1. Plant-Microbe Interactions, Department of Biology, Utrecht University , P.O. Box 800.56, 3508 TB, Utrecht , The Netherlands
2. Microbiology, Radboud Institute for Biological and Environmental Sciences (RIBES), Radboud University , P.O. Box 9010, 6500 GL, Nijmegen , The Netherlands
3. Laboratory of Entomology, Wageningen University , P.O. Box 16, 6700 AA, Wageningen , The Netherlands