Gardiner, M. 2008. Landscape scale and within-field influences on predator abundance and biocontrol services in soybean fields. Ph.D. Dissertation, Michigan State University, East Lansing, MI, USA.
The soybean aphid, Aphis glycines Matumura, is an invasive species first detected in the U.S. in 2000. This pest has now spread throughout soybean growing regions of the U.S. and Canada. Previous work has demonstrated that natural enemies are key drivers of soybean aphid population dynamics within soybean fields. The objectives of my dissertation research were to investigate how landscape diversity and composition influenced the species pool of potential biocontrol agents supplied to soybean fields and determine how within-field interactions between these species affected net biocontrol services. I investigated these objectives in replicate sites across Iowa, Michigan, Minnesota, and Wisconsin from 2005-2007. I measured the impact of predators by experimentally excluding or allowing access to soybean aphid infested plants and comparing aphid population growth over 14 d. I found that while predators significantly suppressed soybean aphid across the region, there was a great deal of site to site variation. To determine how the amount of biocontrol services supplied to a soybean field was influenced by the surrounding landscape, I 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. 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 distribution and abundance of natural and agricultural habitats within the landscape also greatly influenced the abundance of key predator species including Coccinellidae, Carabidae, and Araneae. The activity density of Araneae increased in soybean fields in landscapes with an abundance of forests and grasslands. Relationships with Carabidae activity density varied, with adult-overwintering and predatory species increasing and larval-overwintering species declining with the abundance of grassland in the landscape. The coccinellid community also exhibited varied responses to landscape diversity and composition. Native coccinellids were most abundant in low-diversity landscapes with an abundance of grassland habitat while exotic coccinellid increased with the abundance of forested habitats.
I also investigated how within-field intraguild interactions affected species abundance and biocontrol services in soybean fields. I found that H. axyridis was a significant intraguild predator of both Aphidoletes aphidimyza and Chrysoperla carnea , however, the direct impact of H. axyridis on A. glycines overcame these negative impacts on overall biocontrol. I also measured the extent of IGP of native coccinellid eggs in soybean fields. I found significant IGP occurred and the composition of the potential egg predators varied by state. In Michigan, exotic coccinellids were the most abundant predators while in Iowa native species including C. maculata, Hippodamia parenthesis , and Collops nigriceps dominated the predator community. Thus, in Michigan landscapes, consumption of native coccinellid eggs favors exotic coccinellids while in Iowa, this resource is more likely to sustain other native species.
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