Cold tolerance and winter survival of seasonally‐acclimatised second‐instar larvae of the spruce budworm, Choristoneura fumiferana

Abstract

Abstract Field‐acclimatised spruce budworm larvae, Choristoneura fumiferana, supercool to as low as −41.6°C in winter months. Yet the extent to which they can withstand exposures to temperatures slightly above their supercooling point has never been investigated. In both January and February 2018, we tested various combinations of sub‐zero temperatures (−37 to −42°C) and exposure durations (0.75–12 h) to estimate the combinations of temperature and exposure durations required to kill half the population (LTT50). At −37 or −38°C, the estimated emergence probability was about 0.80 at all exposure durations. In contrast, the LTT50 was reached after 11.4 h at −39°C, 9.4 h at −40°C, and 3 h at −41°C. A temperature of −42°C was fatal to most larvae. During the winters of 2017, 2018 and 2019, survival experiments were conducted at three latitudes (46–48°N) in Québec. Regardless of the year or latitude, none of the daily minimum temperatures recorded in January or February were cold enough to reach the LTT50. However, the sudden drops in temperature that occurred after the winter thaw of March and in early December 2018 were likely responsible for the low proportions of emerged larvae observed. Hence, despite the high capacity of spruce budworm larvae to withstand very low sub‐zero temperatures in winter months, they remain highly vulnerable to cold spells during their early diapause or post‐diapause development. Such climatic disturbances deserve more attention, as they may increase under climate change.