Byers, J.A., Birgersson, G., Löfqvist, J., Appelgren, M., & Bergström, G. 1990a. Isolation of pheromone synergists of bark beetle, Pityogenes chalcographus, from complex insect-plant odors by fractionation and subtractive-combination bioassay. Journal of Chemical Ecology 16:861-876.

Male Pityogenes chalcographus looking for a place to bore into bark of Norway spruce
Abstract-- Capillary gas chromatography with columns of different polarity and two-dimensional fractionation of effluents were used with novel subtractive-combination bioassays to rigorously isolate host- and insect-produced pheromone synergists of the bark beetle Pityogenes chalcographus (Coleoptera: Scolytidae). Methyl (E, Z)-2,4-decadienoate (E, Z-MD) and the previously identified chalcogran were found to be synergistically attractive to both sexes. E, Z-MD was produced sex-specifically in males, and only when they had fed on host-plant tissue. A Norway spruce monoterpene fraction (including alpha-pinene, beta-pinene, and camphene) increased the attractive response to the pheromone components. Dose-response curves for E, Z-MD and chalcogran in the laboratory bioassay indicated the two components are highly synergistic. The isolation methods are important for further progress in identifying certain semiochemical synergists found in trace amounts in complex chemical mixtures, such as when insects must feed in host plants in order to produce pheromone.
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Chemical Ecology