Declining infestations ofDendroctonus frontaliswere studied in terms of within-tree, within-spot, and within-area survival between a severe and mild winter. While brood survival differed dramatically between the 2 winters, the net effect on population trends was the same. Infestations in the study areas neither survived nor proliferated during the following spring and summer. During the severe winter of 1976–77 nearly 100% brood mortality occurred following a period of subfreezing temperatures and this was directly responsible for spot collapse. During the mild winter of 1975–76 survival was dependent on brood stage and location within the tree. Brood survival, which was comparable to that from the previous late summer and autumn, was not the cause of spot mortality. Periodic emergence during intermittent warm periods and a tendency to disperse combined with high dispersal losses were thought to be responsible.