Superparasitism of larval hosts by the walnut fly, Rhagoletis juglandis, and its implications for female and offspring performance.

The oviposition-preference-offspring-performance hypothesis predicts that female insects should prefer to deposit clutches on or in hosts that maximize offspring performance. An important assumption behind this prediction is that female fitness is tightly correlated with the fitness of any one offspring. In this study, we evaluate offspring performance in the walnut fly, Rhagoletis juglandis Cresson (Diptera: Tephritidae), in relation to a previously described oviposition preference for previously exploited host fruit. In particular, we examined how superparasitism of walnut hosts influences offspring survival and weight at pupation under field conditions. We found that superparasitism was common and that increases in larval densities within fruit were associated with reduced larval survival and weight at pupation. In a laboratory experiment, female size was correlated with lifetime fecundity. In this system, oviposition preference is therefore negatively, not positively, correlated with offspring performance. We argue that patterns of female preference in this system reflect direct benefits to females that are traded off against costs in terms of offspring fitness. Because female fitness is a product not only of offspring quality but also of the total number of offspring produced, female walnut flies may be optimizing their fitness by producing many less fecund offspring. Studies examining the preference-performance hypothesis should consider the reproductive conflicts between parents and offspring as potential factors that influence the congruence between parental preference and offspring performance.