Larch casebearer became established in North America in the late 1800s. Cold temperatures are considered a limiting factor for the insect's range, yet the cold hardiness of larch casebearer has never been quantified. We investigated (i) larval survival after acute and prolonged exposure to subzero temperatures and (ii) supercooling points (i.e. temperatures causing the onset of freezing) of overwintering and active larvae from November 2015 to April 2017. We developed models linking exposure temperatures to survival and evaluated them with larvae from field conditions. Historical minimum temperature data were used to estimate changes in survival across time at a site in northern Minnesota. The cold tolerance of larch casebearer changed significantly with season: both lower lethal temperatures and supercooling points were lowest in mid-winter and highest in spring and autumn. For example, 50% survival was estimated after acute exposure to a mean ± SE temperature of −28.9 ± 1.77 °C in October, −40.8 ± 0.77 °C in January and −27.8 ± 1.00 °C in April. A model predicting survival using supercooling points provided conservative estimates of overwintering survival because it overestimated survival by approximately 4% and 8% in 2016 and 2017, respectively. Analysis of climate data suggested that overwintering survival could have increased significantly over the previous half century.
Bibliographical noteFunding Information:
We thank Theresa Cira for instruction on cold tolerance assays; Aubree Kees for assistance with cold tolerance assays and monitoring insect survival; and members of the Salt Institute at the University of Minnesota for helpful discussions on insect overwintering biology. We thank three anonymous reviewers for their helpful comments on an earlier version of this manuscript. Funding was provided by USDA Forest Service award 15-DG-1142004-237; the College of Food, Agricultural and Natural Resource Sciences at the University of Minnesota; Minnesota Agricultural Experiment Station project MIN-17-082; and a University of Minnesota Doctoral Dissertation Fellowship to SW.
© 2018 The Royal Entomological Society
- Climate change
- Coleophora laricella
- lower lethal temperature
- outbreak dynamics