Corn leaf aphids are blue-grey in color. They are extremely small (1/25"). Adults have transparent wings, while nymphs are smaller and wingless. Corn leaf aphids are typically found on or near the tassel or whorl.
Aphids pierce the plant and suck out the juices. This can result in the death of upper leaves in the case of severe infestation, but normally does not have a significant impact on yield. Aphids can also be a vector for diseases, particularly Barley Yellow Dwarf Virus.
Insecticides are available for controlling corn leaf aphids. However, these tiny insects are usually naturally controlled by enviornmental factors and predatory insects. Significant infestations in seed corn fields may justify treatment. Always read and follow all label instructions.