none (Z)-4-Tridecenal was identified as a sex pheromone component of Macrocentrus grandii, a larval parasitoid of the European corn borer. The aldehyde was found to be a common air oxidation product from a series of (Z,Z)-9,13-dienes of 27-33, 35, 37, 39, and 41 carbon atoms in female wasps. Synthetic (Z)-4-tridecenal and (Z,Z)-9,13-heptacosdiene each elicited flight initiation, upwind anemotaxis, casting, landing on the source, walking, wing fanning, and copulatory attempts by male wasps in a wind-tunnel. Field studies demonstrated that both compounds are synergized by a more polar component. Synthetic (E)-4-tridecenal was not attractive.