Introduction

The attraction of thousands of bark beetles an hour in response to the aggregation pheromones released by the first arrivals on a suitable tree is one of the most impressive demonstrations of the power of pheromones. More subtle are the pheromone marks, left by a female parasitic insect on its host, which seem to deter conspecific females from laying in that host.

But why do the first beetles call and why do conspecifics respond so spectacularly to these pheromones? Why do female parasitic insects mark hosts and why should other females take notice? Until relatively recently, many authors suggested that aggregation and host-marking pheromones enabled the most efficient use of resources by a species. However, it is hard to see how such behaviours could evolve unless they benefit individual reproductive success. This chapter investigates the individual advantages for signallers and responders in these pheromone systems.

Aggregation pheromones and Allee effects, the advantages of group living

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). 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 to sex pheromones released by the same sex (Section 4.2.5) (Chapters 3 and 11).