Wing apex melanization variation in cabbage white butterflies (Pieris rapae): Spatial and temporal differences across North American specimens

Pieris rapae was accidentally introduced to North America from Europe in the late nineteenth century and spread westward. Mountain ranges serve as a potential barrier preventing two populations from breeding, possibly magnifying the founder effect for traits genotypically determined and potentially leading to differences between them - including variation in wing apex melanization. This study evaluates differences in the ratio of apex melanization area to whole wing area of P. rapae on opposite sides of North America. We hypothesized there is a difference in the ratios of wing apex melanization for specimens collected from eastern North America versus west of the Rocky Mountains. Specimens were downloaded and analyzed from iDigBio. Both the left and right wing’s length, width, wing spots, and apex melanization were measured. They were also georeferenced. Only specimens with nonzero values for length, width, and apex melanization were included in the final analysis. A linear regression test yielded a correlation between moving westward and the ratio of wing apex melanization. A t-test showed a difference between eastern and western specimens’ average ratios. There are implications when considering the effects of climate change across North America. Cabbage white butterflies, known pollinators, may lack the necessary variation in traits to overcome climate change’s effects, impacting the North American plants adapted to rely on P. rapae for survival.