Conserved cold tolerance of Rhagoletis species from different host fruits and elevations in Colorado, USA

Understanding and characterizing how insects tolerate low temperatures is important for predicting their overwintering survival and subsequent geographic spread. This study characterized the cold tolerance of two members of the Rhagoletis genus (Diptera: Tephritidae) in Colorado, USA. Pupae were collected from the infested fruits in late summer and early fall. For the first time, we show that the rosehip fly Rhagoletis basiola Osten Sacken is freeze‐avoidant; overwintering pupae could supercool to temperatures as low as −26°C and survive. Interestingly, the temperature at which ice forms (supercooling point; SCP) did not vary between R. basiola at high (c. 2900 m above sea level [m a.s.l.]) and lower (c. 1650 m a.s.l.) elevations. We also report the apple maggot R. pomonella Walsh infesting an unusual host fruit, the Dolgo crabapple, in close proximity to infested hawthorn trees. R. pomonella infesting hawthorn fruits and crabapples had similar SCPs and survived temperatures as low as −21°C. Pupae from both host fruits also survived prolonged exposure (2 weeks or more) to mild low temperatures (0 to −5°C). Further study into the mechanisms underlying the impressive and conserved cold tolerance of R. pomonella and R. basiola is an interesting avenue for future research. R. pomonella (apple maggot fly) and R. basiola (rosehip fly) from Colorado overwinter as freeze‐avoidant pupae, surviving temperatures near −20°C. R. basiola populations from elevations of c. 1650 and c. 2900 m a.s.l. have similar cold tolerance, although high‐elevation pupae are smaller. R. pomonella that infest hawthorn and crabapple fruits from a narrow geographic area also have similar cold tolerance and can complete normal development after a simulated winter.