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Fabre, Jean Henri - PHEROMONES

species chemical insects males

[fabruh] (1823–1915) French entomologist.

Always poor, Fabre spent his working life as a science teacher and was aged 50 before he could spend all his time as a field entomologist. In 1878 he bought a small plot of land in Serignan, Provence, to make an open-air laboratory. There he observed and wrote about the insect world in a way which revitalized interest in it by others and which made him the best known of all entomologists. His early research was on parasitic wasps but his close studies of a variety of groups led him to write a 10-volume survey of insects, which remains a classic.

PHEROMONES

Chemical communication is a new branch of chemistry. Pheromones are the best-known examples of that subject. A pheromone is a substance (or mixture of substances) used for communication between individuals of the same species . There are other terms for chemical communication between species (eg between plants and insects, prey and predators, hosts and parasites). These chemical messages belong to the greater subject of semiochemistry (semio = signalling). The word ‘pheromone’ was coined by analogy to ‘hormone’, a chemical messenger within the individual ( pherein = (Greek) I carry, horman = excite).

The subject of pheromones can be traced to the work of , who discovered the attraction of male night-flying moths by females of their species, but it was only in the 1930s that it began to emerge that this attraction was chemical. The first pheromone isolated and identified was bombykol (see structure), the substance produced by females of the silkworm moth ( Bombyx mori ) to attract males for copulation. This work was completed by and co-workers in 1959 after years of investigation. It was followed by that of the gypsy moth ( Lymantria dispar ) in the USA and the isolation of the aggregation pheromone of species of bark beetle that attack and destroy trees, including the vector of Dutch elm disease.

The sexual attractant pheromones of some hundreds of species of moths and butterflies (Lepidoptera) that are agricultural pests have been identified. The pheromone compounds can be synthesized in the laboratory and incorporated into slow-release lures made of rubber or plastic, and these lures placed in traps with a sticky surface to catch adult males. Pheromone traps are used in three ways. In some cases sufficient males can be caught in the traps to prevent many females being inseminated and so the production of the next generation can be blocked, achieving complete control of the pest. In many more cases, in spite of massive capture of males, sufficient females are inseminated to produce a new generation. The traps can still be used for monitoring the state of development of the adult insects and spraying with insecticide can be limited to the time of build-up of the adults. This reduces the use (and cost) of insecticides and the amount of insecticide on the harvested crop. Thirdly, the traps can be used to assess the extent of infestation and decide whether insecticide spraying is necessary or economical. Another technique is mating disruption. Small sandwich-like pieces of plastic with the pheromone impregnated into the central portion are scattered through the crop. The males are unable to locate the females in the ubiquitous aroma. Mating disruption is remarkably effective in controlling the pink bollworm ( Pectinophora gossypiella ) in cotton. The advantage and disadvantage of pheromone traps is that they are effective for only one species. The use of pheromones in agriculture and horticulture is still limited compared to insecticides (perhaps a few million US dollars per annum, compared with billions of US dollars spent on insecticides), but is nevertheless of growing importance in the control of pest insects.

This kind of chemical communication is not confined to insects. Pheromones have been recognized in a wide range of species from bacteria, moulds, fungi, marine and terrestrial invertebrates, fish and mammals, right up to elephants. Today pheromones of hundreds of species have been purified and identified. Chemical communication probably reaches its highest development in the social insects (ants, termites, some bees and some wasps). A whole language of chemicals has evolved to mark sexual attraction, alarm and attack, guidance to food sources, dominance in the colony, to distinguish between colony members and aliens, and many other functions. Knowledge of some of these is already being used in commercial bee-keeping. Looking to all species, the commonest uses of pheromones are sexual attraction, conveying an alarm or flight message, and territorial marking. Many unusual actions in animals (eg the spraying of urine upwards and backwards by tigers and cats, the chinning on grass stalks by rabbits, the rubbing of facial glands on twigs by deer) are now recognised as applying territorial marking pheromones. The cat brushing its cheek against one’s leg is really marking the recipient with chemicals from its cheek glands. Individual recognition pheromones are common among mammals, but their structures are complex and largely unknown.

Pheromone substances may be synthesized from simple starting materials in the organism, or they may be modifications of substances ingested (eg alkaloids or terpenes from plants). Their chemical structures tend to fall into four groups: those derived from fatty acids (bombykol is an example); small molecules of volatile substances, such as 2,5-dimethylpyrazine, a component of the mouse urinary pheromone that delays onset of female puberty, which together with trimethylpyrazine (see structure) is also a component of some ant trail pheromones. Other pheromones are derivatives of amino-acids from dipeptides to large proteins; and steroids, particularly in vertebrates, where sexual hormones and derivatives also function as mating pheromones–eg, 17,20-dihydroxypregnenone, used by the female goldfish, Carassius auratus , to attract males at egg-laying and stimulate them to produce milt.

Pheromones are usually produced in special exocrine glands (those secreting to the external surface) but may also be present in saliva, urine or faeces. Infant animals can recognize their own mothers (as commonly observed in sheep) through pheromones, from birth. While the evidence for pheromones in humans is very unclear, it is now known that babies recognize their mother’s odour from birth.

Fabrizio, Girolamo [next]

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