Immature hoverflies overwinter in cultivated fields and may significantly control aphid populations in autumn

This study was based on broad monitoring of spring emergence of hoverflies and of aphid populations in two French agro-ecosystems. Its aim was to determine whether hoverflies overwinter in agricultural fields and/or in field margins at immature stages and to what extent local and large-scale landscape features influence the abundance of immature overwintering individuals and the associated biological control. Our results show that the influence of the landscape on the abundance of immature overwintering hoverflies varies among trophic groups. Non-aphidophagous species plus some aphidophagous species, which occasionally feed on crop aphids, preferentially overwinter in field margins near wooded land. Conversely, aphidophagous species, which mainly feed on crop aphids, preferentially overwinter within the fields and are less influenced by semi-natural features. This work also shows a strong negative correlation between the abundance of hoverflies that overwinter at immature stages within the fields and the abundance of aphids in the spring, which suggests a biological control by the hoverflies in autumn. Because they may provide a significant service of biological control, the hoverflies that overwinter within agricultural fields should be taken in consideration in the management of croplands, through agricultural practices for example. The landscape configuration also should be considered in order to maximize the hoverflies community in agricultural landscapes, and to maximize the ecosystem services it provides. This study provides valuable insights on overwintering of hoverflies at pre-imaginal stages in agricultural landscapes and on the role of this overwintering strategy in the biological control of aphid populations. It shows that a landscape mostly composed of cultivated land can provide significant service of biological control if semi-natural features are arranged in a convenient way for natural enemies. (C) 2013 Elsevier B.V. All rights reserved.