WHY WALNUT FLIES SUPERPARASITIZE - TIME SAVINGS AS A POSSIBLE EXPLANATION

This study evaluated a possible fitness advantage, specifically time s avings, that might account for an unusual propensity in walnut flies ( Rhagoletis spp.) to superparasitize their walnut hosts and to place eg gs into existing egg-laying cavities. The first part of this study dem onstrated that, in laboratory assays, females of two walnut fly specie s, 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 sta ining of eggs, time saving was not an artifact of differences in the s ize of clutches deposited at new versus existing sites. We further dem onstrated that time savings reflected a reduction in the time required to generate the cavity itself. In the second part of the study, we ev aluated the possibility that, in the field, time saved by reusing exis ting cavities is nullified by extra time spent mating associated with a previously described tendency for males to guard these cavities. Fie ld observations of R. juglandis indicated that use of existing sites w as, as expected, associated with increased mating. Yet, despite the ad ded time spent mating, in observations of similar length females attem pting 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 avoidanc e observed in host-specific insects.