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Semiochemistry

Dateline: 02/01/99

By Alan Bruzel

Semiochemistry is the study of chemical signaling between organisms. The two types of semiochemicals described here are pheromones and kairomones. Pheromones are those molecules secreted by an organism that evoke a response in individuals of the same species. For example, the unsaturated aliphatic alcohol bombykol, (E,Z)-10,12-hexadecadien-1-ol, is a mate-attracting pheromone elaborated by the female silkmoth, Bombyx mori.

Not all pheromones, however, attract potential mates. On the one hand, aggregation pheromones summon conspecific predators to a meal of palatable insects. The aggregation pheromone from the spined soldier bug, Podisus maculiventris, is being marketed as a means of attracting these beneficial insects to your garden. On the other hand, the pursued are not without their own chemical communications devices. When under attack, they emit alarm pheromones that warn others of their kind to stay away. The grain aphid Sitobion avenae produces the alarm pheromone (E)-ß-farnesene when it is disturbed.

But in the perpetual war between the eaters and the eaten, there are chemical messengers that act as double agents. (E)-ß-Farnesene may induce other aphids to flee, but, at the same time, it is believed to point the wasp Aphidius uzbekistanicus to the aphids' whereabouts. To the aphid, this semiochemical is an alarm bell; to the female Aphidius wasp, looking for a suitable host in which to deposit her eggs, it is a dinner bell.

In a similar fashion, (E)-2-decenal, one of the compounds emanating from the southern green stink bug Nezara viridula, attracts the parasitic wasp, Trissolcus basalis. A substance thus emitted by one species that evokes a beneficial response in another species (usually to the detriment of the emitting species) is called a kairomone. Yet another illustration occurs in the dandelion Taraxacum officinale, whose volatile aromas act as attractant kairomones to the hungry beetle Anomala octiescostata.

Before we leave this fascinating play of plot and counterplot, subterfuge and countersubterfuge, there is one more point to ponder: things are even more complex than they appear. It seems that plants do not stoically allow themselves to be eaten without coming to their own defense. Cotton plants chewed upon by caterpillars release volatile compounds that attract the natural enemies of these caterpillars, and the interactions between the grain aphid and the wasp require the intervention of volatile components derived from the grain plant undergoing aphid attack.

Recommended Web resources for additional information:

Analogues of Semiochemicals: Scientific Fancy or Practical Potential?
Abstract from 13th Annual Meeting of the International Society of Chemical Ecology, 1996.

Anti-Inflammatory and Analgesic Sex Pheromone
Aromatic alkaloid from a beetle (Phyllopertha diversa) with putative therapeutic effects in humans.

A Secret Sense in the Human Nose?
From Seeing, Hearing, and Smelling the World. A series from the Howard Hughes Medical Institute.

Chemical Ecology of Phytophagous Scarab Beetles
Review article by Walter Soares Leal characterizing beetle pheromones and plant kairomones.

Fish Behavior and Chemical Communication
Research of chemical attractants used by fish to recognize siblings and to attract mates. From Department of Environmental Toxicology, Uppsala University.

Orientation of the Aphid Parasitoid Aphidius uzbekistanicus (Hymenoptera: Braconidae) Towards Kairomones Derived from Their Aphid Hosts and Synomones from the Hosts' Plants
Abstract from 13th Annual Meeting of the International Society of Chemical Ecology, 1996.

Pherolist
Female moth pheromones that attract male moths.

Subtle Signals Keep Menstrual Clocks in Sync
Evidence of human pheromones in the synchronization of menstrual cycles among women living together. Article by Jonathan Knight in New Scientist.

UK Semiochemistry Network
Scientists sharing interests in mammalian olfaction.

Volatile Semiochemicals Released from Undamaged Cotton Leaves: A Systemic Response of Living Plants to Caterpillar Damage
Abstract from the Insect Attractants, Behavior, and Basic Biology Research Laboratory, US Department of Agriculture.

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