A two-component pheromone blend containing a 1:1 (vol:vol)ratio of isomeric 5,1l-dimethylheptadecane and isomeric 2,5-dimethylheptadecane was tested in high-capacity Unitraps to monitor populations of the western hemlock looper,Lambdina fiscellaria lugubrosa(Hulst), throughout the coastal western hemlock and interior cedar hemlock biogeoclimatic zones of British Columbia. Rubber septa were loaded with each component at doses of 10, 100, 1,000, and 10,000 μ (1992) and 1 and 10 μ (1993). Males demonstrated a dosedependent response, except that captures in traps with the 2 highest doses (1,000 and 10,000μ) did not differ. Unitraps with 10-μ lures captured significantly more males than similarly baited sticky traps at the beginning of the flight and with 1- and 10-μ-baited lures at midflight. Both 1- and 10-μ lures maintained their attractiveness over the 3-mo flight season, which extends from mid-August through October, but catches in traps with l-μ lures were low. In all cases,>80% of the males were captured by the beginning of October. Seasonal trends in trap catches were not closely related to temperature. This research suggests that traps baited with 10-μ lures could be used to monitor populations and detect incipient outbreaks of the western hemlock looper.