Few attempts to control forest insects by means of pheromone mating disruption have<br /> been reported. The first such experiments with the European pine sawfly, Neodiprion sertifer<br /> (Hymenoptera: Diprionidae), resulted in nearly complete trap catch reduction, but no effects on<br /> population density or sex ratio were noted. Unmated females lay eggs, which develop into males<br /> only. Therefore, if mating disruption would be successful, a more male-biased sex ratio is<br /> expected the next generation. One possible explanation for the early results is that mated females<br /> disperse into the treated area, and thus obscure the effects of the treatment. In order to reduce the<br /> effect of such immigrating females, the treated area was increased in the experiment described<br /> here from the earlier used 0.5 – 4 ha to 25 ha. The acetate of erythro-3,7-dimethyl-2-<br /> pentadecanol was used for disruption and released from dispensers every 10 m. The influence on<br /> male orientation was monitored by pheromone traps, baited with the acetate of (2S,3S,7S)-3,7-<br /> dimethyl-2-pentadeanol and placed at 100 m interval along two perpendicular, 1500 m lines<br /> intersecting the treated area. The trap catch reduction was near 100% during the first month, but<br /> then declined to around 90% during the second month. Mating frequencies were checked by<br /> comparing the sex ratio of the next generation from within and outside the treatment area. No<br /> effect of the treatment on the sex ratio was detected, and the frequency of mated females could be<br /> assumed to be independent of treatment. Alternative hypotheses to explain the failure of<br /> pheromone mating disruption in N. sertifer are discussed.