Alarm and trail pheromones

Most social insects use alarm and trail pheromones. Their specificity is not so important, but fast release and dispersal are important. They tend to be small volatile substances for instance, (S)-4-methyl-3-heptanone is an alarm pheromone in the leaf-cutting ant, Atta textata. 

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Insects communicate by releasing pheromones—chemical substances that other insects of the same species detect with their anteimae. There are sex, alarm, and trail pheromones, and many of these are alkenes. Interfering with an insect s ability to send or receive chemical signals is an environmentally safe way to control insect populations. For example, traps with synthetic sex attractants have been used to capture such crop-destroying insects as the gypsy moth and the boll weevil. Many of the flavors and fragrances produced by certain plants also belong to the alkene family. 

Pheromones are often described by function, by the effect they have. For example, sex pheromones describe those involved in mate-finding or attraction. Others include aggregation, alarm, and trail pheromones. Some responses are context-specific - for example in some ant species alarm pheromones cause ants to disperse if released far from the nest but to attack if released close to the nest. In a way, these descriptors allow us simply to describe the range of behaviors mediated by pheromones and then generalize from these to describe patterns in use, say, of sex pheromones across taxa. 

Insect pheromones are organic compounds that are used by insects to communicate with each other. There are sex, trail, alarm and defence pheromones that are given off by insects to inform other insects of opportunities and problems. Pheromones have been exploited by baiting insect traps with an appropriate sex pheromone, which lures a specific insect to its doom. Many of these pheromones are alkcnes, often with Z cis) double bonds. A selection is shown below... 

In many species, the trail pheromone is used in two distinct contexts, namely food retrieval and nest emigration. Hdlldobler and Wilson (1978) have distinguished no less than five types of recruitment in the African weaver ant, O. longinoda. The alarm and territorial pheromones of this species have already been discussed, and these, together with the recruitment systems, constitute the most complex use of chemical communication yet discovered in ants. 

Many different functions of pheromones have been found since Butenandt. Aggregation pheromones attract both sexes to a special location, while sex pheromones are offered by one sex only to attract or arouse the other one. Trail pheromones used by ants mark food trails and alarm pheromones change the state of alertness of conspecifics. These are only some of the functions pheromones can have, and similar different functions can be found in allelochemicals as well. 

Within Hymenoptera, pheromones produced by workers in social colonies are the best studied across many genera, principally in ants [6], with those eliciting trail following most extensively studied. The distinct behavior and the relative ease of the bioassay have resulted in chemical identifications in many species [ 113,114]. Those that have been recently identified are listed in Table 5. In addition, several alarm and recruitment signals have recently been identified. Many of the compounds recently identified in ants have previously been reported as trail or alarm pheromones in other ant species. For example, methyl 4-methylpyrrole-2-carboxylate 64, 3-ethyl-2,5-dimethylpyrazine 65, (9Z)-hexadec-9-enal 66,4-methylheptan-3-ol 67, and methyl 6-methylsalicy-late 68 have been identified as trail pheromone components, and heptan-2-one 69,4-methylheptan-3-one 70, formic acid 71, undecane 61,4-methylheptan-3-ol 67, methyl 6-methylsalicylate 68, and citronellal 72 have been identified as alarm pheromone components [6]. The use of the same chemicals across genera, with some used for very different functions, is an interesting phenomenon. 

Pentadecane is a pheromone of the ant C. obscuripes that seems to calm down ants after the more volatile alarm signals decane and undecane have been evaporated.366 1-Heptyloctyl acetate (A129) is part of the trail pheromone of the ant Leptogenys peuqueti 26... 

Although insect pheromone structures represent a myriad of chemical functionalities [6], the composite pheromones can be classified into six behaviourally functional groups sex, aggregation, dispersal (spacing or epideictic), alarm, recruitment (trail), and maturation. 

Semiochemicals can be placed into two distinct classes Pheromones control intraspecific interactions. These semiochemicals provide information regarding a range of behaviours including mate selection, aggregation, dispersal (alarm pheromones), oviposition and food sources (trail pheromones). Allelochemicals control interspecific interactions such as host and non-host identification by primary colonizers (plant or animal) and whether a food source is already exploited through tritrophic interactions (signals indicating the presence of primary colonizers). 

Wilson (3) and Wilson and Bossert (4) divided pheromones into release pheromones (if the stimulus triggers an immediate behavioral response in the receiving organism) and primer pheromones (if the response is a long lasting one). Substances involved in alarm, territorial marking, trail, social status, sexual attraction and reproductive isolation are release pheromones while the odors involved in puberty acceleration, pregnancy block, estrus synchronization or suppression are primer pheromones. 

Bossert and Wilson (1%3) were the first to use diffusion equations to estimate pheromone concentrations and active space dimensions in still air. They derived equations for three still air situations (i) an instantaneous puff, (ii) a point source emitting at a continuous rate, and (iii) a moving point source such as an ant depositing a trail pheromone. For an instantaneous puff on the reflecting plane surface, such as an ant releasing a momentary burst of alarm pheromone, the active space is a half sphere with a radius (R) at (t) seconds after release of the puff given by ... 

The term recruitment , like the term alarm has become diffuse as the diversity and complexity of social insect behavior has emerged. In this review we will take recruitment to mean the inducement of workers to move from one area to another, either to retrieve food or to move to new territory or a new nest site. The chemical signals involved are usually, but not always, deposited on the substratum, and may therefore also constitute an orientation signal. The two possible functions of trail pheromones , recruitment of nest mates and orientation to food or nest, need to be carefully separated to estimate the importance of these chemical signals in the ecology of each species. 

Compared to alarm pheromones, very few trail pheromones have been characterized chemically this reflects the extremely low levels of compound present in the trail secretions of most species investigated. The confirmed structures are shown in Fig. 15.10. In many cases only a single compound has been identified, usually one which stimulates both recruitment and orientation. These compounds are normally stored and applied in highly dilute aqueous solution, and... 

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