Ambrosia Beetle Host Selection Among Logs of Douglas Fir, Western Hemlock, and Western Red Cedar with Different Ethanol and α-Pinene Concentrations

Logs from Douglas fir, Pseudotsuga menziesii; western hemlock, Tsuga heterophylla; and western red cedar, Thuja plicata, were left in the forest through winter. In April, segments from these logs were removed and randomly positioned adjacent to one another allowing ambrosia beetles to select their preferred host. In early June the tissues of Douglas fir and western hemlock logs contained significantly higher ethanol concentrations and ambrosia beetle (Trypodendron lineatum and Gnathotrichus spp.) densities than logs of western red cedar. Low beetle attack densities in western red cedar were probably a consequence of low ethanol concentrations. Although Douglas fir tissues produced significantly higher ethanol concentrations than western hemlock, the beetles did not effectively discriminate between these two conifer species. Ethanol and α-pinene were significant covariates for the ambrosia beetle densities. α-Pinene concentrations were highest in the phloem of western red cedar, intermediate in Douglas fir, and nearly absent in western hemlock. α-Pinene did not synergize the beetle's response to ethanol or to ethanol + pheromone during host selection, and it may have functioned as a deterrent.