[JayG]

I read that the carrot fly mates at circa 10 ft high!     I have no faith in the height barrier theory as on an allotment they will find any carrots that are exposed regardless of the height.      Enviromesh works

Whilst trying (unsuccessfully) to find more about the carrot fly's "high jump" I did find a question from someone on another forum asking whether they would be safe from carrot fly as they lived at an altitude of 500 feet!   

(Yes, Enviromesh works, every other method depends on how much you like gambling and carrots!)