Blair- best and only example I know was your fellow Canadian Ken Richards and others selecting for a strictly univoltine alfalfa leafcutting bee.  However, it could not be maintained in the field because of genetic swamping by feral conspecifics.  Probably that would be a fate for most attempts at "improving" a native bee except for the case that you imply, rescuing genetically depauperate populations.  Until recently, we lacked tools to track genetic fates of such manipulations in the field.

yours,  jim