These key to Rabdophaga strobiloides in Gagne's book on midge galls. He lists R. persimilis as one of many species that "were reared from willow but without definite reference to particular damage." I see that Felt lists these two species together, without differentiating them--not sure what to make of that; the latter may be an inquiline of the former, whose galls Gagne says are "one of the most common on willow."