Interactions between Pierid butterflies: A new threat to the West Virginia White butterfly?

Background/Question/Methods

Pieris virginiensis, the West Virginia White butterfly, is a rare, native, univoltine butterfly that occupies mature woodlands, completing its lifecycle on native mustard hosts. Since the introduction of Alliaria petiolata (garlic mustard), a biennial invasive forest mustard, there has been concern that P. virginiensis lays its eggs on A. petiolata, but they do not survive. To observe oviposition, we conducted a focal animal survey and recorded behavior at 10-second intervals in two locations: Wooster, OH and Cleveland, OH.  We returned weekly throughout the flight season (April-May 2012). 

During surveys, we noticed Pieris rapae, the European cabbage white butterfly, using woodlands at the Wooster, OH site, an aberrant behavior for this multivoltine pest which can complete its life cycle on weedy mustards such as Brassica and Barbarea spp., and even A. petiolata. After this observation, we conducted behavioral surveys of both P. virginiensis and P. rapae to answer the following questions:  From which plants do these butterflies take nectar? On which plants do these butterflies lay eggs? Do these butterflies interact with each other within or between species? Do the butterflies differ between species in the amount of time that they spend performing each activity? How long does each species persist?

Results/Conclusions

Butterflies took nectar from several genera of spring wildflowers, including Claytonia, Cardamine, Trillium, Phlox, and Viola species. I observed 8 oviposition events by P. virginiensis on Cardamine diphylla, a native mustard, and none on garlic mustard, the invasive. I observed 4 ovipositions by P. rapae, all on C. diphylla. Although these butterflies occupy the same habitat, no direct congeneric interaction was observed. Conspecifics chased each other (3 events), and both Pieris spp. were occasionally chased by the numerous Vanessa atalanta.

Pieris virginiensis butterflies spent 37.8% of their observed time in flight, 29.8% resting, 30.1% nectaring, and just 2.3% engaged in egg-laying activity (58:10 m:s total observation). In contrast, P. rapae spent 65.2% of time in flight, 11.6% resting, 21.9% nectaring, and 1.3% ovipositing (51:40 m:s total observation). As of May 1^st, P. virginiensis were vacant from the southern field site (Wooster), but persisted in Cleveland, and P. rapae still persisted in Wooster. P. rapae may be competing for nectar, oviposition and larval resources with the native P. virginiensis. Given that it can also utilize A. petiolata as a larval host while P. virginiensis apparently cannot, P. rapae may be a previously unrecognized threat to this rare butterfly.