Canadian Forest Service Publications

Impact of short-term exposure to low subzero temperatures on egg hatch in the hemlock looper, Lambdina fiscellaria. 2013. Delisle, J.; Labrecque, A.; Royer, L.; Bernier-Cardou, M.; Bauce, É.; Charest, M.; Larrivée, S. Entomol. Exp. Appl. 149:206-218.

Year: 2013

Issued by: Laurentian Forestry Centre

Catalog ID: 35201

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1111/eea.12123

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The frequency of extreme events, such as cold spells, is expected to increase under global warming. Therefore, the ability of insects to survive rapid changes in temperature is an important aspect to investigate in current population ecology. The hemlock looper (HL), Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae), a defoliator of boreal balsam fir forests in eastern Canada, overwinters at the egg stage on tree trunks and branches where eggs can be exposed to very low subzero air temperatures. Using eggs from the island of Newfoundland (NL) and Quebec mainland (QC), we undertook field and laboratory experiments to determine: (1) their supercooling point (SCP) in mid-January and mid-February; (2) overwintering mortality; (3) cold tolerance to various combinations of subzero temperatures (-25, -30, -33, -35, or-37 °C) and exposure durations (2, 4, 8, 12, or 16 h); and (4) potential causes of death at subzero temperatures above the SCP. Regardless of population or sampling date, eggs supercooled on average at-40.1 °C. In the field, 59% of eggs from either population that overwintered in Sainte-Foy (QC) and Corner Brook (NL) hatched successfully, whereas none did in Armagh (QC) or Epaule (QC). In the laboratory, 50% of eggs survived after 4 h at -34.4 °C or after 14 h at -32.9 °C. In contrast, regardless of exposure duration, >50% of eggs hatched at temperatures ≥33 °C, but <50% did so at ≤35 °C, suggesting high pre-freeze mortality. However, when eggs were attached to thermocouples and exposed to temperatures ranging from -25 to -37 °C for 16 h, 69% froze at temperatures of -35 to -37 °C, but only 2% did at -25 or-30 °C. Time to freeze decreased as subzero temperatures declined, and this was more evident in island eggs than in mainland eggs. Overall, eggs can freeze after a brief exposure to subzero temperatures higher than the standard SCP, and are thus highly vulnerable to cold spells.

Plain Language Summary

An increase in the frequency and intensity of extreme climate events is expected over the coming years, due to climate change. For insects, this means that they will have to adapt in order to withstand abrupt temperature changes, especially during the cold winter months. Among the insects that may be affected is the hemlock looper, a major pest attacking eastern Canada’s balsam fir stands.

The hemlock looper spends the winter in the egg stage on tree trunks or branches. The eggs are therefore subject to the vagaries of the weather, including low temperatures. Researchers conducted laboratory and field tests on hemlock looper eggs in Quebec (Quebec City, Laurentian Wildlife Reserve, Appalachians) and in Newfoundland and Labrador (Corner Brook) to determine their freezing point (-40.1°C), their winter survival capacity, and their cold tolerance level based on duration of exposure.

In Quebec City and Corner Brook, researchers estimated the winter survival capacity of the overall population at 60%, while in the Laurentian Wildlife Reserve (Petit lac à l’Épaule) and in the Appalachians (Armagh), none of the eggs set out in the forest survived the winter. In these last two regions, the temperature reached -40°C on one occasion during the month of January.

The cold tolerance tests revealed that 50% of the eggs did not survive after 4 h of exposure at -35°C, while the percentage rose to 100% after 2 h of exposure at -37°C. This demonstrates the very high vulnerability of hemlock looper eggs to intense cold spells, even those lasting just a few hours.