Mathematical models describing losses of lodgepole pine (Pinus contorta var. latifoliaEngelmann) and survival of mountain pine beetle (Dendroctonus ponderosaeHopkins) by life stage were prepared from data covering a 13-yr period. The greatest survival and emergence of beetles/unit area of bark occurred in trees of large diameter. After most trees of large diam were killed, gallery starts and egg production continued to increase. However, larval survival declined and emergence returned to the endemic level. At high densities of gallery starts and inches, beetle survival was low, probably because of excessive competition among larvae and drying of phloem. Under these conditions of stress, the sex ratio appears to shift further in favor of ♀ after most of the large trees are killed. These observations demonstrate the close association of beetle dynamics with diam structure of lodgepole pine stands, and support the theory that epidemics are strongly dependent upon the presence of large trees having thick phloem.