Pheromones (Insects)

Pheromones are chemical messages that induce a behavioral reaction or developmental process among individuals of the same species. The term is derived from the Greek for “carrier of excitation” and was coined in 1959 by the German biochemist Peter Karlson and the Swiss entomologist Martin Luscher during their investigations of the chemicals that regulate caste development in termites. In 1963, E. O. Wilson and W. H. Bossert of Harvard University formally distinguished two classes of pheromones. Releaser pheromones are messages that induce an immediate behavioral reaction in the receiver. The kinds of behavioral responses evoked in insects are incredibly diverse, and they include alarm, defense, aggregation, attraction, kin and colony recognition, marking of territories and egg deposition sites, mating behaviors, recruitment, trail following, and even thermoregulation. In contrast, primer phero-mones cause a physiological change in the receiver, such as development of a particular caste or sexual maturation, which eventually modifies the organism’s behavior.
All pheromones fall under the broader umbrella classification of semiochemicals—chemicals that are involved in communication. The two major classes of semiochemicals besides pheromones are allomones and kairomones. These are solely interspecific cues, in contrast to pheromones, which are always intraspecific signals. Allomones are chemicals that provide some advantage to the emitter (e.g., defensive secretions), whereas kairomones are cues that confer an advantage to the receiver (e.g., emanations used by a parasite to locate a host). This article only touches on the diversity and complexity of pheromone-mediated behaviors and developmental changes in insects. Communication among social insects, especially among ants, bees, wasps, and termites, involves a highly sophisticated pheromonal language, in which the interpretation of the individual chemical constituents or “words” depends on their particular combinations, ratios, concentrations, and even order of presentation. Context, that is, the recent experiences of the receiver and its physiological state, is all-important in response.


