2-hexenal, against mating attempts by male bedbugs. The latter directly drills a hole into the abdomen of sexually mature female bedbugs and injects its sperm there (traumatic insemination). For mated nymphs, however, such injury can be fatal.[68]
Some insect species emit alarm pheromones when attacked. These trigger either flight or increased aggression. In bees, for example, two alarm pheromone mixtures are known. One is released by the Koshevnikov gland near the sting and contains more than 40 different compounds, such asisoamyl acetate, already described by Butler in its effect, besidesbutyl acetate,1-Hexanol,1-Butanol,1-Octanol,hexyl acetate,octyl acetate and2-Nonanol. These components have lowmolar mass, are volatile, and are the most non-specific of all pheromones. Alarm pheromones are released when a bee stings another animal to attract and entice other bees to attack.Smoke suppresses the effect of alarm pheromones, which is exploited by beekeepers.[69]
The other alarm pheromone of the honey bee contains mainly2-Heptanone, another volatile substance released by the mandibular glands.[70] This component has a repellent effect onpredatory insects. The alarm pheromone cocktail of the bedbug contains unsaturated witch and octenaldehydes, which are perceived as a characteristic sweetish odor in bedbug-infested rooms.[71][72]
Certain insects, such as thecherry fruit fly, mark their oviposition sites in such a way that other females of the same species avoid the site and lay their eggs elsewhere to avoid competition for food among the offspring.[73] Territorial social insects, such as colonies of ants, also mark territories they claim with pheromones.[74]
The marking pheromones include the dispersion pheromones, with which, for example, bark beetles prevent overcolonization of a tree.[75] The females and nymphs of the German cockroach transmit dispersion pheromones in direct contact via their saliva. In the nymphal stage, these serve to deter adult cockroaches and thus protect againstcannibalism. In adults, they prevent the translocation of a habitat.[76]
Trace pheromones are mainly known in insects living incolonies, which mark their paths with low-volatile substances such as higher molecular weighthydrocarbons. Ants in particular often mark the path from a food source to the nest in this way.[77] As long as the food source exists, the trail is renewed. When the food source dries up, the ants spray over the trail pheromone with a repellent pheromone.[78] In 1921, the U.S. naturalistCharles William Beebe reported on theant mill phenomenon, which trace pheromones can trigger inarmy ants: If the animals are separated from the main trail of the colony, the blind ants follow the pheromone trails of ants running in front of them. These run in large circles until complete exhaustion or death, without finding their way back to the colony.[79]
Recruitment pheromones are widely used as an element of chemical communication in social insects and have been demonstrated for bees,termites, and ants. These pheromones are used by insects to stimulate other members of the colony to forage at a food source.[63]Bumblebees perform a dance similar to thewaggle dance primarily to distribute recruitment pheromones.[80]
The orderHymenoptera contains the largest group of eusocial insects, including manybees, especially of the subfamilyApinae, ants, and some species ofvespidae, especially of the subfamily ofVespidaes. Characteristics often include the presence of a reproductive queen andcastes with specializedworkers bees and soldiers. Termites form the second major group of eusocial insects. Colonies are divided into distinct castes, with a queen and king as reproductive individuals, workers, and soldiers defending the colony.[81] Primer pheromones have a major influence on the organization ofhymenopteran states formed by Hymenoptera and of termite colonies. These pheromones influence the hormonal system of the recipient; they often interfere withmetabolism via asignaling cascade or activate proteins that can bind toDNA. In contrast to the releaser pheromones, the primer pheromones are less well studied. For a long time, only one primer pheromone, 9-ODA, was known.
A well-known example of primer pheromones are the queen bee pheromones.[82] These pheromones control social behavior, comb maintenance, swarming, and ovary formation of worker bees. The components arecarboxylic acids andaromatic compounds. (E)-9-oxo-dec-2-enoic acid (9-ODA), for example, suppresses further breeding of queens and inhibits development of ovaries of worker bees. It is also a potent sex pheromone fordrones onnuptial flight.[83]
Brood recognition pheromones are emitted by larvae andpupae and discourage worker bees from leaving the hive while there are still offspring to be cared for. Furthermore, they suppress the formation of ovaries in worker bees. The pheromones consist of a mixture of tenfatty acid esters, includingglyceryl 1,2-dioleate-3-palmitate.[84] Worker pupae contain 2 to 5, drone pupae about 10 and queen pupae 30micrograms of the pheromone.
Older, fouraging worker bees releaseoleic acid ethyl ester, which inhibits the development of nurse bees and makes them care for brood longer.[85] The oleic acid ester acts as a primer pheromone and stabilizes the ratio of brood-caring and food-producing bees. The foragers produce it fromnectar mixed with traces ofethanol, which they feed to the nurse bees. This delays their development until the number of older foragers decreases and with it theexposure of nurse bees to ethyl oleate.
