Aggregation in non-social insects

Insect aggregations are formed and maintained for a number of reasons, and by a number of mechanisms. Some aggregations are simply the result of an uneven distribution of resources: the animals are no more aggregated than would be expected by random distribution over the available resource patches (Parrish and Edelstein-Keshet, 1999; Stephens and Sutherland, 1999). In other cases, animals may be arrested or attracted by stimuli from conspecifics, but may not be interested in the conspecific per se. The stimuli serve, instead, as a reliable indicator of an ephemeral or cryptic resource. There are also those cases where individual fitness is positively dependent on density of conspecifics (Stephens and Sutherland, 1999). A larger group of conspecifics may be more efficient in overcoming active or passive defenses (see e.g. Coulson, 1979; Alcock, 1982; Byers, 1989; Schlyter and Birgersson, 1999; Raffa, 2001), or may otherwise enhance a resource, by mechanisms such as inoculation of a substrate with microorganisms which may render it more suitable as a food source for their larvae (see e.g. Davies, 1962; Ralph, 1976; Lockwood and Story, 1985; Turchin and Kareiva, 1989; McCall and Cameron, 1995; Wertheim et al., 2002b). The subject of this introductory paper is aggregation in nonsocial insects. Mechanisms in formation and maintenance of aggregations are covered, with particular emphasis on pheromones. Interactions between pheromones and semiochemicals from non-conspecifics (e.g. host plants) are explored. Fitness effects of aggregation are also discussed. Suggested fitness costs and benefits are presented, with examples from nature. Special attention is paid to those cases in which the fitness effects of aggregation have been explicitly tested.