Ramirez, C.C. Centro de Investigación en Biotecnología Silvoagropecuaria, Instituto de Biología Vegetal y Biotecnología. Universidad de Talca,Talca, Chile.The response of field-collected Hylastinus obscurus Marsham (Coleoptera: Scolytidae) to volatiles from Trifolium pratense L. of different ages was studied in a four-arm olfactometer. Volatiles from 1.5-, 2-, and 2.5-yr-old plants were more attractive than volatiles from 1-, 3-, and 3.5-yr-old plants. Two-year-old plants were preferred during winter, spring, and summer. One-year-old plants were not preferred in their early stages, but preference increased as they aged. Volatiles from 3-yr-old plants were never preferred. The essential oils obtained from 2-yr-old T pratense elicited an attraction response from H. obscurus. Beetle response is discussed in relation to host-locating behavior.