Local and neighbourhood effects of organic and conventional wheat management on aphids, weeds, and foliar diseases
Résumé
The area under organic farming is increasing in many countries. The effect of a significant increase in the proportion of organic agriculture on pest (sensu lato) populations at the landscape scale is unknown and will depend on both the production of propagules in organic fields and the risk of pest dispersal between fields. In this study, we observed the dynamics of four foliar diseases, aphids, and weeds in 216 wheat fields over 2 years in northern France. We used the survey data to estimate the local effect of how a field was managed (organic or conventional) and the presence or absence of adjacent organic fields (neighbourhood effect) on pest abundance in that field. Because conventional and organic may be considered extremes along a continuum of management practices, a large survey was undertaken of management practices to ensure that the fields were classified according to the actual cropping practices. The presence or absence of organic certification was determined to be the only relevant criterion for classifying cropping practices. The results of proportional odds mixed models showed that some pests responded to local crop management: leaf blotch incidence and aphid density were significantly lower while weed diversity and abundance were higher in organic fields. Only aphids and leaf blotch responded to the neighbourhood effect: the presence of organic fields in the neighbourhood decreased the number of aphids in both organic and conventional fields and decreased leaf blotch incidence but only in conventional fields. These results indicate that the increase in organic acreage in landscapes will not increase pest problems in the short term under the conditions of the study (low disease pressure). (C) 2012 Elsevier B.V. All rights reserved.