Schlyter, F., Birgersson, G., Byers, J.A., & Bakke, A. 1992. The aggregation pheromone of Ips duplicatus and its role in competitive interactions with I. typographus (Coleoptera: Scolytidae). Chemoecology 3:103-112. pdf

Abstract-- Ips duplicatus with I. typographus co-inhabiting Norway spruce (Picea abies (L.) Karst.) would benefit from a pheromone blend distinct from that of the larger competitor I. typographus. GC-MS analysis showed that I. duplicatus males feeding in the host produced ipsdienol (Id), cis-verbenol (cV), trans-verbenol (tV), myrtenol (Mt), and E-myrcenol (EM) and traces of 2-methyl-3-buten-2-ol (MB). I. duplicatus produced Id in approximately racemic form (48.9-54.5% (+)-(S)-isomer). The amounts of Id and EM released over a 9 day period had a maximum of 250 and 5 ng/h/male, respectively, on day 2. Exposure of I. duplicatus males to myrcene and alpha-pinene resulted in the production of small amounts of Id, cV, tV, Mt, and trans-pinocarveol, but not of EM. In laboratory bioassays with walking beetles, the pheromone component Id alone was weakly attractive while EM was inactive, but in binary combination with Id strongly synergized attraction. A combination of EM and Id at a release rate equivalent to 100-200 males was more attractive in the field than 70 unmated males in a spruce log. The addition of myrcene (a suggested pheromone precursor of Id) to Id did not enhance trap catches, while addition of EM increased catches > 10-fold. Subtracting EM from a blend of Id, EM, cV, MB drastically reduced trap catches while subtraction of cV or MB or both had no significant effect. Addition of EM over a wide concentration range to the synthetic pheromone of I. typographus did not reduce the attraction of females of this species in the laboratory. A two-species pheromone interaction field test releasing I. typographus pheromone components (MB+cV) at 10-1000 male equivalents (ME) and I. duplicatus pheromone (Id+EM) at 0, 10-1000 ME in all possible combinations showed both positive intraspecific dos-response effects and an interspecific inhibition. Higher release rates of EM appeared to inhibit I. typographus, especially males. In a tree colonization model, the response of the two competing species to their respective pheromones show a good separation during the mass-attack with a small initial cross-attraction. It remains to be shown whether either of the two pheromone systems have in fact evolved in the present sympatry, or if they are an incidental effect of ancestry of these phylogenetically distant Ips.
Chemical Ecology