Semiochemical inhibitors for protecting conifers from bark beetles (Coleoptera: Curculionidae) in the Western United States: research, development, and application.

In the western U.S., the research, development and application (RD&A) of semiochemical inhibitors for protecting conifers from bark beetles has focused on antiaggregation pheromones and, to a lesser extent, nonhost volatiles. The sequence begins with identifying potential inhibitors by searching relevant literature; sampling the volatile profiles of bark beetle extracts, frass, infested host materials, hosts, nonhosts, and the forest environment; and/or conducting gas chromatographic-electroantennographic detection analyses of bark beetle antennae. Promising inhibitors are then evaluated for levels of inhibition in trapping and tree protection assays under field conditions. Additional assays are required to refine inhibitors and to inform the spacing and distribution of release devices. Factors affecting levels of tree protection include (a) the population density of the target bark beetle species, (b) forest structure and composition, (c) variability in bark beetle responses to inhibitors, (d) the stereochemistry and chemical stability of inhibitors in the forest environment, (e) release rates of inhibitors, (f) strength of inhibition, (g) zone of inhibition, and (h) the presence of other stressors and disturbances. Once an inhibitor has been successfully developed, there are substantial costs and data requirements associated with registration. The commercial availability of inhibitors for protecting conifers from bark beetles is limited but evidence suggests there are favorable benefit-to-cost returns to be realized with additional investments in RD&A. Opportunities include (a) developing new (or better) inhibitors, inhibitory blends, and formulations, (b) automating (e.g., via unmanned aerial vehicles) and optimizing applications of inhibitors in the field, (c) developing cheaper and more efficient means of synthesizing inhibitors, (d) streamlining regulatory processes and incentivizing RD&A to expedite the commercial availability of products, and (e) integrating inhibitors with other management tactics. We review recent advances in the use of inhibitors for western pine beetle, Dendroctonus brevicomis LeConte, southwestern pine beetle, D. barberi Hopkins, mountain pine beetle, D. ponderosae Hopkins, Douglas-fir beetle, D. pseudotsugae Hopkins, spruce beetle, D. rufipennis (Kirby), northern spruce engraver, Ips perturbatus (Eichhoff), and pine engraver, I. pini (Say). Research on these and other bark beetles has informed the RD&A of inhibitors for other forest and agricultural pests.