Patterns of Egg Load in the Walnut Fly Rhagoletis juglandis (Diptera: Tephritidae) in Nature and Their Possible Significance for Distribution of Sexes

Abstract

In a field study of the tephritid fly Rhagoletis juglandis Cresson on its host Juglans major (Torrey) Heller, the density of both sexes on host foliage remained more or less constant throughout the season. The density of each sex on fruit, by contrast, increased steadily over the course of several weeks. The density of males on fruit increased much more sharply than that of females. Associated with the temporal increase in relative fly density on fruit was an increase in the mean number of eggs in a female's ovaries (i.e., egg load). This pattern in egg load was not caused by an increase in the mean egg load of individuals carrying eggs (which remained more or less constant over the season), but rather by an increase in the number of individuals that carried any eggs at all (i.e., the number of reproductively mature individuals). Late in the season, mean egg load of females in the foliage was lower than that of females on fruit for each of 2 yr, but the difference was not statistically significant. Within a given location (fruit versus foliage), egg load was associated with female activity. Females attempting to oviposit on fruit and females found in mating pairs on foliage had high mean egg loads, whereas females feeding on foliage and females dragging their ovipositor (indicative of recent clutch deposition) on fruit had low mean egg loads. We discuss how results for this species conform to a generally held scenario for the distribution of frugivorous tephritid flies in time and space.