Effect of host stimuli on ovariole development in the walnut fly, Rhagoletis juglandis (Diptera, Tephritidae)

Although examples of non-nutritive effects of host resources on ovarian development in insects are known, precise descriptions of effects on ovariole formation and development are rare. In this study, we provide a description with respect to the enhancement of egg production, or egg load, by surrogate fruit stimuli (yellow plastic spheres) in the walnut fly, Rhagoletis juglandis Cresson. A system for staging developing ovarioles was established, and cohorts of females held from emergence with or without surrogate fruit were sampled over time with respect to number of ovarioles as well as number and stage of follicles within ovarioles. As found previously, the presence of surrogate fruit strongly enhanced production of eggs in the first maturation cycle. The effect was probably not due to oosorption in the ovaries of females held without surrogates; in fact, oosorption was never observed in ovaries of females in either treatment. The effect was due in part to an increase in ovariole number in females held with surrogates; in each of two trials, mean ovariole number was slightly but significantly greater for females held with vs. without spheres. However, the main effect was also due in part to an earlier onset and increased rate of vitellogenesis in the first and second follicles of females held with spheres.