Sex pheromone stereochemistry and purity affect field catches of male aphids

Male blackberry-cereal aphids, Sitobion fragariae (Walker), were caught in the field by water traps releasing synthetic (4aS,7S,7aR)-nepetalactone, the major sex pheromone component. The presence of the enantiomer (4aR,7R,7aS)-nepetalactone reduced catches and a plant-extracted (4aS,7S,7aR)-nepetalactone was less effective than the 99% (7S)-lactone. A more extensive trial with the bird cherry-oat aphid, Rhopalosiphum padi, involving the pheromone comprising the stereochemically related (4aS,7S,7aR)-nepetalactol showed a similar trend. The (7R)-isomer caught fewer males than the (7S)-isomer, but in this case the addition of high-purity (7S)-lactol to make a 50% blend caught as many males as pure (7S)-lactol. With plant-derived lactol, further purification did not significantly increase the catch. It is suggested that trace compounds associated with reduced enantiomeric purity in terms of the (7S)-configuration or from plant sources reduce activity of the sex pheromone components. Male damson-hop aphids, Phorodon humuli (Schrank), were also caught in the synthetic lactol traps, and it is suggested that this is due to traces of the (4aR,7S,7aS)-nepetalactols, which comprise the sex pheromone for this species. The significance for aphid chemical ecology studies and pest control strategies is discussed.