ARS | Publication request: Mitochondrial DNA Variation and Range Expansion in Western Bean Cutworm (Lepidoptera: Noctuidae): No Evidence for a Recent Population Bottleneck

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 12, 2008
Publication Date: N/A

Interpretive Summary: The western bean cutworm (Striacosta albicosta) is a pest of both corn and dry bean crops. Until the start of the 21st century, this species was largely confined to the Great Plains. Recently, it has undergone a range expansion and can now be found throughout the Corn Belt as far east as Ohio. This sudden movement is puzzling given the species did not appear able to invade the eastern Corn Belt previously. One possible explanation is that some unknown barrier kept the species out of the eastern Corn Belt until a few individuals managed to cross, after which the population could spread. This scenario was expected to produce a characteristic pattern of genetic variation in the species. New populations were expected to be less genetically variable and have different frequencies of haplotypes (gene variants) compared to old, established populations. We examined DNA sequence variation in the mitochondrial ND1 gene in established populations from Wyoming and Nebraska and new populations in Iowa. The Iowa populations were just as genetically diverse as the Wyoming and Nebraska populations, and haplotype frequencies were similar among all populations. This result indicates that the western bean cutworm¿s current range expansion was not initiated by a small number of individuals that fortuitously managed to cross an existing barrier to movement. Rather, it seems more likely that some change in the environment of the eastern Corn Belt has suddenly made it a more suitable habitat for western bean cutworm. This information will be used by government and university scientists seeking to understand why western bean cutworm is attacking corn crops over a growing area.

Technical Abstract: The western bean cutworm, Striacosta albicosta (Smith) (Lepidoptera: Noctuidae) is a pest of both corn and dry bean crops. At the beginning of the 21st century, the species began to extend its range out of the Great Plains, eastward through the Corn Belt. This rapid range expansion is remarkable because the species distribution had been stable for the previous half century at least, despite the apparent abundance of suitable habitat (i.e., cornfields) immediately to the east. We hypothesized that if the western bean cutworm had to overcome a stable barrier to movement before commencing the current range expansion, it probably experienced a genetic bottleneck in doing so. To test this hypothesis, variation in the mitochondrial ND1 gene was studied in populations from Wyoming, Nebraska, and Iowa. No differences in overall genetic diversity or haplotype frequencies indicative of a bottleneck were observed between the recently founded populations in Iowa and the established populations in Wyoming and Nebraska. This result suggests that the sudden loss of an ecological exclusion mechanism, allowing the species to move east in appreciable numbers, is likely to have triggered the range expansion. This might be due to recent changes in corn farming practices and technology.