Role of colour and shape stimuli in host-enhanced oogenesis in the walnut fly, Rhagoletis juglandis

This study aimed to quantify effects of the host plant on oogenesis in the walnut-husk-infesting fly, Rhagoletis juglandis Cresson (Diptera: Tephritidae), and to assess the role of physical cues in those effects. In laboratory assays, the presence of fruit was manipulated independently of the presence of foliage for newly emerged females, After eight days, in each of two trials, females with fruit were found to have significantly higher egg loads than females without fruit. Foliage presence had little effect. In a second experiment, females held with fruit or a fruit model (plastic yellow sphere of a size similar to fruit) had significantly higher egg loads than females held with neither fruit nor model. Egg loads of females with fruit were not significantly different from those of females with models. In a third experiment, females were held with spheres of various colours or no sphere at all. Females with yellow or green spheres (similar to the colour of walnut fruit) had significantly higher egg loads than females with black, blue or red spheres of other colours or females without spheres. In a fourth experiment, females held with spheres had significantly higher egg loads than females held with cubes of equivalent surface area or females held without a model. Finally, cohorts of newly emerged females held with yellow spheres or without spheres were sampled periodically. In the sphere treatment, mean egg load increased sharply from negligible levels between days 8 and 10. The pattern was similar in the no-sphere treatment, although the increase in egg load appeared to occur a day later. From these experiments, we conclude that physical host fruit stimuli known to be important in host selection in Rhagoletis flies, including colour and shape, also enhance oogenesis in the first egg maturation cycle, and that enhancement of oogenesis via these stimuli requires neither nutritional input from the fruit nor prior egg deposition.