Local or landscape? Pest control and the role of habitat complexity across scales

Background/Question/Methods Pest control by resident populations of natural enemies is influenced by the landscape context of the farm. The degree to which habitat modifications at the farm level improve pest control may depend upon habitat diversity at the landscape level. Densities of cabbage aphids (Brevicoryne brassicae) and their natural enemies in broccoli were measured across a landscape gradient in weekly surveys over three years. Cage studies employed at the extremes of the landscape gradient assessed pest control function. Lab experiments investigated the physiological impact of different food sources on aphids and their predators. Results/Conclusions Natural enemies increased with the proportion of natural habitat (complexity) in the surrounding landscape in all years of the study. Pest control function also increased with landscape complexity, but there appeared to be some degree of substitutability of complexity across landscape and local scales. Despite the enhanced natural enemy community associated with more complex landscapes, the effect of complexity on aphid distributions was not consistent across growing season or years, suggesting that factors other than natural enemies play equally important roles in regulating pest populations. One such factor is the potential for weedy mustard around farms to provide refuge to cabbage aphids.  It is important to consider the interplay between top-down and bottom-up influences when attempting to manage for the ecosystem service of pest control.