The effect of egg load on superparasitism by the snowberry fly

In studies conducted with fruits of the host plant, Symphoricarpus albus (L.), we examine the influence of egg load on the oviposition behaviour of Rhagoletis zephyria Snow (Diptera:Tephritidae). By altering the availability of exogenous protein, three classes of females with progressively increasing egg loads were produced, while keeping confounding factors such as age, experience, and mating status constant. Flies from all three classes were randomly presented with either a pheromone marked fruit, or with an unmarked fruit. Results indicate that increased egg load led to a greater propensity to accept, or superparasitize, the pheromone marked fruit. Upon dissection it was revealed that females which superparasitized had a mean egg load of 19.5 eggs (n=22), while females which rejected marked fruit had a mean egg load of 13.5 eggs (n=26). These results are consistent with the theory of adaptive superparasitism.