The Effects of Shade, Fertilizer, and Pruning on Eastern Hemlock Trees and Hemlock Woolly Adelgid

Abstract

Hemlock woolly adelgid (HWA), Adelges tsugae Annand, an invasive insect native to the Pacific Northwest and Asia, is responsible for widespread health decline and mortality of native hemlocks (Tsuga spp.) in the eastern United States. Shading and fertilizer has been found to affect the survival and health of both HWA and hemlocks. These abiotic factors have been studied separately but not in combination. In this three year study, eastern hemlock trees (1-2 m tall) were treated with pruning, fertilizer, and shade to determine their effects on hemlock tree health and HWA survival and density. Shade cloths were erected over individual trees, granulated fertilizer was applied, and trees were pruned annually. The total number of HWA were counted during the sistens and progrediens adult stages on the low, mid, and high branches on the north, east, south, and west sides of each tree for three years. Survival of aestivating sistens was recorded in artificially, naturally, and unshaded hemlocks. The mean of percent tips alive, branches alive, and foliage density was used to calculate a hemlock health index (scale of 0-100). Shade cloth reduced solar radiation to the trees to levels similar to a naturally-forested hemlock canopy, but did not alter temperature. Trees exposed to shade alone and shade plus fertilizer maintained the greatest HWA density. On unshaded trees, branches on the west side of the tree had lower HWA densities and branches high on the tree had the lowest HWA densities. Pruning plus fertilizer and shading plus fertilizer reduced tree health. Shaded trees had reduced branchlet new growth length. Survival of summer aestivating sistens was nearly twice the survival under artificially-and naturally-shaded trees compared to unshaded trees. There was an inverse density-dependent survival response for aestivating HWA under artificially-shaded and unshaded trees but not naturally-shaded trees. Unshaded hemlock trees had lower HWA densities due to increased mortality of summer aestivating sistens. Unshaded trees had better health and longer new growth branchlets due to increased exposure to solar radiation and lower HWA densities. Silvicultural thinning of hemlocks in forest stands could increase direct sunlight reaching the trees and help decrease HWA densities and improve hemlock health.

Mortality and Recovery of Hemlock Woolly Adelgid (Adelges tsugae) in Response to Winter Temperatures and Predictions for the Future

“…Higher HWA densities in low light conditions were found by Brantley [34], Sussky and Elkinton [69], Mayfield and Jetton [70], and Hickin and Preisser [71]. Mech [33] reported increased mortality of summer aestivating sistens with increases in temperature, particularly above 30 • C. McAvoy [35] reported that HWAs developing under shade had twice the survival rate compared to HWAs growing in full sun. Summer aestivating morality would be lower in the northern latitudes due to lower summer temperatures.…”

Differential sunlight exposure affects settling behaviour of hemlock woolly adelgid crawlers

“…Negative phototaxis has also been observed in other groups of insects including haematophagous bugs (Lazzari et al, 1998) and stored grain beetles (Kim et al, 2013). As noted by McAvoy et al (2017), hemlocks are very shade-tolerant and frequently grow in the forest understory, so if A. tsugae is adapted to cool, shaded environments, sunlight avoidance may help them locate cooler settling sites. This hypothesis is consistent with experiments by Mech et al (2018), who documented an increase in the mortality of aestivating A. tsugae sistens nymphs with increases in temperature, a relationship that strengthened with increasing duration of exposure, particularly above 30 ∘ C. Sussky and Elkinton (2015) also reported high levels of aestivating sistens mortality associated with elevated temperatures.…”

A Subcontinental Analysis of Forest Fragmentation Effects on Insect and Disease Invasion

“…(3) predation risk on pests associated with certain predators and their abundance at different edges (including the "natural enemies hypothesis" prediction that higher plant diversity systems support more insect predators that more effectively control herbivore populations [29,33]), and (4) pest ecophysiological and life-history traits that are associated with ecological features and physical positions (i.e., forest layers) at forest edges [34].…”