The species of greenfly attacking your sweet corn would probably have been the potato aphid, macrosiphum euphorbiae.
This species attacks roses and related plants, then migrates to attack a wide range of other crops including sweet corn, beans,
rape, lettuce and potatoes.
The species can be quite widespread, but is only really numerous and damaging in certain years, probably related to weather
conditions in spring.
Control them by spraying with a suitable insecticide,