Pheromone stereochemical specificity in the biology of the bean beetle Acanthoscelides obtectus

The dried bean beetle, Acanthoscelides obtectus, is a serious pest of legume crops, particularly Phaseolus vulgaris beans, and their management is challenging due to the beetle's cryptic larval stage. The major male-produced pheromone of A. obtectus is methyl (E,R)-2,4,5-tetradecatrienoate, crucial for female attraction, with the (S)-enantiomer emitted in smaller amounts (R:S ca. 9:1). Despite its identification half a century ago, it is still not commercially available for bruchid surveillance due to production and stability issues, with gaps in our knowledge relating to the bioactivity of the stereoisomers. We thus aim to clarify the behavioural specificity of the (R)- and (S)- enantiomers of methyl (E)-2,4,5-tetradecatrienoate, along with two commercially available isomers. Electrophysiological (EAG) assays with virgin females show that stereochemically pure methyl (E,R)-2,4,5-tetradecatrienoate is as EAG-active when part of the natural 9:1 or the racemic 1:1 blends, whereas the stereochemically pure (S)-enantiomer is inactive. Moreover, the structural analogues do not elicit significant EAG responses. Virgin females give positive behavioural responses in the olfactometer only to methyl (E,R)-2,4,5-tetradecatrienoate, but not to its antipode or the structural analogues. Furthermore, stereochemically pure methyl (E,R)-2,4,5-tetradecatrienoate elicits the same level of behavioural activity as the 9:1 blend. Curiously, presence of the (S)-enantiomer in equal proportions synergises female preference for stereochemically pure (R). These findings provide a better understanding of the pheromone biology of A. obtectus and create a platform for the development of pheromone trap-based surveillance with racemic methyl (E)-2,4,5-tetradecatrienoate.