Date of Award

Summer 8-15-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Forest Resources (MFR)

Department

Forest Resources

First Committee Advisor

William H. Livingston

Second Committee Member

Angela Mech

Third Committee Member

Mike Premer

Additional Committee Members

Jay Wason

Abstract

This research focuses on the relationship between a native North American insect, Pissodes strobi (Peck) [Coleoptera; Curculionidae], and eastern white pine (Pinus strobus [L.]; EWP), its preferred host species for eastern North America. Broadly this project serves to describe the role of environmental conditions on variations in P. strobi (white pine weevil; WPW) damage and investigate a potential resistance mechanism in EWP to WPW feeding. Furthermore, this research builds a greater understanding of the relationships between EWP and WPW statewide for Maine, U.S., and regionally, between the southern extent of EWP, the Blue Ridge Plateau in Virginia and North Carolina, U.S.

WPW is a transcontinental species with regionally specific host relationships and naming conventions, commonly referenced in eastern North America as white pine weevil and as spruce weevil in western North America. While environmental relationships and genetic resistance are well documented for Sitka spruce (Picea sitchensis) in the West, there are multiple knowledge gaps regarding the relationship between WPW damage and EWP. Therefore, my research aims to: (1) use risk analysis to identify key tree, site, and environmental variables associated with WPW damage to EWP by utilizing USDA Forest Service Forest Inventory and Analysis (FIA) data and high resolution digital soil maps, climate records, and satellite imagery to derive a suite of previously unavailable site and climate metrics, (2) identify key factors explaining differences between the amount of WPW damage in selected stands of EWP saplings in Maine and the Blue Ridge Plateau based on variation of stem, stand, site, and climate variables, and (3) investigate whether EWP shares similar physical traits in shoot tissue, stone cells, which yield resistance to WPW feeding in some spruce species. Each objective highlights an area for knowledge expansion, where the methodology and analytical techniques used in this study can inform future research on WPW and potentially other EWP-related health issues.

In the quantification of landscape level variations in WPW damage, 70% of 1,757 FIA subplots across Maine with at least one EWP had observed WPW damage. The odds of damage increase for every unit increase in EWP trees per hectare (TPH) in stems ≥ 12.7 cm (5 inches) diameter at breast height and decrease for every unit increase in excess site water. Additionally, WPW damage occurs on all subplots with an EWP TPH value greater than 476 (192 EWP trees per acre; 4% of subplots), where EWP percent composition is on average 75%. Comparatively, most subplots (96%) have EWP TPH values below 476 and lower average percent EWP stand composition at 30%. For field sampling of EWP saplings, 33 of 79 (41%) of the Maine subplots and 14 of 46 (30%) of the Blue Ridge Plateau subplots had observations of WPW damage. When measuring the average percent of trees with WPW damage, Maine’s value (13.1%) was more than double the Blue Ridge Plateau value (5.5%). Although factors within the Blue Ridge Plateau should favor WPW presence (higher % EWP saplings and thicker terminal shoots), the more southern region has notably less WPW damage. Within the Blue Ridge Plateau, the thin duff layer (0.1 cm average) could be limiting WPW survival and damage. Within Maine, WPW damage increases as duff layer thickness increases. Both management strategies (site preparation), and climate factors (soil temperatures) may be influencing WPW survival in the duff layer. Shoot samples collected in Maine and North Carolina (131 samples) had no staining of lignified tissue in the primary cortex which would indicate the presence of stone cells, while 3 (of 24) trees at separate sites in Virginia had positive staining results. A verification process for stone cells using immunohistochemistry, as used for spruce species, was negative for the collected EWP samples.

The focus of EWP management in Maine has long been geared towards reducing WPW damage for stands dominated by EWP, such as the 4% of the stands that averaged 75% EWP in the FIA dataset (EWP TPH > 476) and the field sites in the sapling study. However, percent composition of EWP TPH averages 30% in 96% of the FIA sampled subplots where EWP TPH is less than 476, and WPW damage did not occur on 32% of these subplots. To prioritize growing more EWP with less WPW damage, EWP management in Maine could favor growing EWP within mixed species stands.

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