Parasitoid wasps inflict widespread death upon the insect world. Hundreds of thousands of parasitoid wasp species kill a vast range of insect species, and many such wasps are used globally to control insect pests. Insects have evolved defensive responses to the threat of wasps, some cellular and some behavioral. However, the nature of these responses and the mechanisms that underlie them await further exploration. Here we find an unexpected response of adult Drosophila to the presence of certain parasitoid wasps: accelerated mating behavior. Flies exposed to certain wasp species begin mating more quickly. The effect is mediated via changes in the behavior of the female fly and depends on visual perception. The sight of wasps induces the dramatic upregulation in the fly nervous system of a gene that encodes a 41-amino acid micropeptide. Mutational analysis reveals that the gene is essential to the behavioral response of the fly. Our work provides a foundation in a genetic model system for further exploration of how the activation of visual circuits by the sight of a parasitoid wasp alters both sexual behavior and gene expression.