Landscape heterogeneity impacts aphid suppression while adjacent habitats and aphid abundance impact predator movement in soybean
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Landscape homogenization has resulted in the loss of crop and habitat diversity in agroecosystems, creating resource bottlenecks and higher rates of chemical inputs. This negatively impacts natural enemy communities and natural pest control. Landscape heterogeneity is thought to benefit predator populations (increased landscape complementation) and lower pest colonization (decreased resource concentration); however, few studies have investigated the relative contribution of these two processes in pest suppression. Habitats provide predators with differing floral and/or prey resources. Understanding how predators move between habitats during pest outbreaks is important for assessing how we can improve pest control in agricultural landscapes. My study investigates the direct and indirect impacts of landscape heterogeneity on sentinel soybean aphid populations open to and excluded from predation, in 23 soybean fields in Manitoba in 2017 and 2018 and aims to determine the role of adjacent habitats in predator movement in soybean during an aphid outbreak. Soybean fields were adjacent to alfalfa, canola, spring wheat, or woody vegetation. Five sticky traps were placed in soybean and the adjacent habitat to quantify predator abundance in 2017. Both years, bi-directional Malaise traps were used to quantify predator movement and sweep net samples and plant counts were conducted to quantify predator and aphid abundance in soybean. We found positive effects of crop diversity (1.5 km) on aphid abundance and indirect benefits to aphid suppression in the outbreak year, and positive effects of edge density (1.5 km) on aphid suppression during the low aphid year. Syrphids and coccinellids dominated all samples, were more abundant during the outbreak year, and their immigration was in response to aphid density. Syrphid movement was more impacted by the type of adjacent habitat than coccinellid movement, and wheat and canola were the main contributors of syrphids to soybean. Coccinellid abundance was highest in adjacent wheat. My study suggests planting wheat adjacent to soybean may benefit the early arrival of coccinellids and syrphids for aphid suppression and that landscape heterogeneity benefits aphid suppression through a reduction in resource concentration by crop diversification during outbreaks and through an increase in landscape complementation by edge density in low aphid years.
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