Sheeder Prairie State Preserve, Guthrie Center, Guthrie County, Iowa, USA June 23, 2010 Size: ~35mm tall X 25mm diam
Gall at apex of Rosinweed (Silphium integrifolium) plant in native tallgrass prairie remnant. I recognized this based on a previous sighting (with determination of that by Charley Eiseman) in Missouri last summer: