Populations of the blacklegged tick, Ixodes scapularis Say, are established in western and central Wisconsin in the upper midwestern United States, but appear to be expanding geographically there. Here, we report a previously unknown population in northeastern Wisconsin. Questing I. scapularis nymphs and adults were collected by flagging vegetation from a riverine site in Marinette County, Wisconsin, in spring of 1993 and 1994. Dissection and culture of tick guts in modified Barbour-Stoenner-Kelley II medium showed that some of the ticks were infected with Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwald & Brenner, causative agent of
Lyme disease. Fifteen of 30 white-footed mice, Peromyscus leucopus (Rafinesque), live-trapped at the site on 23-24 August 1994 were infested with immature I. scapularis, and ear-punch biopsies yielded B. burgdorferi cultures from 2 of the mice. However, none of 50 white-tailed deer, Odocoileus virginianus (Zimmermann), shot by hunters in Marinette County in November 1994 had I. scapularis on them, probably because no deer were shot at the same site where the tick population is located. These findings document existence of an established population of I. scapularis and a focus of
Lyme disease in northeastern Wisconsin.