The Cooley spruce gall adelgid (CSGA), Adelges cooleyi, is an insect pest native to the Rocky Mountains and the western coast of the US. It has a complex lifecycle that includes various spruce species (Picea spp.) as primary hosts and Douglas fir (Pseudotsuga menziesii) as the secondary host. To locate the source of introduced populations CSGA were collected on spruce from the native range and the eastern US and mitochondrial DNA sequence analysis was performed on the mitochondrial COI gene. Analysis of 439 base-pairs of COI grouped the insects by shared similarities and the resulting phylogeny identified the northern Rocky Mountain region as the likely source of introductions. Important trends include reduced genetic variation in introduced populations and grouping of samples from Utah with samples from Arizona and western Colorado. These trends were similar to those observed by sampling variation in the nuclear genome with AFLPs and may indicate genetic consequences to introduction and isolation of populations within the native range, respectively.