Changes in key enzymes in the biosynthetic pathways of sex pheromone components can produce differences in component ratios and structures. The sex pheromone communication system is critical to reproduction and the maintenance of a species and so changes in this system can play a major role in the speciation process. Artificial selection of female redbanded leafroller moths that produced either higher or lower ratios of 14-/12-carbon pheromone components was used to study how the biosynthetic pathways were affected in the high and low populations. The results showed that the chain shortening enzymes were selective for the (E) isomer and so left the 14-carbon acyl intermediates enriched in the (Z) isomer. Thus, the high population, which has a higher amount of 12-carbon components, also has a lower ratio of E11-/Z11-14:OAc pheromone components. The data also suggested that chain shortening occurred prior to reduction and acetylation of the 14-carbon components. These changes are not sufficient to isolate the redbanded leafroller populations, but we discuss some cases where significant changes in pheromone component ratios are affected by the chain-shortening enzymes.