Overwintering survival of Drosophila suzukii (Diptera: Drosophilidae) in temperature regimes emulating partly protected winter conditions in a cold–temperate climate of Québec, Canada

Abstract

Field-acclimated Drosophila suzukii (Diptera: Drosophilidae) from the Saguenay–Lac-Saint-Jean region, Québec, Canada, were examined over two years for winter survival, under the hypothesis that flies select protected overwintering microhabitats. In 2016–2017, flies trapped alive in the field or emerged from infested fruits were submitted to four winter regimes of either constant or daily fluctuating temperatures of 5 °C (2–8 °C) or 10 °C (7–13 °C). In 2017–2018, two fluctuating regimes averaging either 1 °C (–2 to 4 °C) or 3 °C (0–6 °C) were tested. Survival was modelled using Cox proportional hazard models testing probability that mortality risk varies with cold winter regime, fly sex, and fly provenance. Hazard ratios were about 1.7 times higher for males than for females. Models indicate that flies in constant and fluctuating 10 °C, in constant 5 °C, or in fluctuating 1 °C with daily exposure to –2 °C would not survive a six-month winter. Female survival extended to the next summer in fluctuating regimes averaging 5 °C or 3 °C. Estimates of 0.95 quantile survival (5%) indicate that overwintering D. suzukii experiencing such cold temperature regimes during winter, with no prolonged sub-zero temperatures, could survive until July of the following year, which is likely at the high population densities observed.

Postdiapause reproduction of spotted-wing drosophila (Diptera: Drosophilidae) in realistically simulated cold climatic springtime conditions of Québec, Canada

“…Despite substantial mortality after 180 days of cold stress, numerous D. suzukii flies of both sexes survived. When transferred to springtime rising temperatures and increased daylength based on regional norms, life expectancy distributions indicated that winter survivors (10-30% of flies tested) would still be alive in June and up to July of next year (Cloutier et al 2021). The flies were from lateseason, high-density populations (Champagne-Cauchon et al 2020), as previously reported (e.g., Thistlewood et al 2018).…”

“…Finding mates in spring would be the most critical factor at the low fly densities that are typical in spring, due to the Allee principle (Jacobs 2009), with decreasing available time and energy (Kramer et al 2009;Yamanaka and Liebhold 2009;Xu and Ma 2020). This would apply mostly to male D. suzukii, for which the odds of winter mortality are nearly twice those of females (Cloutier et al 2021), and as indicated by the results of our 2017 experiment (Supplementary material 1).…”

“…Studies to date in the invasive range of D. suzukii indicate that cold-acclimated flies are chilling intolerant (Tochen et al 2014;Jakobs et al 2015;Ryan et al 2016;Langille et al 2017;Nikolouli et al 2018;Winkler et al 2020), which would preclude winter survival under natural conditions in Canada and in the northeastern United States of America, except in natural microhabitats protected from freezing or in heated anthropic environments (e.g., Zerulla et al 2015;Stockon et al 2019). Cloutier et al (2021) studied D. suzukii's winter survival by exposing field collected flies from Québec to realistically simulated winter regimes of cold stress that lasted six months, followed by a return to springtime conditions, while assuming that fall-acclimated flies select microhabitats with low risk of extensive freezing. Despite substantial mortality after 180 days of cold stress, numerous D. suzukii flies of both sexes survived.…”

“…Despite the substantial mortality that was seen, it cannot be discounted that many D. suzukii might survive local winters to resume reproduction in spring. Even males survived the 180-day cold winter (Cloutier et al 2021), which suggests that both sexes of postdiapause flies could eventually mate after winter to reproduce in the next season. For survivors with no opportunity to mate before diapause, mating after winter would be essential to start a new generation in spring.…”

“…For males, the additional costs of diapause might be reflected in the reduced fertility of their female partners. Knowing the fitness costs of diapause in D. suzukii thus is of considerable interest, given that both sexes can survive cold stress periods equivalent to continental cold-temperate winters (Cloutier et al 2021). Documenting these fitness costs is possible by comparing lifetime fecundity of postdiapause and nondiapause flies in simulated, controlled springtime conditions.…”