Overwintering of two pupal parasitoids of Drosophila under natural conditions.

The overwintering capacity of biocontrol agents is of fundamental relevance for biological control of pests in temperate regions. In this study we tested the cold tolerance of the indigenous Drosophila pupal parasitoids Pachycrepoideus vindemmiae and Trichopria drosophilae at constant low temperature in the laboratory and exposed different preimaginal parasitoid stages in the field during winter. We evaluated whether semi-natural habitats promote overwintering via more favorable microclimatic conditions as well as higher host availability compared to orchards. Further, we studied the parasitoids' phenology in a semi-field experiment during autumn. We found that P. vindemmiae larvae and pupae were most cold tolerant under laboratory and field conditions, while all preimaginal stages of T. drosophilae displayed similar cold tolerance. Semi-natural habitats buffered temperature extremes, yet overwintering survival was not enhanced compared to orchards. Suitable overwintering hosts were present in all habitats at times when parasitoids were still active parasitizing. These results demonstrate that P. vindemmiae overwinters most likely as larva or pupa and that T. drosophilae can overwinter in a preimaginal life stage. Further, we provide evidence that both parasitoids can overwinter in a wide range of habitats and that the availability of hosts for overwintering is unlikely a limiting factor for the parasitoids during fall.