The biogeography of 171 North American species of Euxoa (Lepidoptera: Noctuidae) is reviewed. For biogeographical analysis, species are arranged in groups according to habitat; these are 62 aridland species, 69 forest species, 19 widespread species, and 21 species with special or unknown habitat requirements. Aridland species are distributed primarily in grassland areas in the Great Plains and in sagebrush areas or piñon–juniper woodland in the intermontane region. Ranges in these two regions are connected in arid corridors through the Rocky Mountain region. Forest species in western North America are arranged in two groups: those that occur in both conifer forests and piñon–juniper woodland (17 species), and those that occur only in conifer forests (40 species). Species in the former category occur throughout the mountain ranges of the west including those of the Great Basin wherever suitable habitat occurs. Those of the latter category occur in conifer forests on mountain ranges around the Great Basin but do not occur in the Great Basin, even in mountain ranges that support suitable habitat. Most forest species that occur in the Rocky Mountain region also occur in disjunct woodland areas in the Great Plains.
The effect of Wisconsinan glaciation on the biogeographic regions of western North America is reconstructed from a review of fossil pollen and plant macrofossil studies. From this synthesis it is suggested that during the Wisconsinan glacial maximum, forest species were widely distributed in the Great Plains south of the Laurentian ice sheet, and at low elevations in the Sierra Nevada and Rocky Mountain regions. Piñon–juniper woodland species were widespread in the Great Plains. Forest species that do not occur in piñon–juniper woodland were distributed around the Great Basin but were prevented from reaching suitable habitat in mountain ranges within the Basin by unsuitable habitat in the intervening lowlands. The ranges of aridland species were greatly reduced during Wisconsinan time, particularly in the Great Plains. Aridland corridors through the Rocky Mountains were eliminated during the Wisconsinan glacial maximum.
The possibility of speciation during Wisconsinan time is reviewed. Although the distribution of many sister-species are what would be expected from speciation during the Wisconsinan, the distributions may reflect allopatric distributions during Wisconsinan time and not speciation.