How environmental conditions and changing landscapes influence the survival of a rare woodland butterfly, Pieris virginiensis.

Background/Question/Methods

Rare organisms are often strongly affected by chance, disease, invasive species, and other factors.  Pieris virginiensis (Pieridae), a rare woodland butterfly, flies only in April and May, in often unsuitable weather, and uses the native mustards Cardamine diphylla, C. laciniata, and Arabis laevigata as its primary larval hosts.  P. virginiensis may be adversely affected by the introduction of an invasive mustard, Alliaria petiolata, and other biotic and abiotic stressors.  A. petiolata, which has feeding deterrents known to affect larvae of other Pieris species, has been observed as an oviposition site for P. virginiensis in some locations.  We reexamined a population of P. virginiensis in Ohio that uses A. laevigata as its primary host.  This population was last studied in the 1988 prior to the introduction of A. petiolata to the area.  We sought to test the hypothesis that the introduction of A. petiolata has changed the oviposition behavior of P. virginiensis.  On April 28, 2011, we marked 64 flowering stems of A. laevigata and 54 flowering stems of A. petiolata in the exact location where the previous study was performed and examined them weekly for eggs, larvae, herbivore damage, and other observations. 

Results/Conclusions

We observed 5 P. virginiensis individuals at the research site on 4/21/2011, but none had begun oviposition.  Returning weekly until 5/12/2011, we recovered no P. virginiensis eggs or caterpillars, observed no Pieris-specific damage on monitored plants, and we failed to observe any butterflies.  To examine alternative stressors on the butterfly or host populations, we recorded general damage, deer browsing, and the presence of possible larval predators.  Across all plants, 21.2% had evidence of minor leaf or stem damage, and 10.2 % of plants demonstrated significant deer browsing.  Ants and spiders were observed on 7.6% and 13.6% of the plants, respectively.  Although the invasive A. petiolata is well established at this site, we believe that the primary cause of butterfly mortality for 2011 was weather, with Columbus, Ohio reaching a new record of 18.1 cm precipitation from April 1-28.  From 4/1-5/12, only 33% of the days were marginally acceptable for adult flight (wind < 6.7 m/s, temp. > 10 ^o C).  We conclude that despite pressures due to invasion, predation, and deer browsing, poor flying conditions were the primary stressor in 2011 and apparently prevented any successful reproduction of this butterfly in this area.