According to your responses the aphid you are dealing with is the English grain aphid.
Identification Confirmation
English grain aphids can vary in color from yellow to green or even reddish brown. They are relatively large aphids compared to other aphids found in small grains. They have long black antennae that are more than half their body length. They have long completely black cornicles as well. These cornicles are typically 4 or more times longer than they are wide. Their most distinguishing feature is their black leg joints which other aphids of small grains do not have. This gives them a striped leg appearance.
Known Host Plants
Wheat, Barley, Oats and wild grasses.
Problems and Damage
English grain aphids are not known to cause economically significant damage, but they can spread barley yellow dwarf virus.
Treatment Options
The usage of organophosphate pesticides labeled for this aphid are effective control measures.
For more information on greenbug visit this Kansas State article here.
Image by Kansas Department of Agriculture, Bugwood.org
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