SEX-ATTRACTANT PHEROMONES OF LEPIDOPTERA

The first definitive evidence of pheromone communication dates to experiments performed with the great peacock moth Saturnia pyri by the French naturalist Jean-Henri Fabre in the 1870s. Fabre sequestered a female moth in a screened cage following her morning emergence, to permit her wings to expand and harden. That evening more than 40 male moths arrived at Fabre’s study, “eager to pay their respects to their marriageable bride born that morning.” Further observations showed that cages that had housed virgin females also were attractive; this and other observations led Fabre to conclude that “effluvia of extreme subtlety” mediated attraction. Nearly 90 years would pass before microanalytical techniques would permit identification of the minute quantities of pheromone involved.
The first pheromone to be identified was the sex-attractant pheromone of Bombbyx mori. the commercial silkworm. This silkworm is an entirely domesticated species that is no longer capable of flight; its female-emitted pheromone (Fig. 1) induces upwind walking and courtship behaviors in conspecific males. The German biochemist Adolf Butenandt (who received the Nobel Prize in 1939 for his work identifying the human sex hormones) spent more than two decades in this quest. In 1959 he identified (EZ-10,12-)-hexadecadienol as the single compound causing upwind walking and copulatory attempts and named it “bombykol.” To provide some perspective on this remarkable achievement, Butenandt and his coworkers extracted half a million female moths, finally isolating a few milligrams of the pure pheromone. Today’s modern methods of isolation and characterization (especially coupled gas chromatography and mass spectrometry) were not yet available.
The amount of pheromone that is secreted from or is present in a pheromone-producing gland varies enormously with species and, to some extent, behavioral function. Sex-attractant pheromones may be present in microgram, nanogram, and even picogram (10~12g) quantities per individual. Microanalytical techniques are now so advanced that identifications on occasion can be made from either gland
Representative structures of lepidopteran (moth) phe-romones. (A) Bombyx mori, the commercial silkworm (Bombycidae), (B) Hemileuca electra (Saturniidae), (C) Pectinophora gossypiella, pink bollworm (Gelechiidae), (D) Keiferia lycopersicella, tomato pinworm, (Gelechiidae), (E) Lymantria dispar, gypsy moth (Lymantriidae), and (F) Grapholita molesta, oriental fruit moth (Tortricidae).
FIGURE 1 Representative structures of lepidopteran (moth) phe-romones. (A) Bombyx mori, the commercial silkworm (Bombycidae), (B) Hemileuca electra (Saturniidae), (C) Pectinophora gossypiella, pink bollworm (Gelechiidae), (D) Keiferia lycopersicella, tomato pinworm, (Gelechiidae), (E) Lymantria dispar, gypsy moth (Lymantriidae), and (F) Grapholita molesta, oriental fruit moth (Tortricidae).
extracts or airborne collections from a few individuals, and with just nanogram or even lower quantities of natural chemical. Even the always tedious behavioral bioassays, long used to monitor for behav-iorally active components of gland extracts and airborne collections, have been largely supplanted by using a living insect antenna as a detector. The electroantennogram (EAG) was pioneered in the mid-1950s by Dietrich Schneider, working at the Max Planck Institute near Munich, Germany. In the 1970s, Wendell Roelofs of Cornell University adapted this assay to speed up identifications. A moth antenna was used to monitor which fractions separated by gas chro-matography contained the active components. Later applications mounted the EAG apparatus at the outflow of a gas chromatograph column, and this living detector indicated the presence (and the retention times) of compounds that were likely to be behaviorally active by means of the electrical signal elicited by the interaction of pheromone and the antennal receptors. These advances allowed chemists to zero in quickly on the compounds present in crude extracts that were most likely to comprise the pheromone.
The sensitivity of male silkworm to bombykol is legendary. It has been investigated by recording the electrical response of individual sensory hairs on their antennae (each antenna is equipped with 40,000 such hairs) and by monitoring a male’s change from quiescence to wing fanning and upwind walking. The estimates are astonishing: one bombykol molecule is sufficient to induce the firing of an individual receptor, and a behavioral response can be evoked with only 200 molecules (~10~19g!).
Pheromone structures now have been described for several hundred species of moths. Nearly all these pheromones induce upwind flight by the male to the pheromone-releasing female. The majority of known structures for moth pheromones (examples in Fig. 1) are hydrocarbon chains, usually 10-18 carbons in length, with 1-3 double bonds and terminal acetate, alcohol, or aldehyde. Less common structural motifs in moth pheromones include epoxides, ketones, and hydrocarbons with one or more double bonds or methyl branches; chain lengths known so far range from C10 to C25. Many pheromones, such as those of the moths Hemileuca electra and Grapholita molesta, comprise blends of two, three, or even more components. Specificity of the chemical message is accomplished in many species by females emitting and males responding to precise ratios of their pheromone blend. For example, for males of G. molesta, the ratio of the (Z)-8-and (E)-8-dodecenyl acetate components must be very close to the 95:5 mix produced by the female for maximum attraction. The use of blends and, in some species, precise ratios allows many closely related moth species to have “exclusive” communication channels, even though they share some components of their respective blends. Other strategies for partitioning of the communication channel include restricting sexual activity to specific times of the day or night (Fig. 2).
Pheromones of other types are produced by males of many moths and facilitate close-range recognition and acceptance by the female. In a few species the sexual roles are reversed, with male moths being the attractive sex and recruiting females. Many male butterflies also use pheromones in courtship, disseminating an “aphrodisiac” scent from scales on their wings or, in some butterflies, from specialized paired brushes located at the tip of the abdomen. However, butterflies do
Female of the day-active saturniid moth Hemileuca. electra exposing her pheromone gland, located at the tip of her abdomen. Such pheromone-releasing behavior, termed "calling," and the male's mate-finding behaviors typically occur at set times of the day or night. In the Mojave Desert of California H. electra calls from mid-morning to early afternoon; the closely related species H. burnsi, which shares pheromone components with H. electra, calls from midafter-noon to dusk. Without exclusive times for mating activities, these species would cross-attract.
FIGURE 2 Female of the day-active saturniid moth Hemileuca. electra exposing her pheromone gland, located at the tip of her abdomen. Such pheromone-releasing behavior, termed “calling,” and the male’s mate-finding behaviors typically occur at set times of the day or night. In the Mojave Desert of California H. electra calls from mid-morning to early afternoon; the closely related species H. burnsi, which shares pheromone components with H. electra, calls from midafter-noon to dusk. Without exclusive times for mating activities, these species would cross-attract.
not attract mates with long-distance pheromones; instead they rely on visual signals for mate finding.

