4.3.3. Semiochemicals: kairomones, allomones, and synomones
Communication chemicals (semiochemicals) may function between individuals of the same species (pheromones) or between different species (allelochemicals). Interspecific semiochemicals may be grouped according to the benefits they provide to the producer and receiver. Those that benefit the receiver but disadvantage the producer are kairomones. Allomones benefit the producer by modifying the behavior of the receiver although having a neutral effect on the receiver. Synomones benefit both the producer and the receiver. This terminology has to be applied in the context of the specific behavior induced in the recipient, as seen in the examples discussed below. A particular chemical can act as an intraspecific pheromone and may also fulfill all three categories of interspecific communication, depending on circumstances. The use of the same chemical for two or more functions in different contexts is referred to as semiochemical parsimony.
Myrcene, the terpene produced by a ponderosa pine when it is damaged by the western pine beetle, acts as a synergist with aggregation pheromones that act to lure more beetles. Thus, myrcene and other terpenes produced by damaged conifers can be kairomones, disadvantaging the producer by luring damaging timber beetles. A kairomone need not be a product of insect attack: elm bark beetles (Curculionidae: Scolytinae: Scolytus spp.) respond to α-cubebene, a product of the Dutch elm disease fungus Ceratocystis ulmi that indicates a weakened or dead elm tree (Ulmus). Elm beetles themselves inoculate previously healthy elms with the fungus, but pheromone-induced aggregations of beetles form only when the kairomone (fungal α-cubenene) indicates suitability for colonization. Host-plant detection by phytophagous insects also involves reception of plant chemicals, which therefore are acting as kairomones.
Insects produce many communication chemicals, with clear benefits. However, these semiochemicals also may act as kairomones if other insects recog- nize them. In “hijacking” the chemical messenger for their own use, specialist parasitoids (Chapter 13) use chemicals emitted by the host, or plants attacked by the host, to locate a suitable host for development of its offspring.
Allomones are chemicals that benefit the producer but have neutral effects on the recipient. For example, defensive and/or repellent chemicals are allomones that advertise distastefulness and protect the producer from lethal experiment by prospective predators. The effect on a potential predator is considered to be neutral, as it is warned from wasting energy in seeking a distasteful meal.
The worldwide beetle family Lycidae has many distasteful and warning-colored (aposematic) members (see Plate 6.5, here), including species of Metriorrhynchus that are protected by odorous alkylpyrazine allomones. In Australia, several distantly related beetle families include many mimics that are modeled visually on Metriorrhynchus. Some mimics are remarkably convergent in color and distasteful chemicals, and possess nearly identical alkylpyrazines. Others share the allomones but differ in distasteful chemicals, whereas some have the warning chemical but appear to lack distastefulness. Other insect mimicry complexes involve allomones. Mimicry and insect defenses in general are considered further in Chapter 14.
Some defensive allomones can have a dual function as sex pheromones. Examples include chemicals from the defensive glands of various bugs (Heteroptera), grasshoppers (Acrididae), and beetles (Staphylinidae), as well as plant-derived toxins used by some Lepidoptera (section 4.3.2). Many female ants, bees, and wasps have exploited the secretions of the glands associated with their sting — the poison (or venom) gland and Dufour’s gland — as male attractants and releasers of male sexual activity.
A novel use of allomones occurs in certain orchids, whose flowers produce similar odors to female sex pheromone of the wasp or bee species that acts as their specific pollinator. Male wasps or bees are deceived by this chemical mimicry and also by the color and shape of the flower (see Plates 4.4 & 4.5), with which they attempt to copulate (section 11.3.1). Thus the orchid’s odor acts as an allomone beneficial to the plant by attracting its specific pollinator, whereas the effect on the male insects is near neutral — at most they waste time and effort.
The terpenes produced by damaged pines are kairomones for pest beetles, but if identical chemicals are used by beneficial parasitoids to locate and attack the bark beetles, the terpenes are acting as synomones (by benefiting both the producer and the receiver). Thus α-pinene and myrcene produced by damaged pines are kairomones for species of Dendroctonus but synomones for pteromalid hymenopterans that parasitize these timber beetles. In like manner, α-cubebene produced by Dutch elm fungus is a synomone for the braconid hymenopteran parasitoids of elm bark beetles (for which it is a kairomone).
An insect parasitoid may respond to host-plant odor directly, like the phytophage it seeks to parasitize, but this means of searching cannot guarantee the parasitoid that the phytophage host is actually present. There is a greater chance of success for the parasitoid if it can identify and respond to the specific plant chemical defenses that the phytophage provokes. If an insect-damaged host plant produced a repellent odor, such as a volatile terpenoid, then the chemical could act as:
- an allomone that deters non-specialist phytophages;
- a kairomone that attracts a specialist phytophage;
- a synomone that lures the parasitoid of the phytophage.
Of course, phytophagous, parasitic, and predatory insects rely on more than odors to locate potential hosts or prey, and visual discrimination is implicated in resource location.