Does agricultural landscape structure affect parasitism and parasitoid diversity?

This study evaluates whether previous observations of a higher percentage o f parasitism and parasitoid diversity in a complex agricultural landscape, relative to a simple landscape, represent a general phenomenon. Rates of pa rasitism and parasitoid diversity of the armyworm (Pseudaletia unipuncta) w ere assessed in three replicate (Onondaga, Ingham, and Benton) regions in s outhern Michigan. Within each region, a simple landscape (primarily croplan d) and a complex landscape (cropland intermixed with mid and late successio nal noncrop habitats) were identified through analysis of aerial photograph s. In each landscape, three maize fields were selected, and second to fourt h instar P. unipuncta were released into three replicate plots of maize pla nts. Larvae were recovered after 6 d and reared in the laboratory to record parasitoid emergence. A principal component analysis conducted on landscap e variables indicated that simple and complex landscapes were true replicat es. Extra-field vegetation was similar among the three simple landscapes bu t 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 fou nd previously in this region; rates of parasitism and parasitoid diversity were higher in the complex landscape. However, in Onondaga and Benton, no d ifference 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 d ifferences in rates of parasitism among regions are discussed.