ATTRACTANT AND AGGREGATION PHEROMONES

Although long-distance communication by attractant pherom-ones is well established in nearly all moth lineages, pheromones are widely used by many insect groups in mate finding. Such messages are categorized as either sex-attractant pheromones, if one sex attracts the other (as in moths), or aggregation pheromones, if both sexes are attracted. Feeding on a plant host and release of aggregation pheromones typically are linked, and mating often occurs in such aggregations. Therefore, aggregation pheromones can play a multifunctional role. Representative structures (Fig. 3) of sex attract-ants of insects other than moths include those of the cockroach, aphid, scale insect, caddisfly, sawfly, beetle, and true fruit fly. The chemistries of these messages are diverse, as are the locations of the glands responsible for their production.
 Examples of sex attractant pheromones of insects from nonlepidopteran orders: (G) periplanone B, from the American cockroach, Periplaneta americana, (H) nepetalactone, a pheromone component of a number of aphid species, (I) pheromone of California yellow scale, Aonidiella citrina, (J1 and J2) pheromone components of caddisflies, (K) pheromone component of diprionid sawflies, (L) (2R,3R)-2,3-hexanediol, a pheromone component of the cerambycid beetle Hylotrupes bajulus. and (M) (R)-1,7-dioxaspiro[5,5]decane, from the olive fruit fly Dacus oleae.
FIGURE 3 Examples of sex attractant pheromones of insects from nonlepidopteran orders: (G) periplanone B, from the American cockroach, Periplaneta americana, (H) nepetalactone, a pheromone component of a number of aphid species, (I) pheromone of California yellow scale, Aonidiella citrina, (J1 and J2) pheromone components of caddisflies, (K) pheromone component of diprionid sawflies, (L) (2R,3R)-2,3-hexanediol, a pheromone component of the cerambycid beetle Hylotrupes bajulus. and (M) (R)-1,7-dioxaspiro[5,5]decane, from the olive fruit fly Dacus oleae.
Pheromone components of Dendroctonus brevicomis, the western pine beetle. Myrcene is emitted by the beetle's principal host, ponderosa pine (Pinus ponderosa).
FIGURE 4 Pheromone components of Dendroctonus brevicomis, the western pine beetle. Myrcene is emitted by the beetle’s principal host, ponderosa pine (Pinus ponderosa).

BARK BEETLE PHEROMONES

Bark and ambrosia beetles (Scolytidae) use pheromones to facilitate colonization of host trees (aggregation) and to attract mates. Many scolytid species must attack a tree en masse if they are to overwhelm the tree’s defense, which consists of exuding sap into the tunnel that each beetle bores. The first beetles to arrive may identify the host by means of chemicals emitted by the host tree itself; as they bore into the tree, they release pheromones and increase emission of tree chemicals that together attract both male and female beetles. David Wood of the University of California at Berkeley and Robert Silverstein, then at Stanford Research Institute, worked out these intricate interactions in Dendroctonus brevicomis, the western pine beetle. Infestations begin when females are attracted to their principal host, ponderosa pine, by myrcene, a monoterpene the tree emits as a defensive compound (allo-mone) when injured or stressed, and by the tree’s silhouette. As the “pioneer” females bore into the tree’s bark, they release their pheromone, ( + )-exo-brevicomin (Fig. 4), which is augmented by increased release of myrcene from the host tree. Males are attracted and, after one enters the female’s tunnel, he emits ( — )-frontalin. The combination of host-, female-, and male-released volatiles attracts many more males and females, ensuring that the tree’s sticky sap defense will be insufficient. Once males and females have mated, they alter their chemical message: males and females emit verbenone and trans-verbenol and males release (+ )-ipsdienol (Fig. 4). Together, these three chemicals interrupt further attraction of males and females, thereby helping to regulate the level of infestation and avoiding overexploitation of the tree.

PHEROMONES OF SOCIAL INSECTS

Pheromones mediate many activities of social insects, including defense of the colony, recruitment to food, recognition of individuals and nestmates, and regulation of caste development. The exocrine glands that produce the various pheromones are dispersed throughout the body, as exemplified by those of leafcutter ants (Fig. 5).
Location and function of pheromone-producing glands in leafcutter ants.
FIGURE 5 Location and function of pheromone-producing glands in leafcutter ants.

