Crop diversity and edge density benefit pest suppression through bottom-up and top-down processes, respectively

Abstract

Agricultural intensification has resulted in the homogenization of agricultural landscapes, leading to the loss of crop and habitat diversity, and a higher rate of chemical inputs in the landscape, negatively impacting natural enemy communities and their ability to provide pest control services. Landscape heterogeneity is thought to benefit predator populations (increasing landscape complementation) and lower pest colonization (decreasing resource concentration); however, few studies have determined the relative importance of these two processes in pest suppression. Here, we tease apart direct and indirect impacts of landscape heterogeneity on experimental populations of soybean aphid excluded from and open to predation, in 23 soybean fields in Manitoba, in 2017 and 2018. Field plant counts and sweep net samples were conducted to determine aphid and predator abundances, respectively, and Malaise traps were used to quantify predator movement into soybean. Landscape effects differed between a soybean aphid outbreak year (2017) versus a non-outbreak year (2018). We found that crop diversity at a 1.5 km radius had negative impacts on aphid colonization during the outbreak year, resulting in greater aphid suppression but no impacts on predator movement. Edge density at a 1.5 km radius increased aphid suppression during the low aphid year. Predator movement and abundance were driven by aphid colonization in soybean, and syrphid and coccinellid abundance were 33-fold and 15-fold more abundant in the high compared to the low aphid year. Thus, our study revealed complementary effects of landscape heterogeneity on aphid suppression: a reduction of resource concentration associated with increased crop diversity and resulting in bottom-up effects during an outbreak, and an increase in landscape complementation associated with increased edge density and top-down control in a low aphid year. Increasing crop diversity seems a promising strategy to mitigate aphid outbreaks without compromising food production.