Diet has a profound influence on the fitness of adult tephritid flies. Mass-reared flies are provided yeast hydrolysate as a rich source of nutrition that supports rapid sexual development and mating success. In contrast, wild tephritid flies often live in environments where food may be hard to find, and these are the conditions that sexually immature mass-reared sterile males encounter when released into the field during sterile insect technique campaigns. The effect of natural food sources (bat guano, bird droppings, citrus pollen, and wheat pollen) on the sexual development of adult mass-reared fertile, mass-reared sterile, and wild male Queensland fruit flies, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), was determined by measuring ejaculatory apodeme size. Inclusion of yeast hydrolysate in the adult diet was associated with faster growth of the ejaculatory apodeme in comparison with all other diets. Effects of diet were far less pronounced in mass-reared males, which may indicate reduced nutritional requirements, whereas the ejaculatory apodeme of wild males fed on natural sources of food or sucrose alone did not increase in size over the first 20 days of adult life.