Publilia brunnea (syn. Publilia modesta var. brunnea)
Two color morphs of Publilia brunnea, one typical (yellow with black head and white band on thorax), the other with a brown body and white and yellow circles on the wings. See https://bugguide.net/node/view/575405/bgimage Maybe the darker form is a developmental stage?
On Solidago altissima and being tended by ants (genus Formica).