Remote sensing reveals the importance of seminatural habitat and irrigation on aphid biocontrol in arid agroecosystems.
Abstract
Remote sensing and spatial analyses are increasingly used to understand the role of seminatural habitats in biocontrol, but knowing how to best leverage these tools is a persistent challenge. In arid agroecosystems, irrigation often exaggerates the differences between crop fields and their interspaces. Weeds adjacent to irrigated fields are thus often some of the only persistent vegetation. Remote sensing may be essential to capture the effect of this fine-scale variation on pest biocontrol. We examined the role of land-cover in determining the degree of aphid pest pressure and biocontrol in irrigated alfalfa fields in the Great Basin Desert (Nevada, USA). We experimentally manipulated predator densities to determine true agents of aphid biocontrol in this system. We then assessed land-cover and spatial scale using alternative methods to determine the approaches that best defined the combined direct and indirect effects of seminatural habitat on aphid pest suppression. A remote-sensing approach revealed the importance of land-cover to aphid biocontrol more clearly than conventional vegetation surveys. In particular, remote sensing revealed that weedy patches within alfalfa fields reduced aphid densities, whereas flood irrigation increased aphid densities. Our experiments confirmed that coccinellid beetles are important aphid biocontrol agents. Coccinellids were positively associated with weedy land-cover that was spatially close to alfalfa fields, particularly in the spring. Other predator taxa were also associated with distinct landscape features, though these associations varied between spring and fall. Solution: Weedy seminatural habitat near alfalfa fields, occurring along field margins and the banks of irrigation ditches, was positively associated with the density of coccinellid beetles and associated aphid pest suppression. Remote sensing excelled at identifying these areas and their effects, highlighting its utility as a management tool. Preserving these weedy areas can enhance aphid biocontrol, but farmers must consider potential trade-offs between the control of insect-pests and weeds.