A chiral sex pheromone system in the pea midge, Contarinia pisi

The sex pheromone of the pea midge consists of 2-acetoxytridecane, (2S,11S) -diacetoxytridecane and (2S,12S)-diacetoxytridecane. The responses of male pea midges to the corresponding stereoisomers of (2S,11S)-diacetoxytridecan e and (2S, I 2S)-diacetoxytridecane were tested in field trapping experimen ts and by electroantennographic recordings. When added at 20% of the pherom one component to the sex pheromone blend. the (2S,11R)- and (2R,11S)stereoi somers of (2S, I IS)-diacetoxytridecane, were shown to have a strong inhibi tory effect on male attraction in the field. At the same dose, (2R,11R)diac etoxytridecane, (2R,12R)-diacetoxytridecane, and meso-2,12-diacetoxytrideca ne, did not have a significant effect on male behavior. It was also shown t hat substitution of either (2S, 11S)-diacetoxytridecane or (2S, I 2S)-diace toxytridecane with the related stereoisomers reduced trap catches to the le vel of blank traps. The electroantennographic recordings showed similar dos e-response curves for the pheromone components and the stereoisomers shown to have an inhibitory effect. It seems likely that male antennae have recep tors for both pheromone components and for inhibitory stereoisomers. Scanni ng electron microscopy and transmission electron microscopy of the antennae revealed three types of sensilla involved in chemoreception: sensilla circ umfila, sensilla trichodea. and sensilla coeloconica. The sensilla circumfi la and trichodea are both innervated by two sensory cells, whereas the sens illa coeloconica are innervated by four to five cells.