Four components, (Z)-9-tetradecenal (8.6%), (Z,E)-9,12-tetradecadienal (4.8%), (Z)-11-hexadecenal (49.5%), and (Z)-13-octadecenal (37.1%), were identified in extracts of female pheromone glands of the European sunflower moth,Homoeosoma nebulellum (Lepidoptera: Pyralidae) using GC and GC-MS analyses. EAG and single-cell recordings of male antennal receptors gave strong evidence for (Z,E)-9,12-tetradecadienal as the antennal key compound of sex pheromone detection in this species. This result was confirmed by field trapping; removal of (Z,E)-9,12-tetradecadienal from quaternary blends completely suppressed the male catches. The synthetic blends with this compound as a major component caught five times less males than the blends reproducing the ratio found in the female extracts [5% of (Z,E)-9,12-tetradecadienal only]. The occurrence of a minor component perceived as the most biologically relevant compound is discussed.