The lepidopteran fauna of Massachusetts' offshore islands (USA), particularly Martha’s Vineyard and Nantucket, has been well characterized, and comprises intact assemblages of disjunct, regionally rare, habitat-specialized, and otherwise threatened species that have declined elsewhere in New England. These include the only persistent and extant population of Eacles imperialis Drury (Saturniidae: Ceratocampinae) in New England, one of at least three ceratocampines to have undergone partial or total extirpation from the region. Examining historical and recent records within two groups—Saturniidae and Sphingidae—that have exhibited varying degrees of flux over time, we compare these islands and identify taxonomic and ecological components of their faunas that appear historically volatile. After reviewing suspected agents of these declines, including the introduced generalist parasitoid Compsilura concinnata (Diptera: Tachinidae), we present preliminary tachinid capture data which suggest that C. concinnata has not been established where E. imperialis persists.