Mapping Global Potential Risk of Establishment of Rhagoletis pomonella (Diptera: Tephritidae) Using MaxEnt and CLIMEX Niche Models.

The apple maggot, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), is a major quarantine pest of apples (Malus domestica Borkhausen) in the United States. Apple maggot is found only in North America and negatively impacts the apple industry in the western United States by reducing grower access to export markets. To reduce the threat of apple maggot to export countries and to facilitate the movement of commercial apples, an assessment of potential risk of establishment of apple maggot is needed to predict which regions are suitable or unsuitable for the fly. We used a correlative niche model MaxEnt and a mechanistic model CLIMEX to model global potential risk of establishment of apple maggot. The MaxEnt model was developed by integrating apple maggot occurrences with global climatic variables. Apple (a major host of apple maggot) climatic suitability was used as an additional variable to include species interactions in the MaxEnt model. The CLIMEX model was developed using published apple maggot physiological tolerance thresholds. Both the MaxEnt and CLIMEX models correctly predicted the known distribution of apple maggot in North America, met biological expectations when projected to the world, and mostly agreed on climatic suitability worldwide for the fly. Degree-days at 6.7 °C, elevation, precipitation seasonality, and apple climatic suitability were the most important predictors associated with apple maggot distribution in North America. Our results can be used to make science-based international trade decisions by policy makers, and for monitoring apple maggot potential introductions in countries where it currently does not occur.