The eastern larch beetle, Dendroctonus simplex LeConte (Coleoptera: Curculionidae), is distributed throughout the North American boreal forest sympatric with its primary host, the eastern larch or tamarack, Larix laricina (Du Roi) K. Koch. Outbreaks of eastern larch beetles are typically small and associated with stressed tamaracks. Since 2000, however, an outbreak has killed >90,000 ha of tamarack in Minnesota and surrounding Great Lakes region. Identifying the causes of this epidemic is challenging due to knowledge gaps regarding the insect's biology. We present field data from 2011 to 2014 on degree days associated with spring emergence, dispersal, host colonization, and re-emergence from colonized hosts by mature adult beetles, as well as degree days associated with larval development, and prewinter emergence by adult progeny at study sites in northern Minnesota. After initial host colonization in early spring we found that a second brood was established in early summer by re-emerging parents. In 2012, a third brood was established. Across study years, first broods developed to adults by late summer, with many beetles relocating to the base of the host tree to overwinter. Second broods often reached adulthood and initiated prewinter emergence. The third brood of 2012 overwintered as adults, pupae, and late-instars, resuming development the following spring. Each spring, emergence of adult beetles from all broods established the previous year was highly synchronous. Knowledge of the biology of eastern larch beetles along the southern margin of their range aids in understanding how population dynamics may change with a changing climate.
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Acknowledgments We thank Becky Lein (MNDNR-Forestry) and staff for providing research material; Gretchen Mehmel (MNDNR-Wildlife) and staff of the Red Lake Wildlife Management Area for providing field equipment and accommodations; and Jana Albers, Michael Albers, and Valerie Cervenka (MNDNR-Forestry) for field expertise and logistical support. Funding was provided by a McKnight Land-Grant Professorship to B.H.A. and US Forest Service Evaluation Monitoring Grant NC-EM-B-12-01. Technical assistance was provided by Audrey Zahradka, Erica Nystrom-Santacruz, Michelle Cummings, Jonah Widmer, Andrea Hefty, and Aubree Wilke (U. Minnesota). The authors appreciate the constructive comments of F.R.M.'s doctoral dissertation advisory committee R. Venette (US Forest Service / University of Minnesota), A. D'Amato (U Vermont), and S. Kells (U Minnesota) that helped improved previous versions of this work.
- Dendroctonus simplex
- bark beetle
- degree day
- eastern larch beetle