Pieris oleracea, formerly Pieris napi, was once a widespread pierid butterfly in New England until the introduction of a biological control agent, Cotesia glomerata. It has been suggested that C. glomerata is responsible for the range reduction of P. oleracea. There are been several introductions of a second more specialized biological control agent, Cotesia rubecula, to the United States since the 1960’s. My first goal was to determine the current distribution and status of P. rapae parasitoids and the effectiveness of C. rubecula as a biological control agent since its release. The findings of a survey I conducted of the parasitoid community of P. rapae indicate that C. rubecula now occurs as far west as North Dakota and has become the dominant parasitoid of P. rapae in the northeastern and north central United States and adjacent parts of southeastern Canada, where it has displaced , the previously dominant parasitoid.

Survival of artificially established cohorts of P. rapae larvae was assessed in a collard patch on an organic vegetable farm in western Massachusetts. There was a significant drop in larval survival between the 4th and 5th instar due to parasitism by C. rubecula. This was change from survival curves of P. rapae from a 1985-1986 study, in which there was a significant drop in survival between the 5th instar and pupal stage due to C. glomerata.

The final goal of my thesis work is to try to understand why P. oleracea was able to survive at the focal study site in Lenox, MA despite parasitoid pressure and range reduction elsewhere in New England. In olfactometer tests, there was no difference in attractiveness of naïve C. glomerata females to volatiles of either Cardamine pratensis (cuckooflower) foliage, the host plant of P. oleracea or Brassica olercea (collard) foliage (P = 0.51). In order to determine if overtopping by other vegetation may provide an enemy free space for P. oleracea by affecting detection by C. glomerata, cage experiments were conducted. Overtopping vegetation had a significant effect on parasitism by C. glomerata (F = 12.8, df = 3, P <0.001), and may be the reason P. oleracea has been able to thrive at the Lenox, MA site.