The Resurgence of Larch Casebearer in the Great Lakes Region

Abstract

Larch casebearer, Coleophora laricella Hübner, (Lepidoptera: Coleophoridae), was discovered infesting eastern larch, Larix laricina (Du Roi) K. Koch, in North America in the 1880s. The insect was likely introduced from Europe on shipments of European larch, Larix decidua Mill., stock. Larch casebearer quickly spread through the range of eastern larch and eventually western larch in western North America. Since 2000, larch casebearer defoliation has increased on these two species of Larix across North America despite previous successful biological control by two introduced parasitoids; Agathis pumila Ratzeburg (Hymenoptera: Braconidae) and Chrysocharis laricinellae Ratzeburg (Hymenoptera: Eulophidae) in the Great Lakes region. I sampled eastern larch at multiple sites across Minnesota and Wisconsin, USA in 2018 and 2019, to determine if these parasitoids were still present. I observed the continued presence of A. pumila and C. laricinellae in both states along with 13 other parasitoid species associated with larch casebearer. Cirrospilus pictus Nees (Hymenoptera: Eulophidae), an historically rare parasitoid, was more abundant than the specialist A. pumila. I also examine whether host species affects the overwintering cold tolerance and spring activation of two allopatric populations of larch casebearer from Minnesota and Idaho. Through a common garden experiment, I determined that the supercooling points and the number of degree days accumulated for spring activation for populations of larch casebearer from Minnesota reared on eastern and western larch were similar, suggesting that differences are not likely due to host species. Instead, the different populations on eastern larch and western larch may be two different species or varying autumnal and winter conditions may affect thermal plasticity and adaptation.