Aggregative Behavior is Not Explained by an Allee Effect in the Walnut Infesting Fly, Rhagoletis juglandis

Component Allee effects are considered to be a driving force in the origin and maintenance of aggregative behavior. In this study, we examine whether a pattern of active host reuse by the walnut fly, Rhagoletis juglandis Cresson (Diptera: Tephritidae), involves an Allee effect. We examined how the density of clutches deposited within a fruit, the temporal pattern in which successive clutches are deposited and the spatial distribution of clutches over a fruit's surface influences survival to pupation and pupal size. Within the density range used in this experiment (1 to 7 clutches), increases in larval density strongly reduced pupal weight but not larval survival to pupation. The temporal staggering of clutches into a host strongly reduced offspring survival and, probably owing to competitive release, increased the pupal weight of survivors. Offspring survival and pupal weight were affected relatively little by whether two clutches were deposited within the same oviposition punctures or were evenly spaced. In contrast, in three-clutch treatments offspring survival was higher when clutches were placed within the same oviposition cavity. However, pupal weights did not significantly increase when clutches were placed together and this relatively higher survival rate was not greater than that associated with hosts that contained fewer clutches. The results of the study failed to provide evidence of an Allee effect. We put forward a scenario under which females appear to reuse larval hosts to maximize their own reproductive success, albeit at the expense of the per capita fitness of their offspring.