Landscape diversity enhances the biological control of an introduced crop pest in the north-central U.S

Background/Question/Methods

Arthropod predators and parasitoids provide economically valuable ecosystem services in agricultural crops by suppressing populations of insect herbivores.  The goal of this study was to understand if, and at what spatial scale landscape diversity and composition surrounding agricultural fields affected the arthropod-mediated ecosystem services of biological control.  Here we focus on biological control of soybean aphid, Aphis glycines Matumura, a specialist crop pest recently introduced to the north-central U.S.  We measured the amount of biological control service supplied to soybean in 26 replicate fields across Michigan, Wisconsin, Minnesota and Iowa across two years (2005-06). In n=4 replicated plots per field, 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 d.  We also monitored aphid and natural enemy populations at large in each field.

Results/Conclusions

Predators, principally Coccinellid beetles, dominated the natural enemy community of soybean. In the absence of aphid predators, A. glycines increased significantly, with 5.3 fold higher aphid populations on plants in exclusion cages versus the open field after 14 d.  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 and 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.