Davis goes through the litany of possible and probable causes of population declines for virginiensis — deer browse, garlic mustard as an oviposition sink, shade-tolerant garlic mustard as a direct competitor with native larval host plants, competition with Pieris rapae, and a host of others — in her detailed examination of the life history and population pressures for this species. Her focus was a study site in Morrow Co OH, a site that runs the gamut of presumed populations stressors for West Virginia White.
Davis hypothesizes that the end of the last Ice Age produced an explosion of new habitat for P. virginiensis, which colonized extensively through the upper Midwest and down the Appalachian Spine. However, she notes, available habitat in Ohio (and elsewhere on the southern periphery of the species’ occurrence) is disappearing rapidly, leading to many local extinction events that will likely increase in the future.
She also affirmed the hypothesis of introduced, invasive garlic mustard as an inappropriate oviposition sink for WV White — the females will readily lay eggs on garlic mustard, but it kills 100% of the larvae that try to eat it. However, she and other workers have not been able to quantify exactly how strong a sink this is, especially as virginiensis also seems to use another spring cress in Ohio in addition to toothwort. We believe that Maryland populations of virginiensis use the toothwort (C. diphylla) exclusively unless tricked into ovipositing on garlic mustard.
The most concerning trend Davis uncovered is the possible uncoupling of the phenology of the butterfly and its host plant. Examination of flight records for the past hundred years for the butterfly suggest that its emergence is mainly tied to temperature, and rising average temperatures have meant that West Virginia White emerges earlier and earlier in the spring over time, presumably tied to climate change. Toothwort, however, does not show the same trend — its emergence and growth period has stayed the same, apparently cued to some factor not affected by climate change. Eventually, Davis suspects, the butterfly may simply emerge too early to be able to find and oviposit on toothwort.
Samantha Davis’ dissertation is available from the Wright State repository.