Why walnut flies superparasitize: time savings as a possible explanation

Abstract

 This study evaluated a possible fitness advantage, specifically time savings, that might account for an unusual propensity in walnut flies (Rhagoletis spp.) to superparasitize their walnut hosts and to place eggs into existing egg-laying cavities. The first part of this study demonstrated that, in laboratory assays, females of two walnut fly species, R. boycei and R. juglandis, save time when cavities are reused and that in R. juglandis, where it was examined in detail with in vivo staining of eggs, time saving was not an artifact of differences in the size of clutches deposited at new versus existing sites. We further demonstrated that time savings reflected a reduction in the time required to generate the cavity itself. In the second part of the study, we evaluated the possibility that, in the field, time saved by reusing existing cavities is nullified by extra time spent mating associated with a previously described tendency for males to guard these cavities. Field observations of R. juglandis indicated that use of existing sites was, as expected, associated with increased mating. Yet, despite the added time spent mating, in observations of similar length females attempting to lay eggs at existing sites deposited clutches more often than females attempting to lay eggs at new sites. We discuss these results in the context of the more common pattern of superparasitism avoidance observed in host-specific insects.