The open bar is closed: restructuration of a native parasitoid community following successful control of an invasive pest

When an invasive species establishes in a new area and reaches high densities, antagonists may benefit from this bonanza situation. How such a newly formed community may then be challenged after the regulation of the invaders by external outputs; e.g. the deliberate introduction of specialized natural enemies; remains however poorly documented. To investigate this issue, we used the case study of the invasion of the Asian chestnut gall-wasp Dryocosmus kuriphilus Yasumatsu in France and its subsequent control by the exotic parasitoid Torymus sinensis. From the survey of 26 locations during 5 years after the T. sinensis introduction, we analyzed how the patterns of co-occurrence between the different native parasitoid species changed through time. Our results demonstrate that native parasitoid communities experienced increased competition as the D. kuriphilus levels of infestation decreased. During the last year of the survey, two alternative patterns were observed: either native parasitoid communities were almost inexistent, or they were dominated by one main parasitoid: Mesopolobus sericeus. We observe that the two patterns correlate with the habitat and that they may be explained by environmental features such as differences in the natural reservoirs for native parasitoids. These results highlight how the boom-and-bust dynamics of an invasive pest followed by successful biological control can deeply alter the structure of native communities of natural enemies.