EFFECT OF LANDSCAPE STRUCTURE ON PARASITOID DIVERSITY AND PARASITISM IN AGROECOSYSTEMS

The armyworm (Pseudaletia unipuncta) was used as a model host insect t o explore the influence of agricultural landscape structure at two spa tial scales on larval parasitoid species richness and rates of larval parasitism in southern Michigan. First, within fields, we compared par asitoid communities in maize fields near, and distant from, a hedgerow edge. Second, we replicated these studies within a complex landscape (agricultural fields of small size embedded in a landscape with abunda nt hedgerows and woodlots) vs. a simple landscape (agricultural fields of large size embedded in a landscape with few hedgerows and woodlots ). The structural differences between the simple and complex agricultu ral landscapes were characterized by analysis of aerial photographs an d digital land-use data. After landscape analysis, three maize fields from each area were selected for the experimental studies. The complex landscape contained fields that were 75% smaller, had 63% more perime ter of wooded field edge per hectare of field area, and had 81% more f ield edge in wide hedgerow than fields in the simple landscape. Fields in the simple landscape had 74% and 53% more field edge in herbaceous roadside and crop-to-crop interfaces, respectively, than did fields i n the complex landscape. In the six selected maize fields, third and f ifth instar P. unipuncta were released individually onto maize plants 5 m and 90 m from a hedgerow edge. Larvae were recovered after 4-5 d a nd reared in the laboratory to record parasitoid emergence. Parasitoid species diversity was similar in both landscape types (simple landsca pe: four species; complex landscape: five species). Mean percentage pa rasitism was significantly higher in the complex landscape than in the simple landscape(13.1% vs. 2.4%) but was not affected by the location within fields (near hedgerows vs. distant from hedgerows) in either l andscape type.