PATTERNS OF EGG LOAD IN THE WALNUT FLY RHAGOLETIS-JUGLANDIS (DIPTERA,TEPHRITIDAE) IN NATURE AND THEIR POSSIBLE SIGNIFICANCE FOR DISTRIBUTION OF SEXES

In a field study of the tephritid fly Rhagoletis juglandis Cresson on its host Juglans major (Torrey) Heller, the density of both sexes on h ost 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 incre ase in relative fly density on fruit was an increase in the mean numbe r 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 a ny eggs at all (i.e., the number of reproductively mature individuals) . Late in the season, mean egg load of females in the foliage was lowe r than that of females on fruit for each of 2 yr, but the difference w as not statistically significant. Within a given location (fruit versu s foliage), egg load was associated with female activity. Females atte mpting to oviposit on fruit and females found in mating pairs on folia ge had high mean egg loads, whereas females feeding on foliage and fem ales dragging their ovipositor (indicative of recent clutch deposition ) on fruit had low mean egg loads. We discuss how results for this spe cies conform to a generally held scenario for the distribution of frug ivorous tephritid flies in time and space.