The role of climate in outbreaks of larch casebearer

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The role of climate in outbreaks of larch casebearer

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Larch casebearer, Coleophora laricella Hübner (Lepidoptera: Coleophoridae), is an invasive insect in North America that was introduced from Europe in the late 1800s and defoliates larch trees. Once combatted successfully with an importation biological control program, it has resurged in several parts of its invaded range. This dissertation describes several laboratory, field, and landscape studies to examine the effects of photoperiod and temperature on survival and development of larch casebearer. First, I characterized patterns in defoliation. From 1997-2016, defoliation fluctuated synchronously between forests of western larch (Larix occidentalis) in Oregon/Washington and eastern larch (Larix laricina) in Minnesota, suggesting that climate acting across large spatial scales may drive similar patterns among allopatric casebearer populations. In Minnesota, I found that warmer minimum winter temperatures appeared to have contributed to the resurgence of larch casebearer in Minnesota. Spring phenological synchrony between bud break by eastern larch and larval activation did not appear to drive the resurgence; all larvae were estimated to activate several days after the average day of bud break. Field studies suggested that, in cooler growing seasons, some larvae may not accumulate sufficient degree-days to reach the overwintering stage. I applied degree-day models to historical climate data to elucidate changes in 1) spring phenological synchrony (i.e., the number of days between bud break by eastern larch and subsequent spring activation by larch casebearer larvae) and 2) the proportion of larvae estimated to reach the overwintering stage. I found that larch casebearer consistently activates several days after eastern larch has broken bud and that higher estimated proportions of larvae have reached the overwintering stage over the previous ~50 years. The importance of degree-day accumulation for outbreaks of larch casebearer was corroborated by a spatiotemporal autologistic model developed on defoliation data from aerial surveys. The analysis of landscape patterns also demonstrated that feeding by larch casebearer may predispose eastern larch to mortality from eastern larch beetle Dendroctonus simplex LeConte (Coleoptera: Curculionidae). In summary, the resurgence of larch casebearer in Minnesota appears to have been facilitated by increasingly warmer winters and growing seasons.



University of Minnesota Ph.D. dissertation. July 2018. Major: Entomology. Advisor: Brian Aukema. 1 computer file (PDF); xxiii, 180 pages.

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Ward, Samuel. (2018). The role of climate in outbreaks of larch casebearer. Retrieved from the University Digital Conservancy,

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