The Evolution of Gregariousness in Distasteful Insects as a Defense Against Predators

Abstract

Many insect species form aggregations in either the larval or adult stage. The evolution of gregariousness in insects may seem disadvantageous because aggregations are easily discovered by a predator and because insects are small and slow in relation to many potential predators that then have the capacity to consume an entire group. For animals in general, a benefit of group living may be a decreased risk of predator attack on any particular individual. For insects, this effect, called dilution, can be a consequence of distastefulness, which limits the number of prey that a predator can take. A model is developed in which we vary the risk of detection in relation to group size and the degree of prey distastefulness and aposematic coloration, both of which affect the number of prey a predator samples during avoidance learning. According to the model, members of small groups may have a higher rate of death from predaiton than solitary individuals, but above a certain minimum group size, group members do better than solitary individuals. As group size increases above the minimum value, group members suffer fewer and fewer deaths from predation. However, other factors, unrelated to predation, constrain group size. For gregariousness to be advantageous, such constraints must be weak enough to allow groups larger than the minimum size. Our model predicts that (1) gregarious insect species should be distasteful, and the evolution of distastefulness should precede that of gregariousness; (2) the distribution of group sizes among species should show a bimodal form; and (3) the largest aggregations should be found in forms that have fewer constraints unrelated to predation on group size. The agreement between these predictions and data is discussed.