Plant-derived pyrrolizidine alkaloids play an important role in the biology of the salt marsh moth, Estigmene acrea (Lepidoptera: Arctiidae). They are phagostimulants for larvae and they stimulate the growth and development of adult male androconial organs called coremata. In this study, we have shown that the pyrrolizidine alkaloid monocrotaline N-oxide (MNO) fed to larvae also affects the courtship behavior of adult males. Males fed a diet containing MNO display their coremata while males fed on the same diet without alkaloid rarely display. This explains why it has been difficult to replicate field observations of the “lekking” behavior of this species in the laboratory where animals are frequently raised on commercially available diets devoid of alkaloids. Corematal inflation was observed in isolated males and in laboratory leks. The effect of larvae feeding on pyrrolizidine alkaloid on the reproductive behavior of adults suggests that this substance may modify the development of the moth's nervous system and contribute to their unusual dual mating strategies. MNO was also shown to be an adequate precursor for the production of the courtship pheromone hydroxydanaidal.