Levels of predator movement between crop and neighboring habitats explain pest suppression in soybean across a gradient of agricultural landscape complexity

Abstract

Landscape complexity has been shown to play an important role in determining the levels of pests and predators found in agricultural fields. Although movement of predators between landscape habitats and crop fields is a crucial mechanism mediating landscape effects on pest control services, this has rarely been quantified in agroecosystems. Here we evaluated the relationship among agricultural landscape complexity, levels of predator movement and the suppression of soybean aphid, Aphis glycines Matsumara, in 27 soybean fields in Manitoba. Over a two-year period, we quantified soybean aphid suppression using predator manipulation treatments, predator movement using bi-directional Malaise traps, and landscape complexity using digital maps of the area within a 2 km radius of the focal fields studied. When aphids were exposed to predation, population growth was reduced by 73.7% on average (range: 38.3%–95.6%) compared to aphid populations protected with predator exclusion cages. Bi-directional Malaise trap and sweep-net sampling indicated that hover flies (Diptera: Syrphidae), followed by minute pirate bugs (Hemiptera: Anthocoridae), lady beetles (Coleoptera: Coccinellidae) and green lacewings (Neuroptera: Chrysopidae) were the numerically dominant predators. Focal fields were located in landscapes with a range of 0.3–40.3% of woodland, with soybean, cereals, and canola as the other major land-cover types present. Final aphid population size showed a negative association with the proportion of cereals and positive associations with the proportion of woodland and field border grass in the landscape. Levels of predator movement between soybean and neighboring habitats had negative associations with final aphid population size, and were the best predictors in the statistical models, either alone or combined with independent landscape complexity variables. Our results provide the first empirical evidence that landscape effects on pest suppression can be explained by the contribution of predators from extra-field habitats. From a management perspective, these results suggest that higher levels of pest suppression can be achieved by designing landscapes that facilitate predator movement to crops from extra-field habitats.