Density-dependent patterns of parasitism are critically important to population-level outcomes of host–parasite interactions. For parasitoids that primarily obtain nutrients from their hosts—such as those that attack soft-bodied, plant-feeding insects—lifespan is predicted to be correlated with host density. To test this hypothesis, we assessed the survivorship of Aphelinus certus, an invasive species that parasitizes soybean aphid (Aphis glycines) in North America. We found that higher host densities resulted in longer-lived parasitoids and that 9.87 hosts per day are required for a parasitoid to reach its maximum lifespan. Parasitoids from this assay were also used to develop a wing wear index that relates setae damage to age, allowing us to estimate the life expectancy of field-collected parasitoids (14.63 days) compared to laboratory-reared parasitoids (19.75 days). Finally, the effects of density dependence on parasitoid lifespan were explored using a matrix population model. Overall, our approach to modeling the survivorship of a parasitoid offers a high degree of ecological realism, and our development of an index to estimate the age of field-collected parasitoids provides valuable opportunities for life table analyses and can be applied to gain insight into the demographic trends of parasitoid populations.