Effet des pratiques agricoles sur une communauté d’insectes utiles (les coccinelles), à l’échelle de la parcelle et du paysage

The use of pesticides in agriculture is a major threat for the environment. Because organic farming does not use synthetic pesticides, it is a promising alternative in the use of biological control of pests by beneficial insects. But, the functions that deliver this ecosystem service are not fully understood, specifically concerning the role of farming practices. The objective of this work was to analyze the effects of organic farming on the distribution of beneficial. Twenty pairs of organic and conventional wheat fields distributed in landscape with 6 to 37% of organically farmed land were sampled. In each field, aphids and coccinelidae have been sampled during four month, in spring. Farming practices are known from farmers interview. The results show that beyond the differences between the two systems in terms of pesticide use, they both exhibit a high diversity of practices (e.g. tillage). The abundance of coccinelidae is higher in organic fields. This difference is explained by practices with a direct effect (pesticide use) but also via indirect effect of practices as the higher density in organic fields which provide a better microclimate. This suggests that biological control may be more efficient in organic versus conventional fields. In contrast, the study does not show any effect of the surrounding landscape. These results are only valid for a group of beneficial a given year. I participated to the 2013 sampling of coccilenidae and also carabidae and parasitoids. The latter two were sampled in 2012 but I did not have time to analyze the data.