Aphids and their natural enemies are differently affected by habitat features at local and landscape scales

Abstract:
Biological control, an essential ecosystem service to agriculture, can be affected by ecological processes operating at landscape scales. Here we assessed the effect of landscape complexity, measured as proportion of arable land (PAL), on the abundance of aphids, parasitoids, and specialist and generalist predators. In addition we set up cage experiments to test the ability of these groups of enemies to suppress aphid densities. Landscape context did not significantly explain differences in aphid or parasitoid densities between fields. However, aphid densities were significantly higher in field interior compared to the margin. Coccinellid (specialist predator) abundance showed a similar pattern, with higher density in the field interior, indicating an aggregative response to aphid prey. In addition, Coccinellid abundance increased with PAL but only in field interiors and not at the field margins. The abundance of carabids (generalist predators) increased with PAL, suggesting that they benefit from landscape simplification. The cage experiment revealed that specialist as well as generalist predators were able to reduce the number of aphids on barley tillers and that a combination of both guilds did not provide a greater reduction of aphids. Our results suggest higher densities of generalist predators with increasing PAL Nonetheless, the greater abundance of coccinellids and carabid beetles in cereal fields embedded in simple landscapes does not necessarily imply better pest control since natural enemies may compete, thereby limiting their ability to control pests. (C) 2012 Elsevier Inc. All rights reserved.