The apple maggot fly, Rhagoletis pomonella Walsh (Diptera: Tephritidae), is a major pest of commercially grown domesticated apple (Malus domestica Borkhausen) in North America. The shift of the fly from its native host hawthorn (Crataegus mollis) to apple in the eastern United States is often cited as an example of incipient sympatric speciation in action. However, R. pomonella is also present in the Pacific Northwest region of the United States where it infests apple, native black hawthorn (Crataegus douglasii), and introduced English ornamental hawthorn (C. monogyna). It is believed that R. pomonella was introduced to the Portland, OR, area via larval-infested apples from the east. The fly subsequently spread through the region, shifting onto black hawthorn and ornamental hawthorn as additional hosts as it spread. It is also possible, however, that R. pomonella is native to black hawthorn in the Pacific Northwest and switched to infest apple and ornamental hawthorn after the introduction of these two alternative hosts to the region. Here, we document the distribution of R. pomonella through the Pacific Northwest and Rocky Mountain states of the western United States to help clarify the origin (s) of the fly outside the eastern United States. We report a distribution pattern consistent with the hypothesis that R. pomonella was introduced to the Pacific Northwest via infested apples. In particular, the low levels or lack of C. dougliasii-infesting R. pomonella east of the Cascade Mountains in the states of Washington, OR, and Idaho implies that the fly is not native on black hawthorn and is of recent origin. We discuss the evolutionary and applied implications of the results with respect to our current understanding of host race formation and control for R. pomonella.
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