Corthylus columbianus Hopkins became permanently established in. the study area in southern Indiana during 1940–62. Three epicenters were present in 1945. Two of these shifted, expanded, and merged the following year. The two remaining epicenters merged during the first outbreak. As the population declined after the first outbreak and began retreating toward the center of the stand, two epicenters were again definable. One of these alternately expanded and contracted for several years demonstrating, qualitatively, that epicenters are definable and dynamic in time and space. That is, any particular locus undergoes some degree of areal shift each year and varies in size through time.

Quantitatively, attack and egg cradle densities within the epicenters follow the same pattern. Densities were generally lowest in the newly invaded areas. Except during epiphytotics, highest density areas were usually aggregated near the center of the stand, but shifted from year to year. For example, only one tree contained a high density in 1954; by 1956 the high density area had shifted spatially and encompassed 10% of the study trees.

An “indigenous population” of C. columbianus is an areal concept. The insect withdrew to certain areas, not certain trees, during times of low population numbers. The “brood” tree concept–a single tree containing all or a majority of the population during times of low population numbers–is rejected in the case of the Columbian timber beetle for the study area in southern Indiana.