The spruce coneworm, Dioryctria abietella Denis & Schiffermu " ller ( Lepidoptera: Pyralidae), is a major pest in spruce seed orchards in Europe. Initial work on its pheromone ecology reported significant attraction of males to ( 9Z, 11E)- tetradecadienyl acetate ( 9Z, 11E- 14: OAc), but this compound appeared to be a very weak attractant in field trials. In this study, we show that a second female- produced compound, the polyunsaturated long- chain hydrocarbon ( 3Z, 6Z, 9Z, 12Z, 15Z)- pentacosapentaene ( C25 pentaene), is a strong synergist to 9Z, 11E- 14: OAc, and that both compounds are needed for significant attraction of males. In field- trapping experiments, the highest catches were obtained with high amounts of pentaene added to the acetate ( acetate: pentaene ratio from 1 : 10 to 1 : 30) and the highest doses ( 1- 3 mg per rubber septum dispenser) were the most attractive. Two potential behavioural synergists, ( Z)- 9- tetradecenyl acetate and ( 9Z, 12E)- tetradecadienyl acetate, turned out to be behavioural antagonists and both reduced trap catch of D. abietella in a dose- dependent way when added individually to the binary blend. The recent discovery of the C25 pentaene as a pheromone component of D. abietella, as well as of three North American congeners, suggests that the use of a mixture of so- called type I and type II pheromone compounds is a widespread motif of sexual communication within the genus. The identification of a highly attractive sex pheromone will help in developing efficient strategies for monitoring and control of D. abietella populations in European spruce seed orchards.