Distribution Patterns of Rhagoletis pomonella (Diptera: Tephritidae) Eggs in Hawthorn

Abstract

The dispersion of eggs of the apple maggot fly, Rhagoletis pomonella (Walsh), among fruits sampled from two Crataegus mollis (Torr. et Gr.) Scheele hawthorn trees changed over the season, showing first an aggregated dispersion, subsequently a random dispersion, and finally an even dispersion. We suggest that eggs initially were aggregated because when host fruits were very hard and green (mid-August), females deposited eggs in the few sites that could be penetrated with the ovipositor. After the fruit ripened (September), eggs were dispersed evenly, suggesting that under conditions of abundant suitable fruits, females avoid ovipositing in fruit already containing an egg, probably in response to the host-marking pheromone deposited by females following egg-laying. As a result of the placement of eggs, most ripe fruits contained a single egg, thus larvae should have been assured ample developmental resources and minimal intraspecific interactions. Before fruit ripening, there were no significant distributional differences of eggs between the two trees or between the upper and lower half of the canopy. For the entire season, distribution among the tree quadrants was similar. As the season progressed, differences between the trees and crown levels within one of the trees appeared. We conclude that any dispersion study of a species that uses host discrimination cues should consider population density and comparative quality, quantity, and distribution of suitable available resources, as well as changes that may occur in these parameters over time.