Low-temperature threshold for egg survival of a post-diapause and non-diapause European aedine strain, Aedes albopictus (Diptera: Culicidae)

Background: The interplay between global warming and invasive arthropods in temperate zones is of utmost interest in terms of the potential expansions of vector-borne diseases. Up to now, investigations on the recent establishment of mosquito vectors have focused on temperatures during their phases of activity. However, cold temperatures may also act as a strong ecological constraint. Projected changes in winter climate indicate an increase of mean minimum temperatures of the coldest quarter, less frequent days with frost and a shorter frost-season in Europe at the end of the century. Nevertheless, single cold extremes are also expected to persist under warming scenarios, which have a strong impact on reproduction success. Methods: Here, the temperature constraints of European Aedes albopictus eggs, which had passed through a diapause, compared to non-diapausing eggs were examined systematically under controlled laboratory conditions. Additionally, one tropical strain of Ae. albopictus and of Ae. aegypti was used in the comparison. Results: The lower temperature threshold tolerated by the European eggs of Ae. albopictus which have undergone a diapause, was -10 degrees C for long term exposures (12 and 24h) and -12 degrees C for 1h exposure. Non-diapausing eggs of European Ae. albopictus were found to hatch after a -7 degrees C cold treatment (8, 12 and 24h exposure). Both tropical aedine species only tolerated the long term treatment at -2 degrees C. Neither Ae. albopictus nor Ae. aegypti eggs hatched after being exposed to -15 degrees C. Survival was mainly influenced by temperature (F = 329.2, df = 1, p < 0.001), whereas the duration of the cold treatment only significantly influenced the hatching response at the thermal limits of survival (F = 5.6, df = 1, p = 0.031) but not at 0 degrees C (F = 0.1, df = 1, p = 0.730). Hatching success after the cold treatment was significantly increased in European eggs, which have undergone a diapause compared to non-diapausing eggs (F = 14.7, df = 3, p < 0.001). These results illustrate rapid adaptation. Conclusions: Here, low temperature thresholds for aedine mosquito egg survival were detected. The compilation of risk maps for temperate regions can substantially be improved by considering areas where an establishment of a vector population is unlikely due to winter conditions.