Aggregation pheromones lead to the formation of animal groups near the pheromone source, either by attracting animals from a distance or stopping (“arresting”) passing conspecifics (Chapter 10) (Wertheim et al. 2005). In contrast to sex pheromones (which attract only the opposite sex), aggregation pheromones by definition attract both sexes (and/or, possibly, larvae). However, the benefits of aggregation to individuals may be complex and two rather different mechanisms may apply. First, individuals may be aggregating for the benefits of living in a group. Second, what we call aggregation pheromones may be the response of eavesdropping conspecifics, for example males responding to sex pheromones released by other males to attract females, in order to mate with those females (Chapters 3 and 11).

By contrast, host-marking pheromones deposited on hosts by insects parasitizing fruits or caterpillars, for example, lead to dispersion as host-marking pheromones deter other females from laying eggs in that host. How host-marking pheromones evolved is discussed in Section 4.2.