Fitness and physiology ofAdelges tsugae(Hemiptera: Adelgidae) in relation to the health of the eastern hemlock

Abstract

The hemlock woolly adelgid, Adelges tsugae Annand is an invasive insect that frequently causes hemlock (Tsuga spp.) mortality in the eastern United States. Studies have shown that once healthy hemlocks become infested by the adelgid, nutrients are depleted from the tree, leading to both tree decline and a reduction of the adelgid population. Since A. tsugae is dependent on hemlock for nutrients, feeding on trees in poor health may affect the ability of the insect to obtain necessary nutrients and may consequently affect their physiological and population health. Trees were categorized as lightly or moderately impacted by A. tsugae based on quantitative and qualitative tree health measurements. Population health of A. tsugae on each tree was determined by measuring insect density and peak mean fecundity; A. tsugae physiological health was determined by measuring insect biomass, total carbon, carbohydrate, total nitrogen, and amino nitrogen levels. Adelges tsugae from moderately impacted trees exhibited significantly greater fecundity than from lightly impacted trees. However, A. tsugae from lightly impacted hemlocks contained significantly greater levels of carbohydrates, total nitrogen, and amino nitrogen. While the results of the physiological analysis generally support our hypothesis that A. tsugae on lightly impacted trees are healthier than those on moderately impacted trees, this was not reflected in the population health measurements. Adelges tsugae egg health in response to tree health should be verified. This study provides the first examination of A. tsugae physiological health in relation to standard A. tsugae population health measures on hemlocks of different health levels.

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

“…Joseph et al [37] found conflicting results of fertilization of four hemlock species with varying responses of oviposition rates and densities. Jones et al [42] examined the effect of tree health on health and fecundity of HWA and found results somewhat in conflict of McClure [36] and Joseph et al [37]. In the Jones et al [42] study, moderately HWA-impacted trees had greater HWA fecundity but were less healthy (based on nutrient content) than lightly-impacted trees.…”

“…But the higher successful settlement rate of sistens in low light plus our findings of higher survival of aestivating sistens in low light, would add credence to there being overall greater densities of HWA in low light conditions. Jones et al [42] reported survival of 40% of aestivating sistens in a forested setting. A very similar survival rate (41%) was found in our study.…”

“…All of the parameters were rated on a scale of 0-100% in 5% increments so that a high value would indicate a healthy condition while a low value would indicate an unhealthy condition. The mean of these three measures (percent tips alive, branches alive, and foliage density) was used to calculate a hemlock health index (0-100) [42]. Percent live branch tips was the estimate of live hemlock branch tips relative to the branch tips that have no needles.…”

Evaluating a potential area-wide IPM strategy for managing hemlock woolly adelgid in the eastern United States

“…Statistical analyses were performed using a linear mixed model ANOVA with repeated measures (LMMARM), with a first-order autoregressive covariance structure (Littell et al 2000, Preisser and Elkinton 2008, Jones et al 2016) (Appendix 11). For each analysis, treatment, year, and the interaction of treatment × year were the fixed effects factors.…”

“…Each of these measures was rated 0 -100. The average of these five measures was then used to establish a tree crown health index of 0 -100 (Jones et al 2016). …”