none Macrocentrus grandii; PHEROMONES-; Synthetic lactone compound enantiomers, behavioural response assays; ORIENTATION-; Flight initiation & anemotaxis response to synthetic pheromone enantiomers In a previous study we reported identification of (3R*,5S*,6R*)3,5-dimethyl-6-(-ethylethyl)-3,4,5,6-tetrahydropyran-2-one as a component of the pheromone of Macrocentrus grandii Goidanich. The lactone was present in male and female wasps, and laboratory and field bioassays demonstrated that both sources of the lactone elicit flight initiation, upwind anemotaxis, and casting in male wasps. In the present study, the synthetic (3R,5S,6R)- and (3S,5R,6S)-lactone enantiomers (RSR and SRS, respectively) were bioassayed for biological activity. In wind tunnel studies the SRS enantiomer elicited flight initiation, upwind anemotaxis, and casting by male wasps comparable to lactone derived from male and female wasps. Flight response to the RSR enantiomer averaged 14 percent of the SRS enantiomer. No specific ratio of the stereoisomers was found more attractive than the SRS enantiomer alone. Field studies demonstrated the SRS enantiomer was active alone in attracting male wasps. When paired with (Z)-4-tridecenal (a previously identified female-derived sex pheromone), the SRS enantiomer yielded a synergistic response comparable to (Z)-4-tridecenal plus female-derived lactone.