ABSTRACT Lespesia archippivora is a widespread generalist parasitoid whose hosts include monarch butterfly larvae. We report parasitism rates by this tachinid fly in wild captured monarchs, using data collected over 7 y by 77 volunteers in the Monarch Larva Monitoring Project, a North American Citizen Science Program. Data were collected in 21 U.S. states and one Canadian province, with focus sites in Wisconsin, Michigan and North Carolina. Overall, approximately 13% of monarch larvae contained tachinid flies. Parasitism rates varied by year, and this variation was, to a large extent, region-wide. For example, larvae collected in 2000–2002 throughout the Upper Midwestern U.S. suffered significantly more parasitism than those collected in 1999 or 2003–2005. There were no consistent patterns with respect to date of collection within years. The number of flies per parasitized monarch ranged from one to 10 and comparison to a Poisson distribution indicated that flies were more aggregated within hosts than exp...