Insect herbivore damages on white spruce growing in plantations and naturally regenerated under-canopy forest stands

This data was prepared to compare insect damage on white spruce (Picea glauca (Moench) Voss, Pinaceae) growing in plantations with naturally regenerated trees under mature forest canopies in the boreal forest (Québec, Canada). We selected ten sites in the naturally regenerated forest and small, multispecies plantations and sampled ten young trees (per site) in late summer 2020 and again in early and late summer 2021. We recorded overall rates of damage for galls, damage by spruce budworm (Choristoneura fumiferana (Clemens, 1865)), spruce bud midge, spruce budmoth, spruce gall midge, cooley adelgid, defoliation from sawflies and other caterpillars.

Methods
For our study, we selected ten white spruce trees per site, and ten sites at both plantation and forest sites (naturally regerated under mature forest canopies), ensuring each tree was 2 to 3.5 meters tall and was spaced a minimum of 5 meters apart to avoid overlapping effects. These sites ranged in distance from each other, extending from 100 to 2500 meters, allowing us to capture a representative sample of the habitat variability. We commenced our initial sampling during the late summer of July-August 2020 and conducted follow-up samplings in early (May-early June) and late (July) summer of 2021. Our investigation focused on assessing the damage to the trees' current-year growth (only), by first observing the buds as they began to open in the spring and then assessing the elongated shoots.

To capture the extent of damage, we employed two main approaches: branch sampling and whole-tree surveys during the growing season.

Branch Data: In the late summers of both years (2020 and 2021), we cut a 40 cm branch from the mid-crown of each tree for detailed herbivory scoring (damage patterns), performed within 24 hours of collection. We evaluated damage types including bud loss—particularly from the spruce gall midge— pineapple gall formations by Eastern spruce gall adelgid, and shoot damage from moth larvae (Spruce budworm was assessed separately from other caterpillars) and sawflies (yellow-headed sawfly). Additionally, using the visual estimation technique known as the Fettes method, as adapted by Kanoti in 2019, we assessed the total defoliation for each shoot in late summer 2021. This method involved estimating the percentage of needles missing from a current year shoot averaging these across all the buds per branch and deriving a percent defoliation per site. We also recorded the total number of developed and undeveloped buds on each branch.

Whole Tree Survey Data: During the early and late summers of 2021, we conducted timed visual surveys on the whole trees, focusing on the branches around 1.5 meters from the ground for a set period of three minutes by a single trained person. These surveys cataloged damage according to predefined categories, deliberately excluding damage by the spruce gall midge and spruce bud moth (as these two damage-type buds need to be dissected or cut open to check the presence of the relevant insects) to maintain consistency in our data. Along with noting the damages, we counted the total number of buds that had developed on the branches within view. This method, while allowing for a quicker assessment of a larger number of shoots, might be less precise in detecting smaller, more subtle damages.