Identification of host fruit volatiles from domestic apple (Malus domestica), native black hawthorn (Crataegus douglasii) and introduced ornamental hawthorn (C. monogyna) attractive to Rhagoletis pomonella flies from the western United States.
The apple maggot fly, Rhagoletis pomonella, infests apple (Malus domestica) and hawthorn species (most notably the downy hawthorn, Crataegus mollis) in the eastern USA. Evidence suggests that the fly was introduced into the western USA sometime in the last 60 years. In addition to apple, R. pomonella also infests two species of hawthorns in the western USA as major hosts: the native black hawthorn (C. douglasii) and the introduced ornamental English hawthorn, C. monogyna. Apple and downy hawthorn-origin flies in the eastern USA use volatile blends emitted from the surface of their respective ripening fruit to find and discriminate among host trees. To test whether the same is true for western flies, we used coupled gas chromatography and electroantennographic detection (GC-EAD) and developed a 7-component apple fruit blend for western apple-origin flies, an 8-component black hawthorn fruit blend for flies infesting C. douglasii, and a 9-component ornamental hawthorn blend for flies from C. monogyna. Crataegus douglasii and C. monogyna-origin flies showed similar levels of upwind directed flight to their respective natal synthetic fruit blends in flight tunnel assays compared to whole fruit adsorbent extracts, indicating that the blends contain all the behaviorally relevant fruit volatiles to induce maximal response levels. The black and ornamental hawthorn blends shared four compounds in common including 3-methylbutan-1-ol, which appears to be a key volatile for R. pomonella populations in the eastern, southern, and western USA that show a preference for fruit from different Crataegus species. However, the blends also differed from one another and from domesticated apple in several respects that make it possible that western R. pomonella flies behaviorally discriminate among fruit volatiles and form ecologically differentiated host races, as is the case for eastern apple and hawthorn flies.