Conclusions
This study revealed that a host specialized aphid successfully achieved feeding on a new host plant when the new host plant was pre-infected with a phytopathogen. The feeding experience enabled the host-specialized aphid to fully adapt to the new host plant that was healthy. This is a new pathway of insect adapting to host plant, which may be widespread in other polyphagous aphids with host specialization. The results were of great significance for the management of polyphagous aphids. For instance, plant pathogen control should be effective to prevent A. gossypii on overwintering hosts from infesting cucurbit crops. In order to further understand the host adaptation pathway of aphid mediated by phytopathogen, we suggest to compare aphid-derived and biotrophic phytopathogen-derived molecules (effectors) in overcoming plant defenses, and to find and analyze differentially expressed genes during the rapid adaptation process mediated by phytopathogen.