1. Organic Research Centre; Elm Farm, Hamstead Marshall; Newbury RG20 0HR UK
2. Faculty of Agriculture and Horticulture; Humboldt University Berlin; Albrecht Thaer-Weg 5 14195 Berlin Germany
3. Forest Pathology and Dendrology; Institute of Integrative Biology; ETHZ Zurich Switzerland
4. Department of Animal Science; Aarhus University; DK-8830 Tjele Denmark
5. Ecological Plant Protection Group, Faculty of Organic Agricultural Sciences; University of Kassel; 37213 Witzenhausen Germany