Ecological Applications, 16(4), 2006, pp. 1619–1628 Ó 2006 by the Ecological Society of America PREDATORS EXERT TOP-DOWN CONTROL OF SOYBEAN APHID ACROSS A GRADIENT OF AGRICULTURAL MANAGEMENT SYSTEMS ALEJANDRO C. COSTAMAGNA 1 AND DOUGLAS A. LANDIS Department of Entomology, 204 Center for Integrated Plant Systems, Michigan State University, East Lansing, Michigan 48824-1311 USA Abstract. The discovery of soybean aphid, Aphis glycines Matusumura, in North America in 2000 provided the opportunity to investigate the relative strength of top-down and bottom- up forces in regulating populations of this new invasive herbivore. At the Kellogg Biological Station Long Term Ecological Research site in agroecology, we contrasted A. glycines establishment and population growth under three agricultural production systems that differed markedly in disturbance and fertility regimes. Agricultural treatments consisted of a conventional-tillage high-input system, a no-tillage high-input system, and a zero-chemical- input system under conventional tillage. By selectively restricting or allowing predator access we simultaneously determined aphid response to top-down and bottom-up influences. Irrespective of predator exclusion, our agricultural manipulations did not result in bottom-up control of A. glycines intrinsic rate of increase or realized population growth. In contrast, we observed strong evidence for top-down control of A. glycines establishment and overall population growth in all production systems. Abundant predators, including Harmonia axyridis, Coccinella septempunctata, Orius insidiosus, and various predaceous fly larvae, significantly reduced A. glycines establishment and population increase in all trials. In contrast to other systems in which bottom-up forces control herbivore populations, we conclude that A. glycines is primarily controlled via top-down influences of generalist predators under a wide range of agricultural management systems. Understanding the role of top-down and bottom- up forces in this context allows agricultural managers to focus on effective strategies for control of this invasive pest. Key words: agroecosystems; aphid; Aphis glycines; biological control; Coccinella septempunctata; generalist predators; Harmonia axyridis; invasive herbivore; lady beetles; Michigan, USA; Orius insidiosus; top-down vs. bottom-up effects. INTRODUCTION Ecologists have long been interested in understanding the relative strength of top-down and bottom-up forces in shaping natural communities (Hairston et al. 1960, Hunter and Price 1992, Power 1992, Walker and Jones 2001). In terrestrial ecosystems both forces are wide- spread (Gruner 2004), and several factors have been shown to significantly affect the relative strength of their effects, including plant morphology (Kareiva and Sahakian 1990), productivity (Power 1992, Fraser and Grime 1998, 1999, Dyer and Letourneau 1999, Moran and Scheidler 2002), consumer efficiency (Power 1992), temporal vs. spatial variation of herbivore populations (Hunter et al. 1997), herbivore life stages (Walker and Jones 2001), seasonal and year-to-year changes in abiotic and biotic conditions (Walker and Jones 2001, Boyer et al. 2003, Gratton and Denno 2003a, b), differential predation risk among herbivore species (Denno et al. 2003), and herbivore body size (Sinclair et al. 2003). Although not explicitly framed in a bottom- up vs. top-down context, several studies in agroecosys- tems have tested the effects of fertility regimes and host plant resistance vs. the effects of natural enemies on pest control (Walker and Jones 2001). Agroecosystems often consist of highly simplified food webs with strong reciprocal influences between adjacent trophic levels (Polis et al. 2000), resulting in ideal systems to manipulate bottom-up vs. top-down forces. Despite this potential, no simultaneous manipulations of both forces have been conducted in agroecosystems (but see Dyer and Stireman 2003). Furthermore, understanding the relative strength of top-down vs. bottom-up forces in agricultural systems has the potential to provide insights into fundamental ecological questions, as well as help guide agroecosystem management. We tested the relative strength of top-down vs. bottom-up influences on the population growth of soybean aphid, Aphis glycines Matsumura (Homoptera: Aphididae). Aphis glycines is an important pest of soybean (Glycine max L.) in China and Southeast Asia, causing yield losses of up to 70% (Wang et al. 1962, Hirano et al. 1996, Van den Berg et al. 1997, Wu et al. 2004). In the year 2000, A. glycines was discovered for the first time in North America and as of 2004 was reported present in 21 states in the United States and Manuscript received 26 May 2005; revised 4 November 2005; accepted 6 December 2005. Corresponding Editor: M. P. Ayres. 1 E-mail: costamag@msu.edu 1619