Closely related phytophagous insects that specialize on different host plants may have divergent responses to environmental factors. Rhagoletis pomonella (Walsh) and Rhagoletis zephyria Snow (Diptera: Tephritidae) are sibling, sympatric fly species found in western North America that attack and mate on plants of Rosaceae (∼60 taxa) and Caprifoliaceae (three taxa), respectively, likely contributing to partial reproductive isolation. Rhagoletis zephyria evolved from R. pomonella and is native to western North America, whereas R. pomonella was introduced there. Given that key features of the flies' ecology, breeding compatibility, and evolution differ, we predicted that adult eclosion patterns of the two flies from Washington State, USA are also distinct. When puparia were chilled, eclosion of apple- and black hawthorn-origin R. pomonella was significantly more dispersed, with less pronounced peaks, than of snowberry-origin R. zephyria within sympatric and nonsympatric site comparisons. Percentages of chilled puparia that produced adults were ≥67% for both species. However, when puparia were not chilled, from 13.5 to 21.9% of apple-origin R. pomonella versus only 1.2% to 1.9% of R. zephyria eclosed.The distinct differences in eclosion traits of R. pomonella and R. zephyria could be due to greater genetic variation in R. pomonella, associated with its use of a wider range of host plants than R. zephyria.