In June 2015, a huge, adult butterfly feeding aggregation of thousands of individuals in seven species, dominated by Asterocampa celtis (northern-race Hackberry Emperor), occurred in Douglas and Jefferson Counties, Kansas. This aggregation differed in species composition from smaller aggregations in the same observation area in 1970–1971. A rare A. celtis melanic form was in both the 1970 and 2015 aggregations. Vehicles killed thousands of butterflies in the 2015 aggregation. This study also summarizes data on local population sizes of A. celtis and Asterocampa clyton (Tawny Emperor) in the U.S. and provides new adult food records obtained from eight of the butterfly species in the aggregations.