Assessment of risk posed by introduced braconid wasps to Pieris virginiensis, a native woodland butterfly in New England

The range of the native butterfly Pieris virginiensis Edwards has decreased in New York and Ontario since the 1940s. Loss of habitat and harm from Cotesia glomerata (L.), a braconid parasitoid introduced to North America in the late nineteenth century for biological control of the invasive pest butterfly Pieris rapae (L.), have been suggested as the causes of this decline. In western Massachusetts, extensive habitat suitable for P. virginiensis remains and its principal host plant, Cardamine diphylla, is common. We found P. virginiensis to be widely present, occurring at 39% of host plant patches. In laboratory tests, we found that both C. glomerata and Cotesia rubecula (Marshall) are able to parasitize and successfully develop in P. virginiensis larvae when these are presented on detached leaves of C. diphylla. However, when we exposed laboratory-reared first or second instars of either P. rapae or P. napi (both suitable hosts for these Cotesia spp.) as sentinel larvae on leaves of either Brassica oleracea or C. diphylla at sites where P. virginiensis was present, no parasitism by either Cotesia sp. was detected. Some sentinel larvae were parasitized by an unidentified ichneumonid in the genus Hyposoter. We conclude that in western Massachusetts, P. virginiensis is widely distributed at low densities and that while it is physiologically an acceptable and suitable host for both C. glomerata and C. rubecula, larvae of field populations of this butterfly are not attacked because these parasitoids do not forage in forested habitats, even when they are locally present in adjacent meadows. Consequently, we report that this butterfly is not threatened by these parasitoids contrary to early suggestions in the literature.