Alarm and Defense

Charles Butler was the first to describe the behavioral effects of an insect alarm signal. He recognized in 1609 that the stinger of a honey bee (Apis) worker impaled on human skin or clothing attracts more bees to sting that site. The multifunctional role of this signal is shown by the reactions to the same chemical signal of guard honey bees at the entrance to their hive. The presence of an intruder can cause a guard bee to release alarm pheromone from her sting chamber; she disseminates this message into the hive by wing fanning, thereby summoning many bees. These alerted bees seem to be “agitated, ” with rapid movements and mandibles agape, poised to defend their colony. The sting chamber has more than 20 known pheromone components, most of which induce either alerting or stinging. The other behavioral reactions evoked by components of the sting chamber include lowering the number of foragers departing the hive and repelling foragers that have arrived at a food source. The main constituent of this mixture-evoked alerting and, to a lesser extent, stinging is isopentyl acetate, and the amount per worker changes dramatically with the behavioral task that bees are performing. In the first few days of an adult worker’s life, when she is confined to housekeeping and brood-tending tasks inside the hive, essentially no isopentyl acetate is present. The amount rises to 4-5 |ig at several weeks of age, when she either assumes guard duties or starts foraging outside the hive. For the remainder of her life as a forager, the amount falls to approximately 2 |ig per stinger. Stinging itself seems to be released by many components of the sting chamber, including isopentyl acetate, and especially n-butyl acetate and 1-pentanol. As with many reactions of social Hymenoptera, the context in which the signal is released is crucial to the kinds of behavior evoked. Context must be taken into account in interpreting behavior and in devising diagnostic behavioral bioassays. For assays of honey bee stinging, for example, one standardized procedure is to provide a vibration of set intensity to the hive, followed by presentation in front of the hive entrance of a swinging target such as a cotton ball or piece of leather containing a candidate alarm pheromone. The number of bees attacking and stinging the target is used to score the level of response.
Defensive behaviors of social insects are quite varied and difficult to categorize into mutually exclusive behaviors. Detection of an alarm pheromone by an ant, for example, can cause it to splay its mandibles, raise its head, bite, and spray an odoriferous and irritating defensive secretion toward a perceived enemy. Unlike attractant and aggregation pheromones, which typically are carried downwind in a turbulent wind flow, alarm pheromones are often released either in relatively still air within the confines of the colony or at ground level, where wind is attenuated. In such situations, molecular diffusion largely or exclusively governs the distribution of pheromone, and the resulting concentration gradient of pheromone supplies potentially useful information about the direction toward the source of alarm. Thus, ants may run toward the source of pheromone (essentially up the odor gradient) or, at lower concentrations farther away from the odor source, movements may seem to be undirected with respect to the odor source. There are many kinds of defensive reactions and, in leafcutter ants, several glandular sources for alarm pheromones (Fig. 5).
An instructive example is provided by the defensive reactions evoked by some of the more than 30 mandibular gland components of the weaver ant, Oecophylla longinoda, worked out in considerable detail by John Bradshaw, Philip Howse, and Ray Baker at the University of Southampton. If a droplet of mandibular gland secretion is daubed onto a flat surface in still air, the volatile pheromone diffuses outward at a rate that is dependent on its molecular weight and its diffusion coefficient. A region in which the concentration of pheromone is above the minimum required to elicit a particular behavioral reaction is termed an “active space.” The active spaces of each of the four principal components of the mandibular gland secretion 20 s after its deposition are shown in Fig. 6. The sequence of defensive activities seems to be ordered by proximity to the odor source. At the outer limit
Active spaces in still air of the principal components from the mandibular gland of the weaver ant, Oecophylla longinoda. The pheromone has been deposited in the center and the boundaries of behavioral activity of each component 20 s later are represented.
FIGURE 6 Active spaces in still air of the principal components from the mandibular gland of the weaver ant, Oecophylla longinoda. The pheromone has been deposited in the center and the boundaries of behavioral activity of each component 20 s later are represented.
of the active space, worker ants encounter only hexanal above threshold levels. Ants show heightened levels of running with open mandibles, but their trajectory is not aimed toward the odor’s source. Ants that enter the 1-hexanol region, however, move up the odor gradient toward the odor’s source. Once they have reached the active space of 3-undecanone, this compound further facilitates orientation and lowers the threshold for biting, as does 2-butyl-2-octenal. Together these four compounds ensure that the ants are recruited to the site at which the alarm pheromone was released, and that they attack an adversary that has been marked with this secretion. Mandibular gland components involved in defense, including other active constituents in addition to the four main constituents, vary within a colony among castes and even between major and minor workers. There also is substantial variation in a given caste among colonies, suggesting that different colonies may have unique defensive codes.

