Does the native West Virginia White butterfly (Pieris virginiensis) oviposit on invasive, toxic garlic mustard (Alliaria petiolata)?

Background/Question/Methods

Pieris virginiensis Edwards, the West Virginia White butterfly, is a rare, univoltine butterfly  that resides in mature riparian forests in the eastern United States.  This butterfly uses Brassicaceae family members Cardamine diphylla, C. concatenata, and Arabis laevigata as its primary host plants, and nectars on a wide variety of spring ephemerals.  Evidence indicates that P. virginiensis occasionally oviposit on the exotic invasive plant Alliaria petiolata, garlic mustard, but caterpillars do not survive.  However, the frequency of oviposition “mistakes” in natural habitats has not been quantified.  We systematically surveyed host plants at a site in Morrow Co., OH, that was surveyed in 1988 for P. virginiensis oviposition, to determine when and on which larval host plants oviposition occurs.  We examined host plants for eggs and caterpillars, and quantified the presence of predators and damage present.  In addition, we examined oviposition preference through oviposition choice tests in Allegany State Park, Salamanca, NY.   Butterflies were offered three host plants: first or second year Alliaria petiolata, first or second year Arabis laevigata, or flowering Cardamine diphylla to determine which age and species of plant is most preferred by ovipositing butterflies. 

Results/Conclusions

In Morrow Co., OH; we found two adult butterflies during the first field visit in 2011, and no flying adults during any field visit in 2012.  We found no eggs or caterpillars in 2011 and just one egg in 2012; in contrast, the 1988 systematic search of the same area revealed 123 eggs.  The lack of flying adults and eggs suggests that this population is severely reduced and possibly extirpated from this site, however, climate change over the last twenty years may have contributed greatly to this site’s declining butterfly population: flight season wind speed, minimum temperature, and maximum temperature all have increased significantly at this site over the last 20 years.   During preliminary oviposition trials with two butterflies, the butterflies oviposited all eggs on Alliaria petiolata , even when offered native host plants Cardamine diphylla and Arabis laevigata. 

Given our preliminary results as well as anecdotal and historical observations, we expect that 2013 field data will confirm and extend the observation that P. virginiensis prefers to lay its eggs on the invasive A. petiolata.  There might be a link between increasing Alliaria petiolata in a forested site and decreasing P. virginiensis populations, however, more work needs to be done.