Saltational evolution of contact sex pheromone compounds of Callosobruchus rhodesianus (Pic)

Callosobruchus rhodesianus (Coleoptera: Chrysomelidae: Bruchinae) is a pest of stored legumes occurring throughout the Afro-tropical region. The females of these beetles produce a contact sex pheromone that elicits copulation behavior in males. To identify the contact sex pheromone compound, cuticular compounds of virgin females were collected using a filter paper method. The compounds were fractionated by acid-base partition and chromatography techniques and assayed for their ability to elicit male copulatory activity. Gas chromatography-mass spectrometry analysis of the active fraction revealed that the main active compounds were hexahydrofarnesyl acetone (6,10,14-trimethyl-2-pentadecanone) (1) and 2-nonadecanone (2), and the synergists were C27-C33 hydrocarbons (HCs), n-alkanes, and a homologous series of mono- and di-methyl branched alkanes. The main compounds (1) and (2) were substantially different from the contact sex pheromones (monoterpene dicarboxylic acids) previously identified from congeneric species. Copulatory assays using synthetic standards together with the natural HCs revealed that both ketone compounds were needed for a full male response. The present results explain the mate recognition specificity of C. rhodesianus, and the differences between the pheromones in the beetles may be explained by saltational evolution.