426 release a pheromone in their feces that causes aggregation under cowpats and rotting wood where food and shelter occur. Likewise, Collembola aggregate in moist places in response to a pheromone to avoid desiccation and to reproduce; the latter is especially significant as these arthropods do not copulate (Chapter 19, Section 4.1). In Coleoptera the pheromones serve primarily to aggregate the beetles to a food source that may be isolated (e.g., stored grain) or require the collaborative efforts of a large number of insects to overcome host resistance (e.g., bark beetles attacking healthy trees). As noted in Section 4.1.1, juvenile locusts produce an aggregation pheromone whose function is to keep the marching swarm intact. However, gregarious adult females also produce a volatile aggregation pheromone in their egg pod froth (Saini et al., 1995). The pheromone, whose most important components are acetophenone and veratrole (Rai et al., 1997), attracts egg-laying females to a common site, resulting in very high egg-pod densities. In addition, it predisposes the hatchlings to take on gregarious characteristics of color, physiology, and behavior.

Aggregation pheromones are also common in a wide range of blood-feeding insects, serving to bring conspecifics together for mating, oviposition, and larviposition (McCall, 2002). For these specialized insects, the pheromones facilitate the concentration of populations in sites where mating and egg-laying can occur in relative safety (e.g., cracks and crevices in walls for bedbugs), or in locations where conditions are suitable for egg-laying. For example, the eggs of Culex mosquitoes, Simulium (black flies), and sand flies release a pheromone that triggers mass oviposition. Likewise, tsetse flies larviposit in large aggregations, especially in the dry season when suitable sites may be rare, as a result of a pheromone released as an anal exudate by the burrowing larva.

Compared to sex pheromones, relatively little work has been done to establish the chemical nature of aggregation pheromones. Those studied mostly appear to be mixtures of compounds, often including terpenoid compounds and cyclic alcohols or aldehydes, that act synergistically. Given their function, it is not surprising to discover that the aggregation pheromones of many beetles are metabolites of inhaled or ingested host-plant substances and that host odors synergize their effect (Borden, in Kerkut and Gilbert, 1985). Like sex pheromones, aggregation pheromones are typically highly specific, particular stereo and optical isomers being attractive to a given species. Beetle aggregation pheromones are rarely produced by exocrine glands. Commonly they are released in the feces though the precise sites of synthesis remain unclear. Pheromones produced from ingested material could be produced by cells in the gut wall or, as has been suggested for a few species, by gut microorganisms; those derived from host-plant vapors taken up via the tracheal system are perhaps synthesized in the hemolymph (fat body?) or Malpighian tubules. It has been suggested that some pheromones arose initially as detoxification products, their precursors being toxic to the beetles so that their modern function of promoting aggregation developed secondarily (Borden, 1982).

4.1.4. Alarm Pheromones

As their name indicates, alarm pheromones warn members of a species of impending danger. They are produced by mites and insects that live in groups, including social forms, for example, cockroaches, treehoppers, aphids, bedbugs, termites, and social Hymenoptera (Blum, in Kerkut and Gilbert, 1985; Blum, 1996). In termites it is only soldiers that produce (and respond to) the pheromone. Among Hymenoptera, hornets and honey bees, but not bumble bees, produce alarm pheromones as do almost all ants investigated. The pheromones may originate internally, being released as in treehoppers when the body wall is broken open, or in exocrine glands. Corpses (but not, living specimens) of P. americana release material that repels conspecifics and members of some other species. Thus, the repellent may be a cue that cockroaches use to avoid areas where others have died (Rollo et al., 1995). Some Hymenoptera have more than one pheromone-producing gland; for example, in Formica species of ants there are mandibular glands, Dufour's glands, and poison glands. The chemical nature of alarm pheromones is highly varied but tends to be specific for each group. Mites, aphids, and termite soldiers produce terpenoid compounds, while honey bees produce a mixture of acetates, an alcohol, and a ketone. Formicine ants also produce terpenoids such as citral and citronellal and, in addition, formic acid, undecane (Figure 13.7D), and various ketones. These compounds often act synergistically. Typically, the alarm pheromones of non-social insects and mites stimulate dispersal (escape behavior). Such behavior is also seen in social species away from the nest. However, when the pheromone is released near the nest, the insects are attracted toward the source and may subsequently attack the intruder. In honey bees, for example, the pheromone is released from the sting shaft (embedded in the intruder!) causing more bees to attack.

Alarm pheromones frequently have other additional functions especially as allomones, leading Holldobler (1984) to speculate that they evolved originally as defensive chemicals. Those of ants, for example, may also serve as defensive compounds, repelling intruders, or release digging behavior (perhaps an adaptation for excavating ants buried in a cave-in), or act as trail pheromones. In the honey bee the alarm pheromone may be released to indicate a depleted food source.

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