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.

Cold tolerance of the drosophila pupal parasitoid Trichopria drosophilae.

Trichopria drosophilae (Perkins) (Hymenoptera: Diapriidae) is a pupal parasitoid of drosophila flies recorded from several parts of the world. It is currently considered for augmentative biological programs to control the severe agricultural pest Drosophila suzukii (Matsumura) (Diptera: Drosophilidae). Since D. suzukii has invaded regions that experience zero and sub-zero winter temperatures, cold tolerance of the parasitoid is an important aspect to consider. We characterized low temperature tolerance and overwintering capacity of a colony of T. drosophilae collected in Northern Switzerland. We used copper-constantan thermocouples to determine the supercooling point and pre-freeze mortality. Moreover, we subjected honey-fed and unfed adult T. drosophilae as well as developing stages within their drosophila host to short- and long-term acclimation conditions and assessed the duration of their survival at low temperatures. Finally, we exposed adult and sub-adult stages to winter conditions in a semi-field experiment and evaluated their survival. We found that T. drosophilae is chill susceptible like D. suzukii, but adults froze and survived at colder temperatures than those reported for D. suzukii. Adult parasitoids could tolerate several days of exposure to sub-zero temperatures and could reproduce afterwards, whereas sub-adult stages could survive longer periods under these conditions. The provision of honey and water enhanced the survival of adults and long-term acclimation led to longer survival in all stages. The semi-field experiment supported the results of the laboratory tests. Based on these results we suggest that in Central Europe, T. drosophilae survives winters mainly in developing stages but adults are likely able to tolerate short periods of low spring temperatures.