Ecological Applications, 19(1), 2009, pp. 143–154 Ó 2009 by the Ecological Society of America Landscape diversity enhances biological control of an introduced crop pest in the north-central USA M. M. GARDINER, 1,5 D. A. LANDIS, 1 C. GRATTON, 2 C. D. DIFONZO, 1 M. O’NEAL, 3 J. M. CHACON, 4 M. T. WAYO, 1 N. P. SCHMIDT, 3 E. E. MUELLER, 2 AND G. E. HEIMPEL 4 1 Department of Entomology, 204 Center for Integrated Plant Systems, Michigan State University, East Lansing, Michigan 48824-1311 USA 2 Department of Entomology, 1630 Linden Dr., 237 Russell Labs, University of Wisconsin, Madison, Wisconsin 53706 USA 3 Department of Entomology, 177 Insectary, Iowa State University, Ames, Iowa 50011-3140 USA 4 Department of Entomology, 219 Hodson Hall, 1980 Folwell Ave., University of Minnesota, St. Paul, Minnesota 55108 USA Abstract. Arthropod predators and parasitoids provide valuable ecosystem services in agricultural crops by suppressing populations of insect herbivores. Many natural enemies are influenced by non-crop habitat surrounding agricultural fields, and understanding if, and at what scales, land use patterns influence natural enemies is essential to predicting how landscape alters biological control services. Here we focus on biological control of soybean aphid, Aphis glycines Matumura, a specialist crop pest recently introduced to the north-central United States. We measured the amount of biological control service supplied to soybean in 26 replicate fields across Michigan, Wisconsin, Iowa, and Minnesota across two years (2005– 2006). We measured the impact of natural enemies by experimentally excluding or allowing access to soybean aphid infested plants and comparing aphid population growth over 14 days. We also monitored aphid and natural enemy populations at large in each field. Predators, principally coccinellid beetles, dominated the natural enemy community of soybean in both years. In the absence of aphid predators, A. glycines increased significantly, with 5.3-fold higher aphid populations on plants in exclusion cages vs. the open field after 14 days. We calculated a biological control services index (BSI) based on relative suppression of aphid populations and related it to landscape diversity and composition at multiple spatial scales surrounding each site. We found that BSI values increased with landscape diversity, measured as Simpson’s D. Landscapes dominated by corn and soybean fields provided less biocontrol service to soybean compared with landscapes with an abundance of crop and non-crop habitats. The abundance of Coccinellidae was related to landscape composition, with beetles being more abundant in landscapes with an abundance of forest and grassland compared with landscapes dominated by agricultural crops. Landscape diversity and composition at a scale of 1.5 km surrounding the focal field explained the greatest proportion of the variation in BSI and Coccinellidae abundance. This study indicates that natural enemies provide a regionally important ecosystem service by suppressing a key soybean pest, reducing the need for insecticide applications. Furthermore, it suggests that management to maintain or enhance landscape diversity has the potential to stabilize or increase biocontrol services. Key words: Aphis glycines; biological control; ecosystem services; introduced crop pests; landscape diversity; natural enemies; predators; soybean aphid. INTRODUCTION Insects provide vital ecosystem services including decomposition, pollination, and biological control of crop pests (Losey and Vaughan 2006). In agroecosys- tems, the diversity and abundance of natural enemies that provide biological control in crop fields are influenced by the structure and composition of the surrounding landscape (Marino and Landis 1996, Colunga-Garcia et al. 1997, Bommarco 1998, Elliott et al. 1999, Thies et al. 2003, Schmidt and Tscharntke 2005, Tscharntke et al. 2005). Landscape variables such as habitat complexity, quality, and patchiness as well as an organism’s dispersal capability all impact the ability of a landscape to support biological control in agricultural croplands (Elliott et al. 1999, Thies et al. 2003, Schmidt and Tscharntke 2005). Moreover, agricultural crop fields are ephemeral habitats in which anthropogenic distur- bances such as tillage, pesticide application, and harvesting require arthropods to frequently recolonize crops (Wissinger 1997). The surrounding landscape provides the local species pool of arthropods for this recolonization and thus may influence the level of biological control in frequently disturbed crop fields (Lee et al. 2001). Manuscript received 1 August 2007; revised 5 March 2008; accepted 15 April 2008; final version received 8 May 2008. Corresponding Editor: S. M. Altizer. 5 E-mail: gardin18@msu.edu 143