Divergent preferences of Rhagoletis pomonella host races for olfactory and visual fruit cues

The apple and hawthorn-infesting host races of Rhagoletis pomonella Walsh (Diptera: Tephritidae) represent a model for incipient sympatric speciation. Recent studies have shown that apple and hawthorn flies differ in their responses to host-fruit odor. Each host race preferentially orients to the volatile chemicals emanating from the surface of its natal fruit and is antagonized by non-natal fruit volatiles. Fruit odor discrimination is evolutionarily important due to its contributory role in host-specific mating, generating partial premating reproductive isolation between the races in sympatry. Visual cues are also involved in R. pomonella host searching, yet the extent to which they are used to discriminate between hosts has not been fully resolved, nor has their interaction with olfactory cues. Here, we report results from a series of two-way choice experiments conducted in the field on resident apple and hawthorn flies at a site near Fennville, MI, USA. Four key findings emerged from the study: (1) both olfactory and (2) visual cues increased host discrimination of apple and hawthorn flies for their respective natal fruit, (3) the presence of a single appropriate volatile blend on a natal or non-natal visual model generally increased the ability of flies to discriminate for their natal fruit, and (4) the presence of both natal and non-natal olfactory cues on fruit models did not enhance host discrimination above levels observed for visual models alone. We discuss the implications of our results for interpreting the findings of two-way host choice experiments and for sympatric speciation in phytophagous insects.