There's no occurrence of Lartz, Laertz or Laartz indexed from the local German newspapers in "Milwaukee's German Newspapers; an index of death notices and related items" (1844-1950).
re: Lartz in Waukesha Co.?
There may be entries for the name in an index of the Waukesha Freeman at the Waukesha Public Library.
Gary Rebholz, compiler/editor
Indexing Milwaukee's German newspapers continuously since 2007 ...