Elevated light levels reduce hemlock woolly adelgid infestation and improve carbon balance of infested eastern hemlock seedlings

Abstract

The rapid loss of eastern hemlock (Tsuga canadensis) due to infestation with hemlock woolly adelgid (Adelges tsugae, HWA) has greatly altered structure and function of eastern forests. Numerous control strategies including local pesticide use and biocontrol with predator beetles have been implemented with considerable cost and varying success. Silviculture treatments that increase incident light on surviving hemlock trees to reduce infestation and ameliorate carbon starvation have been proposed as another possible conservation strategy, yet no controlled studies have evaluated the coupled plant-insect responses to increased light. We conducted a nursery experiment on artificially infested eastern hemlock seedlings under varying levels of ambient light, ranging from 0-90% shade. We measured HWA infestation (ovisac density), short-and long-term indicators of carbon balance (leaf chlorophyll fluorescence, net photosynthesis, total nonstructural carbohydrate content, and shoot growth), and nutrition (leaf N content). We hypothesized that higher light would result in reduced HWA densities, higher C assimilation rates, and improved tissue non-structural carbohydrate balance; and these effects would ameliorate the effects of infestation on C balance and lead to improved seedling growth. HWA density decreased with increasing light, and was highest in the 90% shade treatments. However, photosystem II efficiency and net assimilation were also lower under the higher light treatment. Despite tradeoffs between reduced infestation and reduced leaf function from higher light, and little variation in sugar content among treatments, both leaf and root starch content and seedling growth were higher under the higher light treatment. Increasing light levels improves long-term carbon balance for hemlock seedlings in the presence of HWA. Although hemlock typically occurs in deeply shaded 3 forests, our results suggest that silvicultural treatments such as forest thinning that increase light exposure may reduce HWA abundance and lead to better tree C balance, and may be an effective component of large-scale conservation and restoration strategies.

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

“…This reduced new branchlet growth is supported by Brantley et al [35] where they reported reduced photosynthetic efficiency and shorter branchlet length with reduced light levels. The effect of increased fertilizer in the current study was not as great as the effect of shade in increasing HWA density.…”

“…Brantley et al [35] also made a similar recommendation based on their studies of the effect of light on HWA and hemlocks. This could increase the mortality of summer aestivating sistens by exposing them to more solar radiation and increased temperatures leading to reduced HWA densities.…”

“…Shade or lack of shade may have an impact on HWA densities, with reports of higher HWA densities in low light conditions by Mayfield and Jetton [33], Sussky and Elkinton [31], and Hickin and Preisser [34]. Mech [32] reported increased mortality of aestivating sistens with increases in temperature, particularly above 30 • C. Brantely et al [35] found higher HWA densities with lower light levels but also lower long-term carbon balance, suggesting that seedling health improves as light levels increase.…”

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

“…Another mortality factor impacting HWA occurs during the summer due to high temperatures and possibly solar radiation. 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.…”

Differential sunlight exposure affects settling behaviour of hemlock woolly adelgid crawlers

“…Thus, subsequent generations of adelgids may also have been influenced by longer-term, indirect effects of sunlight mediated through the host plant. In artificial shade studies by Hickin and Preisser (2015) and Brantley et al (2017), eastern hemlock seedlings were exposed to shade treatments for several weeks or months prior to infestation, during which time host plants experienced sunlight-related effects on host physiology and chemistry. Hickin and Preisser (2015) found that shade increased the relative water content and water potential of eastern hemlock seedlings, and depending on time of measurement, decreased rates of transpiration and photosynthesis.…”