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 expected by chance. Larvae collected during later stadia were both more likely to be parasitized and contained more flies per larva. Whereas the first pattern is likely to be the result of longer exposure to parasitoids, the latter suggests that females either respond to larger hosts by laying more eggs or that superparasitism (parasitism by more than one individual) occurs. When more than four flies emerged from a single host, their mean pupa mass was smaller, suggesting a cost to superparasitism. We report six cases of hyperparasitism by a wasp in the family Perilampidae. When three milkweed species with varying levels of cardenolides occurred in one area, monarchs found on a species with low cardenolide levels (Asclepias syriaca) were most likely to be parasitized, but those found on a species with very high levels of cardenolides (A. curassavica) showed higher levels of parasitism than those on found on the less toxic A. incarnata. Potential impacts of L. archippivora on monarch butterflies are discussed in light of these findings.