Trail Following and Recruitment

Social insects use trails of varying permanence to exploit food resources and sometimes for colony movement and relocation. E. O. Wilson’s exhaustive study of what governs the persistence of the food trail of the fire ant, Solenopsis invicta, provides an example of how such systems function. A foraging worker that has encountered a suitable food source lays down a chemical trail by dragging its stinger sporadically along the ground as it returns to the nest. The trail phe-romone is a mixture of farnesenes [mainly (Z,E)-a-farnesene] from their Dufour’s gland, and at any given moment each ant contains only about a nanogram of trail pheromone. The trail from one individual does not persist for long—the active space falls below threshold in less than 2 min, and its effective length is not much more than a meter. Solenopsis can even adjust the amount of pheromone deposited on the trail by altering how firmly it drags its sting. The amount of pheromone on the trail is regulated by three factors: the number of ants returning, the proportion of ants laying a trail, and the amount that each ant contributes to the trail. When the food is gone or an ant cannot reach the food source because of other ants, any ant that cannot feed simply does not reinforce the trail.
The number of ants recruited to leave the nest for foraging is a direct function of the quantity of trail pheromone released by a returning forager: to induce nestmates to leave the nest and forage along the trail, the returning forager releases much more pheromone than is found along the trail itself. These simple rules followed by individuals allow mass recruitment, a sophisticated system whereby one group of ants transfers information about the quality of a food source some distance away to another group of ants. The seeming disadvantage of the impermanence of such trails is actually a useful feature of the system that permits ants to match the number of foragers to the quantity of the resource.
Trail communication also is used in relocation of Solenopsis colonies. Scouts that have discovered a suitable nest location lay down a trail that other workers follow to the same site. If the location is indeed deemed favorable by new workers inspecting the site, these workers add pheromone to the trail upon their return trip to the nest; this leads to an exponential increase in traffic. Eventually the brood is transferred to the new nest, and the queen follows. Trails close to the nest of some ant species can be relatively permanent, lasting days, and these are called trunk trails. The constituents of the trail pheromones are known for many ant groups, and they are produced from a variety of glandular sources, including the Dufour’s, poison, and sternal glands, and the hindgut. Trail pheromones also are widely used by termites in foraging activities. Chemical trails are also important in regulating foraging activities of colonial tent caterpillars (Lasiocampidae). Caterpillars that have located food on distant branches add pheromone to the silken trail on their return trip to the silk nest at which they spend their nonfeeding time. These marked paths are then followed preferentially by future foragers.

