Synergistic effect of a pheromone and a kairomone on host selection and colonisation by Ips avulsus

WE report for the first time a synergistic effect on the pheromone response of a bark beetle species (Coleoptera: Scolytidae) by the pheromone of a sympatric species. Ips avulsus Eichh. and I. grandicollis Eichh. are two sympatric Ips species often found colonising the same host tree in the southern USA. Male I. avulsus produce the pheromone ipsdienol (2-methyl-6-methylene-2,7-octadiene-4-ol) 1,2 and both sexes are attracted to this compound in the field 3 . Male I. grandicollis produce the pheromone ipsenol (2-methyl-6-methylene-7-octene-4-ol) 1,2,4 and both sexes respond to the S-(–)-isomer 5 . I. avulsus is known to respond in low numbers to both the crude attractant 6 and infested host material 7 of I. grandicollis . To investigate this relationship, a series of tests was carried out in east Texas. Two to four tree trunk-simulating olfactometers 8 set 10 m apart adjacent to a loblolly pine stand at Lufkin, Texas, were baited with test material during the afternoon, when flight of I. avulsus occurred. Test materials included the optically pure enantiomers of ipsenol 9 , racemic mixtures of ipsenol and ipsdienol, and billets infested with male I. avulsus and I. grandicollis . All test materials used in a test series were offered simultaneously.