Sex Pheromone of the Introduced Pine Sawfly, Diprion similis, Revisited to Define a Useful Monitoring Lure: Deviating Chiral Composition and Behavioural Responses Compared to Earlier Reports

SIMPLE SUMMARY: Larvae of sawflies within the family Diprionidae feed on conifer needles and can cause significant damage to a tree by reducing its growth rate and even causing its death. In forest protection, it is therefore important to make use of various tools to detect the potentially harmful species, and the chemical signals emitted by female sawflies to attracts males, i.e., sex pheromones, have been identified for several species. However, a very precise natural pheromone is often expensive to produce and formulate, and in this study we investigated if the previously reported superior mixtures of similar substances (stereoisomers) actually improved the trap catches of the introduced pine sawfly, Diprion similis. Our field tests performed in Ontario, Canada, did not verify the necessity of adding other stereoisomers to the main pheromone component, the propanoate of (2S,3R,7R)-3,7-dimethylpentadecan-2-ol, in order to obtain maximum catch. Thus, the main component alone can be used in monitoring programs aiming at detection of the introduced pine sawfly. When testing the threo four-isomer blend, it was as attractive as the main component alone, suggesting that monitoring programs can use this more easily synthesized mixture without losing efficiency. We also highlight the need for renewed investigation of male attraction to various isomeric mixtures previously proposed as the sex pheromones for other diprionids. ABSTRACT: Extracts of Diprion similis females contained about 15 ng of the sex pheromone precursor 3,7-dimethylpentadecan-2-ol per female. After derivatisation with (S)-2-acetoxypropanoyl chloride, we found that the major stereoisomer in the extract was (2S,3R,7R)-3,7-dimethylpentadecan-2-ol. Small amounts of other stereoisomers of 3,7-dimethylpentadecan-2-ol were also identified in the extract, namely 1% of (2R,3S,7S), 0.3% (2R,3R,7R) and 0.4% of (2R,3R,7S). An unknown fifth substance showed a very similar spectrum to 3,7-dimethylpentadecan-2-ol, both in SIM and full scan mode. None of the earlier suggested behavioural synergistic isomers ((2S,3S,7S), (2S,3S,7R) and (2S,3R,7S)) were detected in the extracts. In field tests in Ontario, Canada, the earlier identified main pheromone component, viz. the propanoate of (2S,3R,7R)-3,7-dimethylpentadecan-2-ol, was tested alone and in combination with other stereoisomers, earlier reported to be synergistic. No synergistic effects were detected and the threo four-isomer blend was as attractive as the pure main compound. Thus, one of the few examples of a diprionid sawfly using more than one substance in its sex pheromone could not be confirmed. The results also suggest that monitoring programs can use the more easily synthesized threo-blend without losing efficiency. Furthermore, the study suggests that other diprionid pheromones may benefit from a reinvestigation, to clarify possible synergistic effects of stereoisomers.