The antiaggregation pheromone verbenone was operationally tested for 5 yr to deter mass attack by the mountain pine beetle on lodgepole pine in campgrounds and administrative areas surrounding Redfish and Little Redfish Lakes at the Sawtooth National Recreation Area in central Idaho. Each year, five-gram verbenone pouches were evenly distributed (-10 m apart) within seven of 14 0.2-ha plots. During the first 2 yr of the study a median of 12% of the host trees >13 cm dbh were attacked and killed on the treated plots, whereas trees on the untreated plots incurred a median mortality of 59%. When ?50% of the trees on the untreated plots were killed a detectable beetle response to verbenone on the treated plots dramatically declined. After 5 yr, mountain pine beetle had killed a median of 87% of the lodgepole pine trees >13 cm in untreated plots and 67% in plots containing verbenone pouches. Beetle pressure was higher on untreated plots in 2000 and 2001, nearly equal between treatments in 2002, higher on verbenone-treated plots in 2003, and similar between treatments in 2004. It is hypothesized that the lack of response to verbenone after 2 yr may be related to both population size and spatial scale, i.e., large numbers of vigorous beetles in a local area with a reduced number of preferred large-diameter trees become crowded and stressed, causing a decline in the response to verbenone. The 2-yr delay in widespread pine mortality caused by verbenone would have given land managers time to use other management tactics to deter catastrophic loss of trees caused by mountain pine beetle.