Influence of noncrop habitats on the distribution of Eriborus terebrans (Hymenoptera: Ichneumonidae) in cornfields

Eriborus terebrans (Gravenhorst) is the most abundant parasitoid of the European cornborer, Ostrinia nubilalis (Hubner), in Michigan. The within-field distribution of adult E, terebrans was sampled in 1991 and 1992 by placing malaise traps near a wooded edge, near an herbaceous edge, and in the field interior in each of 4 fields of corn, Zea mays L. During the 1st generation of 1991, significantly more females were captured in wooded-edge traps than in herbaceous-edge or interior traps. In the Ist generation of 1992, more females were again captured in wooded-edge traps in 2 fields, whereas in the other 2 fields, more were captured in both herbaceous-edge and wooded-edge traps than interior traps. In the 2nd generation of both years, there was no consistent pattern of distribution of adult females among sites. The distribution pattern of adult females among fields was not consistently correlated with the distribution of O. nubilalis larvae or percentage parasitism. Overall, fen er males were captured than females, and in the Ist generation of both years, more males were captured near wooded edges of cornfields in which the previous crop had been corn, than any other sites. Both male and female E, terebrans were captured in malaise traps at the interface of a woodlot canopy and a cornfield with capture zones from 1 to 4.3 m high. We propose that E. terebrans distribution in cornfields is influenced by resources such as sugar and a moderate microclimate present in adjacent woodlots but unavailable in early-season cornfields. Perennial noncrop habitats may play an important role in conservation of natural enemies for biological control.