Corn leaf aphids can be common in Iowa corn, but they rarely cause significant economic losses. They prefer sorghum as a host plant, but will also feed on corn, barley, millet and many grasses. Although infestations become apparent when tassels emerge, colonies can start to develop in late June.
Iowa State University
Hodgson, Erin W., "Scout for Corn Leaf Aphids Before Tasseling" (2009). Integrated Crop Management News. 594.