Sample data from 215 infested lodgepole pines from 11 separate locations in the United States and Canada were analyzed to determine the relative effect of the measured variables on mountain pine beetle emergence density (production). From this analysis a model was developed which expresses production as a function of attack density and basic habitat suitability. The measured variables having the most influence on basic habitat suitability were lodgepole pine phloem thickness, cortical resin canals, host resistance, and predation by woodpeckers. Host resistance was also found to influence the intensity of attack. A theoretical model was constructed to examine the relative effects of lodgepole pine phloem thickness, and resistance to attack, on the dynamics of the beetle population. Analysis indicated that mountain pine beetle outbreaks are triggered by rapid declines in stand resistance resulting from climatic disturbances, insect defoliation, etc.