TheReticulitermes flavipes usesterpenes such asγ-cadinene and γ-cadinenal as caste-stimulating or -inhibiting primerpheromones. These assist the juvenile hormone in determining the position oftotipotent workers in the caste system.[86] In ants, female larvae possessbipotentiality for some time and thus the possibility of developing as either queens or workers. At some point in larval development, continued feeding determines the fate of the larva. If the juvenile hormone titer is raised above a certain threshold,gynomorphs develop; otherwise, workers develop. Control of larval feeding is governed by a primerpheromone of the queen ant.[87]
In the 19th century,Lymantria dispar escaped from entomologistÉtienne Léopold Trouvelot inMassachusetts and spread throughout the United States by the mid-20th century, becoming one of the most fearedpests today. As early as 1898, Edward Forbush and Charles Fernald made attempts to control the population of the gypsy moth by luring males into traps set with attractant females.[88]The United States Department of Agriculture continued these attempts in the 1930s, using extracts of female abdominal spikes to attract male moths.[89] The United States Department of Agriculture was the first to use this method. The application of insect pheromones inpest control has been intensively studied, especially since the first syntheses, with the aim of developing environmentally friendly methods to controlpopulation dynamics.[90]
In pest control, the use of pheromones inattractant traps to control insects is common practice. This can involve attracting the insects to kill them with aninsecticide or physically to trap them or formonitoring.Bark beetles are attracted with aggregation pheromones to trap them. The attractant is usually released when drilling into spruce wood, signaling that the tree can be colonized. The bark beetle trap is an important tool for controlling bark beetles.[91] However, the use of attractant traps poses the problem that the pheromone may act as akairomone and thus attract predatory insects. By reducing the population of natural predators of the bark beetle, the pheromone trap has a counterproductive effect in this case.[92] Monitoring by means of attractant traps, such aswindow traps, is used for the quantitative detection of pests to control them more specifically with insecticides depending on the activity detected. In addition, they are used in the identification of new species.[93]
Trap beetles work on the same principle as attractant traps. The bark beetles of the initial infestation attract further conspecifics by aggregation pheromones. Windthrow is suitable as trap beetles, which can be equipped withpheromone dispensers to enhance the attraction effect. Trees prepared in this way divert approaching bark beetles from the stand and bind them to controllable stems. The use of trap trees requires regular inspection of the trees. When larval galleries appear, the trees are debarked and larvae and pupae dry out. If necessary, the infested tree may be treated with insecticides or burned to prevent escape of the next generation.[94]
Egg-laying-preventing marking pheromones are widely used in the insect world. Various experiments have demonstrated the possibility of controlling population dynamics by these pheromones.[95] Application of the oviposition-preventing marking pheromone ofRhagoletis cerasi, which cannot be controlled withyellow boards, for example, reduced infestations of cherries by 90%.[96]
Another application is the confusion method or mating disruption. Here, a high substance concentration of artificially produced pheromones is applied. This makes it impossible for the males to follow the pheromones of the females, thus hindering the reproduction of the pest. The confusion method has a species-specific effect.[97] It is usually successful with respect to one species if sufficient dispensers are applied, but in some cases related species occupy theecological niche that is released.
Bees use theNasanov pheromone to guide worker bees back to the hive. The pheromone contains terpenes such asgeraniol andcitral. Beekeepers use a man-made product to attract bees to an unused hive.[98] The process is suitable for trappingAfricanized honey bees in trap boxes.[99]
Toxicological studies were mainly carried out in connection with the approval of pheromone traps and dispensers. A health hazard cannot be generally assessed due to the large chemical diversity of pheromones, but is usually excluded because only small amounts are emitted. However, in higher doses, orally administered pheromones such ascantharidin cause death in rare cases.[100]
Commercial application in crop protection intensified the study of pheromones and led to the development of highly sensitiveanalytical methods.[32] The identification of a pheromone proceeds through several steps. First, an extract of the pheromone is obtained. This is done by the method already used by Butenandt of extracting glands or whole animals with an easily evaporatedsolvent, ideally at the time of high pheromone production. Alternatively, the pheromone isadsorbed onactivated carbon from the gas phase and an extract is obtained with little solvent.[101] For very small traces,solid-phase microextraction is suitable. For identification, the extracts or the solid-phase microextraction samples are analyzed bygas chromatography-mass spectrometry.[101]
The electroantennogram technique is suitable for studying thebiological activity of insect pheromones.[55][102] In this technique, anelectrode inserted into the antenna main stem and an antenna branch measures the change inelectrical voltage as a function of theconcentration of pheromone molecules impinging on the antenna, which are transported to the antenna in a defined manner by an air current. [58] By varying the pheromone molecule, the influence of certainfunctional groups interacting with thechiral elements of the receptors can be determined.[55]
The coupling ofgas chromatography and electroantennogram allows the biological activity of the compounds present in an extract to be verified.[55] The shape of the electroantennogram depends on the fragrance component in the air stream, and the amplitude increases with the concentration and flow velocity of the air.[103]
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