Effects of temperature and humidity on the survival and hatching response of diapausing and non-diapausing Aedes aegypti eggs

In seasonally varying environments, diapause, which is induced by a short photoperiod, favors overwintering of many insects. In Aedine mosquitoes, embryonic diapause is associated with higher survival and resistance to low temperature and humidity. Aedes aegypti, the main vector of dengue and other arboviruses, has recently expanded its distribution towards temperate regions. One of the mechanisms that might have favored this expansion in South America is the ability to induce embryonic diapause. This type of diapause has been recently discovered in populations from Argentina, associated with hatching inhibition and increased amounts of lipids in the eggs. The aim of this study was to assess the four-month survival of diapausing (D) and non-diapausing (ND) eggs stored at different humidity and temperature conditions. Two populations from the temperate region of Argentina were analyzed: one from Buenos Aires (BA), a city with a relatively mild and short winter, and another from San Bernardo (SB), a locality with a harsher and longer winter. For both populations, D and ND eggs were obtained from colonies maintained under 10:14 L:D and 14:10 L:D hours respectively. Eggs were exposed to six different conditions of humidity and temperature for 85 days. After exposure, egg survival and hatching response were analyzed. D eggs showed significantly higher survival at low humidity (both populations), and at medium and high humidity and at low temperatures (SB population). In addition, D eggs showed a significantly lower hatching response at high humidity and low temperatures, and higher proportion of not hatched eggs remaining viable after two immersions under all conditions. D eggs from SB were significantly more tolerant to low temperatures than those from BA. ND eggs from SB were significantly more tolerant to low temperatures, while those from BA were more tolerant to low humidity. Overall, the effect of diapause was a significant increase in the number of not hatched, viable embryos after immersion. Results suggest that the ability of Ae. aegypti to induce egg diapause increases the probability of successful overwintering and further expansion of its distribution range, and as a consequence the risk of arbovirus transmission might increase in temperate areas.