Landscape configuration as well as composition influences coccinellid abundances in agricultural landscapes

Background/Question/Methods

Landscapes provide ecosystem services to agro-ecosystems by supporting insect predators of crop pests, a service valued at $US 4.5 billion. It is generally accepted that landscape composition influences biocontrol services, as numerous studies over the last decade have found that predator abundance and diversity are positively related to the amount of non-crop habitat within landscapes. However, the arrangement of cover types within the landscape may also be important, as modeling studies predict that the average size and interspersion of patches affect the rate at which predators colonize crop fields. To examine the influences of configuration and composition on biocontrol services in agroecosystems, I examined coccinellid abundance and aphid suppression data collected in Midwestern soybean fields in 2005-06 and 2008-09. Coccinellid abundances were measured with yellow sticky traps and aphid suppression was measured by comparing aphid numbers on plants from which predators were excluded to numbers on plants to which predators had access. Landscape metrics were calculated for circles of 0.5, 1.0, 1.5, and 2.0 km radius around each soybean field to measure abundance of non-crop habitat, mesh size and interspersion of cover types. AIC model comparisons were used to examine the relative influences of composition and configuration metrics.

Results/Conclusions

Response of coccinellids to landscape composition and configuration differed by species and by landscape extent considered. Preliminary analyses suggest that non-crop habitat best explained H. axyridis abundances at extents of 2.0, 1.5, and 1.0 km, while mesh size best explained variation in H. axyridis abundance within 0.5 km landscapes.  In contrast, models containing cover type interspersion best explained C. septempunctata abundance at extents of 1.5 and 1.0 km. Furthermore, early season abundances of these two coccinellid species were best predicted by cover type interspersion, and aphid suppression was inversely related to mesh size. The effects of mesh size and cover type interspersion on coccinellid abundances and aphid suppression suggests a need to consider the arrangement as well as the identity of land cover when managing landscapes to improve biocontrol services. In other words, it is not only important what we are growing in agroecosystems but also where we are growing it. Furthermore, the inverse relationship between aphid suppression and mesh size suggests that in addition to known effects of reduced habitat diversity on predator populations, agricultural intensification may lead to reduced biocontrol services through the effects of increasing field size on the dispersal of beneficial insects.