Assessing the risk of establishment of western cherry fruit fly (Diptera: Tephritidae) in commercial cherry-growing areas of California

The western cherry fruit fly, Rhagoletis indifferens Curran (Diptera: Tephritidae), is a major quarantine pest of sweet cherry, Prunus avium (L.) L., in the western U.S. that potentially could be an invasive pest within areas of the U.S. that currently do not have the fly.  We used a correlative ecological niche model Maxent and a processes-based mechanistic model CLIMEX to assess the potential risk of establishment of western cherry fruit fly, R. indifferens, in the commercial cherry-growing areas of California. Model validation was done using independently collected R. indifferens presence-absence data from California. Threshold independent (i.e. AUC) and threshold dependent measures (i.e. sensitivity, specificity, and Kappa) were used to evaluate predictive power of the models. Most of the high to very high risk areas for R. indifferens were predicted in northern parts of California and the Sierra Nevada Mountains, where the fly exists in association with its native host, bitter cherry [Prunus emarginata (Douglas) Eaton].  Human footprint index (human-associated plantings of sweet cherry and irrigation), bitter cherry distribution, and direct solar radiation (a surrogate for photoperiod) were some of the strongest predictors of R. indifferens distribution in the Maxent model. Overall results suggest that R. indifferens is unlikely to establish in the commercial cherry-growing areas in the San Joaquin Valley of California, because (1)  chilling requirements in those areas are not met, and (2) heat stress and dry stress are too high.