Disentangling local agronomic practices from agricultural landscape effects on pest biological control
Abstract: The biological control of crop pests is a valuable service provided by various beneficial organisms that are naturally present in agricultural landscapes. Semi-natural habitats has long been recognized as essential to preserve beneficial insects, but proof of their efficiency to enhance biological control of pests remains non conclusive. Here, we examined the variability of landscape effect on biological pest control and the way local agronomic practices may modulate it. Biological pest control was monitored in 80 commercial fields (arable crops and orchards) during three consecutive years in four contrasting French agricultural landscapes distributed along a double gradient of pesticide use and landscape complexity measured in a 1 km2 area centred on each field. Biocontrol efficacy in each field was assessed using three types of sentinel preys (weed seeds, moth eggs, and aphids). The effects of landscape simplification, pesticide use intensity and their interaction on the predation of each sentinel prey were analysed using generalised linear mixed models. At local level, the intensity of pesticide use reduced the predation of weed seeds and aphids. At landscape level, the proportion of the monitored crop reduced the predation of weed seeds and moth eggs. Finally, significant interactions between local and landscape factors were detected for each sentinel prey: (i) the predation of moth eggs was negatively influenced by landscape simplification at low pesticide use intensity only, whereas the effect was positive at high pesticide use intensity; (ii) aphid predation significantly decreased with increasing crop-wood interface length and proportion of meadow, but only at high pesticide use intensity; (iii) weed seed predation significantly decreased when land use diversity around the fields increased under high pesticide use intensity and, inversely, increased with land cover diversity around the fields at of low pesticide use intensity. These results suggest that reduction in pesticide use should be associated with the reinforcement of semi-natural habitats in agricultural landscape to enhance natural biocontrol.