Practices of conservation agriculture prevail over cropping systems and landscape heterogeneity in understanding the ecosystem service of aphid biocontrol
Résumé
Aphidophagous natural enemies significantly lower yield losses induced by aphids in cereals. This natural biocontrol is threatened, and conserving this ecosystem service requires better understanding of the factors that influence it. Natural enemies are mainly responsive to habitat diversity in the surrounding landscape. However, their positive response to landscape heterogeneity does not necessarily translate into more effective pest control. In this study, we investigated combined effects of production situation (including landscape heterogeneity) and agricultural practices, especially soil management, on an aerial natural biocontrol. Grain aphid populations and predation by a hoverfly species were assessed in 52 wheat fields under plowed, reduced-tillage or direct seeding systems, under conventional or organic management, at two nearby locations. The hoverfly populations that prey on aphids were studied by counting their pupae shells on ear barbs. Crop management and production situation were studied to describe pest occurrence and natural biocontrol. Both integrated (e.g. tillage type, organic management) and specific (e.g. nitrogen fertilization, rotation duration) variables were compared. Our main finding was that the integrated variables did not sufficiently explain aphid abundance and their biocontrol. Conversely, specific variables related to types of management were informative. Nitrogen fertilization induced an increase in aphid populations and the number of hoverfly pupae as the time since last plowing increased. Semi-natural habitats had considerable influence on the pest and its natural enemy; however, the strong influence of location observed in this study was not related to landscape composition. We showed that although the landscape did influence the potential for biological regulation, crop management enabled its expression and thus its effectiveness. Categorizing crop management practices too coarsely may masks effects of individual practices. Future studies on biological control of pests in agroecosystems will gain to finely describe cropping systems as well as landscape heterogeneity, if they are to unravel the explanatory variables on this utmost important ecosystem service for ecological intensification of agriculture.