Queen Pheromone of the Honey Bee

As emphasized for alarm pheromones, a pheromone can communicate many meanings depending on context. The queen pheromone of the honey bee (Apis mellifera) exemplifies this principle, which has been termed “pheromonal parsimony” by Murray Blum of the University of Georgia. The queen pheromone is produced by the queen’s mandibu-lar glands and its five known components are 9-oxo-(E)-2-decenoic acid, ( + )- and (— )-9-hydroxy (E)-2-decenoic acid, methyl p-hydroxybenzoate, and 4-hydroxy-3-methoxyphenylethanol. The queen produces about 500 |ig of this mixture daily, most of which is picked up by the continually changing retinue of a dozen or so workers that constantly groom the queen. Trophallaxis (interchange of food), antennation, and grooming among colony members in turn disperse queen pheromone throughout the colony. The releasing functions of the queen pheromone include the “retinue” behavior (attendance and grooming of the queen—these behaviors require all five components), suppression of construction of new queen cells, and a delay in swarming.
Outside the hive, swarming bees without a queen are attracted to a source of 9-oxo-(E)-2-decenoic acid, but they will not form a cluster without the addition of 9-hydroxy (E)-2-decenoic acid. Drones (males) are attracted to virgin queens flying 10 or so meters above ground level by her release of 9-oxo-(E)-2-decenoic acid. Perhaps the most dramatic effect of the queen pheromone is in its governance of colony productivity. Without queen or queen pheromone in the colony, many workers remain idle. Queen pheromone stimulates comb construction, brood rearing, foraging, and food storage. Artificial application of queen pheromone increases all these activities. The queen pheromone also has a clear primer effect inasmuch as it inhibits development of the workers’ ovaries. In the absence of the queen (and the queen pheromone), egg production is triggered in up to one quarter of the workers, and these “false queens” and other workers in turn produce some queen pheromone.

Pheromones of Honey Bee Workers

Honey bee workers produce diverse messages from a number of exocrine glands. The Nasonov gland (which queens and drones lack) is situated on the seventh abdominal tergite. A worker exposes this gland by flexing its abdomen, usually while wing fanning and elevating its abdomen. The secretion contains mainly geraniol, geranial, and geranic acid, and these influence foraging, marking, and, when coupled with queen pheromone, clustering. Nasonov pheromone also is important to swarming. After a swarm has departed the hive, the first workers to arrive at a clustering site expose their Nasonov gland, and the scent attracts other flying workers. Nasonov secretions also are used in “house hunting.” A scout that has found a potentially suitable nest site returns to the swarm and communicates direction and distance by the dance language. Scouts release Nasonov phe-romone at the site, thereby helping to attract more bees to evaluate its suitability. Nasonov pheromone also is released by bees fanning at the hive’s entry; this odor (probably mixed with odors from the hive) seems to aid disoriented foragers in finding the hive’s entrance.
The mandibular gland produces mainly 2-heptanone, which is released by guard bees as an alerting pheromone, and possibly to mark an intruder. This compound, perhaps in combination with other components of the mandibular gland, also may be used to mark flowers that are no longer productive, thereby improving foraging efficiency. Other pheromones labeled “footprint” pheromones mark the nest entrance, and a thermoregulatory pheromone causes nurse bees that are incubating pupae to raise their body temperature by means of muscle contractions. Capping of brood cells is induced by a phe-romone consisting of mixture of four fatty acid esters. The examples of pheromonal communication in A. mellifera considered here provide a glimpse into the pervasiveness of pheromonal communication in this insect, and the diversity of reactions that can be mediated by pheromonal messages.

Termite Pheromones

The development of castes in termites seems to be governed by complex interactions between juvenile hormone, pheromones, and environmental conditions such as food availability and time of year. For example, in a colony of Kalotermes (a “lower termite”), the absence of a king and queen in the colony causes development of replacement (supplementary) reproductives from pseudergate workers, but with the establishment of a reproductive pair (or more), they secrete pheromones that induce pseudergates to eat the excess of reproductives. A queen-produced pheromone inhibits female pseu-dergates from becoming reproductively competent, and a male-produced pheromone similarly inhibits male pseudergates from becoming reproductive. In the absence of a queen, the king secretes a pheromone that stimulates production of females. The proportion of soldiers in the colony is also regulated by similar interactions. The identity of the pheromones that modulate the proportion of castes in a colony remains unknown.

