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Invasive parasites are a major threat to biodiversity worldwide, so understanding the factors that control them is necessary to improve the health of affected host species. In the Galápagos Islands, the invasive nest ectoparasite, the avian vampire fly (Philornis downsi), is causing up to 100% mortality in nestling Darwin’s finches. However, urban finch nests have fewer flies than non-urban finch nests. One explanation is that finches incorporate cigarette butts into their nests, which can decrease nest parasite abundance for other bird species. For our study, we exposed larval flies to cigarette tobacco-treated (concentrated or diluted) or untreated cotton, then characterized pupation success, pupal deformities and success, and adult fly eclosure success and size. The influence of moisture on the effect of tobacco treatment on fly health was also determined. Flies reared in the tobacco treatments as larvae had lower pupation success, larger pupal volume, and a higher prevalence of pupal deformities compared to control flies, regardless of moisture treatment. Furthermore, we found that tobacco-treated flies had lower eclosure success. In fact, very few tobacco-treated flies survived to adulthood. We also collected finch nests and quantified the prevalence and mass of cigarette butts and abundance of flies in the nests. Although most urban finch nests contain cigarette butts (73%), the mass of cigarette butts was very low and did not correlate with fly abundance. Compared to past studies, finch nests require ten times as many cigarette butts to affect fly survival. Although tobacco can negatively affect vampire flies, finches likely do not incorporate enough cigarette butts to affect fly fitness.