Basic and Applied Ecology 5 (2004) 347–355 Host density influences parasitism of the armyworm Pseudaletia unipuncta in agricultural landscapes Alejandro C. Costamagna à , Fabia ´n D. Menalled 1 , Douglas A. Landis Department of Entomology, 204 Center for Integrated Plant Systems, Michigan State University, East Lansing, MI 48824-1311, USA Received 5 February 2004; accepted 27 April 2004 Summary Several studies have shown positive responses of parasitism to either host density or landscape complexity. However, no experiments have manipulated host density in landscapes of differing complexity. Here we report the results of a field experiment conducted to determine how host density and agricultural landscape structure jointly affect parasitism and parasitoid diversity of Pseudaletia unipuncta (Haworth) (Lepidoptera: Noctuidae). Parasitism was assessed by experimentally adding P. unipuncta sentinel larvae at low (1 larvae/plant) and high (3 larvae/plant) densities to detect parasitism in commercial cornfields located in a complex and a simple agricultural landscape. The braconid wasps Glyptapanteles militaris (Walsh) and Meteorus spp. accounted for 98.4% of the observed parasitism. Landscape structure did not influence parasitism (80.2% on average) and contrary to expecta- tions, showed a trend towards increased parasitoid richness and diversity in the simple landscape. Increasing host density revealed a trend of increasing parasitoid richness and diversity, and differentially affected parasitism at the parasitoid specific level. G. militaris parasitized a significantly greater proportion of hosts at low host density, while the opposite occurred for Meteorus spp. (primarily M. communis). These offsetting responses of parasitoids resulted in the lack of an overall host density effect on parasitism. The differential response of these parasitoids to host density is discussed in relation to differences in morphological and life history characteristics. Our results suggest that the specific composition of parasitoid assemblages could significantly alter parasitism at different host densities independently of landscape structural complexity. r 2004 Elsevier GmbH. All rights reserved. ARTICLE IN PRESS www.elsevier.de/baae KEYWORDS Agroecosystems; Glyptapanteles militaris; Meteorus communis; Landscape structure; Parasitoid assemblages 1439-1791/$ - see front matter r 2004 Elsevier GmbH. All rights reserved. doi:10.1016/j.baae.2004.04.009 à Corresponding author. Tel.: +1-517-432-5282; fax: +1-517-353-5598. E-mail address: costamag@msu.edu (A.C. Costamagna). 1 Present address: Department of Land Resources and Environmental Sciences, 719 Leon Johnson Hall, Montana State University, Bozeman, MT 59717-3120, USA.