Does Agricultural Landscape Structure Affect Parasitism and Parasitoid Diversity?

This study evaluates whether previous observations of a higher percentage of parasitism and parasitoid diversity in a complex agricultural landscape, relative to a simple landscape, represent a general phenomenon. Rates of parasitism and parasitoid diversity of the armyworm (Pseudaletia unipuncta) were assessed in three replicate (Onondaga, Ingham, and Benton) regions in southern Michigan. Within each region, a simple landscape (primarily cropland) and a complex landscape (cropland intermixed with mid and late successional noncrop habitats) were identified through analysis of aerial photographs. In each landscape, three maize fields were selected, and second to fourth instar P. unipuncta were released into three replicate plots of maize plants. Larvae were recovered after 6 d and reared in the laboratory to record parasitoid emergence. A principal component analysis conducted on landscape variables indicated that simple and complex landscapes were true replicates. Extra-field vegetation was similar among the three simple landscapes but differed among complex sites. Parasitoid species diversity differed among regions, with six species recovered in Onondaga and two species from both Benton and Ingham. Rates of parasitism were only partially consistent with previous experimental results. In Ingham, results were similar to those found previously in this region; rates of parasitism and parasitoid diversity were higher in the complex landscape. However, in Onondaga and Benton, no difference in rates of parasitism or parasitoid diversity were found. Thus, the hypothesis that landscape complexity increases parasitoid diversity and rates of parasitism was not supported. Possible reasons for the observed differences in rates of parasitism among regions are discussed.