Effects of pre-winter conditions on diapause phenologies and mortality rates of host-associated populations of Rhagoletis pomonella (Diptera: Tephritidae) in western Washington

The apple maggot, Rhagoletis pomonella (Diptera: Tephritidae), is a host-specialized fruit fly endemic to the eastern United States known for its propensity to form host races in sympatry. The fly was introduced to the Pacific Northwestern U.S. <40 ya via larval-infested apples. There, its host range has expanded from apple (Malus domestica) to early-fruiting native black hawthorn (Crataegus douglasii), and late-fruiting, introduced ornamental hawthorn (C. monogyna). In the field, we recently found seasonal distributions of R. pomonella populations are offset: eclosion times mirror host fruits’ ripening, despite spatial overlap among host species. Thus, adaptive shifts in diapause timing appeared rapidly, repeating patterns observed in eastern populations.   

Whether divergent phenologies are maintained by environmental or genetic mechanisms in the Pacific Northwest is not known. We manipulated the pre-winter (= pre-diapause) environment under standardized laboratory conditions to test two hypotheses: (1) Pre-winter thermal accumulation modulates post-winter eclosion timing; (2) Mortality increases when pre-winters are longer or shorter than host-specific intervals in nature. Pupae were subjected to 15, 30, 45, or 60 days of 22°C “pre-winter” followed by 25-weeks of 5°C “winter”­. Mean days to eclosion were unaffected by pre-winter treatments, remaining fixed within all host populations (45.3, 49.2, 54.1, and 64.8 for black hawthorn-, early-apple-, late-apple-, and ornamental hawthorn-associated flies, respectively). These recently derived, divergent eclosion phenologies might lead to reproductive isolation and potentially cladogenesis. Mortality rates were also unaffected by treatments, though consistently lower for hawthorn-origin (~32%) than apple-origin (~50%) flies. Local hawthorn hosts may engender greater survivorship than apple hosts.