According to your responses the aphid you are dealing with is the Russian wheat aphid.
Identification Confirmation
Russian wheat aphids are light green in color. Their antennae are black tipped along with their tarsi (feet). Cornicles are not present on this aphid. Russian wheat aphids have a distinct elongated tear drop body shape that distinguishes them from other aphids.
Known Host Plants
Wheat, Barley, Oats and other grasses
Problems and Damage
Russian wheat aphids feed on leaf fluids and commonly inject toxins into the leaf. Severe damage can cause leaf edges to roll inward and streaks of purple or white. These aphids tend to feed on the youngest uppermost leaves of the plant.
Treatment Options
Visit pages 10-12 of this document to learn more about Russian wheat aphid thresholds and to read a list of effective insecticides.
More Images
First Russian Wheat Aphid Photo by Gary Puterka,USDA-ARS. Final image by Peggy Greb, USDA-ARS.
More information about this pest can be found on pages 10-12 of this document.