WHEN PHEROMONES

BECOME KAIROMONES

It is also worth noting that parasitoids and predators have coevolved to exploit and manipulate the pheromones of their prey. For example, a group of clerid beetles uses the pheromonal signals of bark beetles to locate and invade the tunnels of their prey in the bark and cambium layers of conifers. Similarly, pentatomid bug species frequently suffer high levels of parasitism from parasitic flies from several families, or from specialist wasp predators. It has been unequivocally demonstrated that these parasitoids use the bugs’ pheromones as kairomonal cues to locate hosts. The flies and wasps are attracted specifically to components of their host’s pheromone blend. For both the predatory clerids attacking bark beetles and the fly and wasp species attacking pentatomid bugs, the attraction to the pheromones of their prey can be so strong that traps baited with the prey pheromones actually catch more of the parasitoids or predators than the target species.
However, illicit use of the pheromones of prey can go well beyond simply eavesdropping on pheromonal signals. In a fascinating example of coevolution, bolas spiders in the genus Mastophora (and several other genera) produce the pheromonal signals of their prey to lure the prey close enough to be caught by a swinging, sticky thread of silk. The prey are male noctuid moths responding to copies of the female pheromone. Even more extraordinary, there is evidence to suggest that within an hour or so, the spiders can change the phe-romone lures they produce, to maximally exploit the different times of flight of prey species that respond to differing pheromone blends.
A variety of insects that live inside nests and sometimes parasitize colonies of social insects such as ants and termites also have developed the ability to aggressively mimic the pheromonal signals of their hosts. For example, several staphylinid beetle species live inside termite nests, where they receive all their food from their hosts, and are groomed and cared for by their hosts as though they were termite brood. The chemical cues that both prevent the termites from recognizing the inquilines and induce the feeding and grooming behaviors closely mimic the true pheromones used by the termites for these functions. In an even more aggressive example, the larvae of some syrphid flies are obligate predators on the brood of their ant hosts. The fly larvae produce a blend of cuticular hydrocarbons that closely matches the hydrocarbon profile of the host’s brood, effectively camouflaging the fly. The camouflage is so good that if the nest is attacked, the worker ants will carry the fly larvae to safety as though they were ant brood.

APPLICATION OF PHEROMONES IN PEST MANAGEMENT

Insect pheromones have proven useful in pest control. Most of these applications use synthetic copies of pheromones that mediate either attraction or aggregation. Compounds are formulated in protective matrices or reservoirs that emit the pheromone over weeks or months. Pheromone-baited traps are used to detect exotic invaders, to decide whether pest levels are sufficient to warrant intervention, and to time the application of conventional insecticides or other control measures. For example, the spread of the gypsy moth (Lymantria dispar) in the United States is monitored with inexpensive pheromone-baited traps whose sticky internal surface ensnares males. Approximately 350,000 traps are deployed yearly to determine the extent of spread of the European strain of the gypsy moth from the eastern portions of the United States to the Midwest and South, or, especially along the west coast, to signal the occasional invasion of the Asian gypsy moth strain.
Pheromones also are used for direct population control. The tomato pinworm (Keiferia lycopersicella), for example, is a devastating pest of tomatoes in Mexico, largely because this moth is highly resistant to insecticides. Application of ” cocktails ” (mixtures of two or more insecticides) as many as 40 times during a crop cycle may not prevent complete crop loss. However, if a tomato field is blanketed with plastic dispensers that release micrograms per hour of synthetic pheromone, the emitted pheromone will interfere with normal mate-finding activities of males, even if the pinworm population is initially at high density. Just how mating disruption works is not fully established, but efficacy likely involves the additive effects of habituation of responsiveness (a presumed central nervous system phenomenon) and some competition for the male’s attention between the natural emitters, i.e. females, and the numerous sources of synthetic pheromone. The amount of synthetic pheromone needed to disrupt mate finding is quite small: typical application rates are several grams per hectare per week, and nearly all pheromones are nontoxic and nonpersistent. This technique is now commonly used to control several dozen moth species.

CONCLUSION

Pheromones are a dominant form of communication in most insects, and the messages conveyed serve myriad behavioral and physiological functions. Current microanalytical techniques permit identification of many of these messages, even though they occur in minuscule quantities. Studies expanding our understanding of the chemistry of these messages continue, although in many pheromone systems our ability to characterize the chemicals produced is in advance of our progress in understanding the evoked behaviors, particularly among the complex communication systems of social insects. Current frontiers of investigation in insect phe-romones include establishing how genes control biosynthesis, how these chemicals are transduced into an electrical signal in the receptor cells of the responder, and how the signals are processed in the brain, leading to a behavioral output. Only a few structures of primer pheromones have been elucidated, and consequently much remains to be learned about their mode of action.

Next post:

Previous post: