Density-Dependent Parasitism and the Evolution of Aggregated Nesting in the Solitary Hymenoptera

Abstract

Aggregated nesting is widespread among the solitary Hymenoptera and may represent an important step toward the evolution of eusociality via the semisocial pathway. Five hypotheses have been proposed to explain the evolution of nesting aggregations: first, aggregation may occur in response to a restricted distribution of some key resource; second, aggregation may increase foraging efficiency; third, aggregation may confer improved protection from natural enemies; fourth, aggregation may decrease the cost of nest site search; and fifth, aggregation may increase opportunities to reuse old nests. Proximate explanations for the formation of nesting aggregations, including the philopatry hypothesis, do not compete with these ultimate explanations but, rather, complement them. The role of parasite pressure in favoring or disfavoring aggregated nesting has become an area of active research and debate. This paper reviews field studies of density-dependent foraging by nest parasites and resulting patterns of nest exploitation to test generalizations concerning the effects of parasites. Nest parasites foraged either independently of host nest density (n = 2) or they concentrated in areas of high nest density (n = 4). Nest parasitism was directly density dependent (n = 8), density independent (n = 4), or inversely density dependent (n = 2). Thus, parasite pressure may favor either aggregated or dispersed nesting. Diversity in patterns of density dependence appears to result from the interaction of variable parasite foraging strategies and host defensive strategies. Inversely density-dependent parasitism may result from parasites being limited by egg supply or handling time, or from improved defense capabilities of grouped hosts. Improved defense may be a product of increased parasite confusion, active group defense, selfish herding, or improved parasite detection. Data supporting these hypotheses are critically reviewed and further research needs are identified.