Adelges Piceae Research Articles

Quantifying current and potential future impacts of balsam woolly adelgid infestation on forest biomass

Balsam woolly adelgid (Adelges piceae; BWA) is an invasive forest pest in the US whose infestation in fir forests can cause widespread tree mortality. A growing body of evidence suggests that the severity of BWA’s effects is linked to climatic conditions, where sites featuring seasonally warmer temperatures tend to demonstrate higher degrees of insect-induced forest degradation. Thus, a warming climate may promote the expansion of BWA-driven damage, particularly in highly susceptible subalpine fir (Abies lasiocarpa) forests. The degree to which climate change may foster BWA infestation and the relationship between current and future infestation damage severity are largely unknown. To understand BWA’s potential climate-driven impacts on subalpine fir forests now and in the future, we built a field-validated, spatially explicit predictive model that estimates BWA infestation severity as a function of locally downscaled temperature variables. Using only four seasonal temperature variables, our model was able to explain over 78% of variance in measured severity. We applied that model to the prediction of severity for five different time periods (2020, 2040, 2060, 2080, and 2100), with the 2040 and onward predictions being driven by four separate climatic shared socioeconomic pathways (SSP126, SSP245, SSP370, and SSP585). These high resolution (∼30 m) maps were compared to spatially coincident modeled estimates of subalpine fir aboveground biomass, the results of which provide valuable insight into the degree to which BWA may drive forest degradation, loss of ecosystem services, and increase in available fuels. In northern Utah, the southeasternmost extent of BWA infestation in the western US, the two major mountain ranges, Wasatch and Uinta, demonstrated very different climatic susceptibility to infestation damage, with the former being highly exposed to BWA-induced degradation under even moderate climate projections, and the latter being fairly robust against BWA’s effects in all but the most extreme climate scenarios. In 2020, 41% of the study area’s subalpine fir biomass is climatically exposed to some level of damage from BWA. By 2100, even under moderate climate projections (SSP245), 79% will be exposed, with 37% predicted to feature relatively high severity. By placing our results within the context of the Resist, Accept, Direct land management framework, we offer spatially explicit guidance to inform current and future silvicultural practices to mitigate future damage to subalpine fir forests at the leading edge of BWA expansion in the western US.

Whole-genome sequence of the Cooley spruce gall adelgid, Adelges cooleyi (Hemiptera: Sternorrhyncha: Adelgidae).

The adelgids (Adelgidae) are a small family of sap-feeding insects, which, together with true aphids (Aphididae) and phylloxerans (Phylloxeridae), make up the infraorder Aphidomorpha. Some adelgid species are highly destructive to forest ecosystems such as Adelges tsugae, Adelges piceae, Adelges laricis, Pineus pini, and Pineus boerneri. Despite this, there are no high-quality genomic resources for adelgids, hindering advanced genomic analyses within Adelgidae and among Aphidomorpha. Here, we used PacBio continuous long-read and Illumina RNA-sequencing to construct a high-quality draft genome assembly for the Cooley spruce gall adelgid, Adelges cooleyi (Gillette), a gall-forming species endemic to North America. The assembled genome is 270.2 Mb in total size and has scaffold and contig N50 statistics of 14.87 and 7.18 Mb, respectively. There are 24,967 predicted coding sequences, and the assembly completeness is estimated at 98.1 and 99.6% with core BUSCO gene sets of Arthropoda and Hemiptera, respectively. Phylogenomic analysis using the A. cooleyi genome, 3 publicly available adelgid transcriptomes, 4 phylloxera transcriptomes, the Daktulosphaira vitifoliae (grape phylloxera) genome, 4 aphid genomes, and 2 outgroup coccoid genomes fully resolves adelgids and phylloxerans as sister taxa. The mitochondrial genome is 24 kb, among the largest in insects sampled to date, with 39.4% composed of noncoding regions. This genome assembly is currently the only genome-scale, annotated assembly for adelgids and will be a valuable resource for understanding the ecology and evolution of Aphidomorpha.

Using Remote Sensing and Climate Data to Map the Extent and Severity of Balsam Woolly Adelgid Infestation in Northern Utah, USA

Balsam woolly adelgid (Hemiptera: Adelges picea Ratzeburg; BWA) is a nonnative, invasive insect that has infested fir trees in the US for over a century, yet robust methods for mapping BWA have remained elusive. We compare three approaches to mapping BWA in the subalpine fir forests of northern Utah, the forefront of BWA spread in the western US: (1) using moderate-resolution, multispectral satellite imagery; (2) using terrain and climate data; and (3) using a combination of imagery, terrain, and climate data. While the spectral data successfully detected forest degradation, they failed to distinguish between causal agents of change (R2mean = 0.482; RMSEmean = 0.112). Terrain and climate data identified landscape conditions that promote BWA infestation but lacked the ability to characterize local-scale tree damage (R2mean = 0.746; RMSEmean = 0.078). By combining spectral, terrain, and climate data, we present a repeatable approach for accurately mapping infestation severity that captures both regional abiotic drivers and the local damage signals of BWA (R2mean = 0.836; RMSEmean = 0.065). Highly infested areas featured increased visible and shortwave infrared reflectance over time in the spectral data. The terrain bore little influence on severity, but climate variables indicated that warmer areas are more prone to severe infestation. This research study presents an analytical framework upon which future BWA monitoring efforts can be built.

An evaluation of climate influences on balsam woolly adelgid infestations in Idaho

Balsam woolly adelgid (Adelges piceae) (BWA) was first reported in western North America in 1928 and continues to expand its range eastward into the Rocky Mountains, causing substantial damage and mortality in firs (Abies spp.) across the region. Climate is thought to influence BWA in multiple ways, yet few studies have addressed this region. Here we compared mean climate variables at 28 field sites in Idaho, USA, where stand and BWA conditions were measured in 2018. We evaluated BWA conditions using two field-based metrics, BWA presence and host damage. Using information from published studies from Canada, we found that winters were almost always warm enough to support BWA at the Idaho sites, and summers were always warm enough to support completion of at least one or two generations of BWA. Application of a previously published climate risk index indicated no relationship with observed BWA damage. We used statistical models to assess the influences of mean climate variables and stand characteristics on BWA presence and damage observed at our sites. Warmer summers or wetter conditions were associated with greater BWA presence and damage. At sites with both colder and drier conditions, BWA was recorded as present but damage levels were substantially reduced. Given that our Idaho sites are representative of climate conditions across most of the ranges of subalpine fir and grand fir in the western US, we suggest (with lower confidence) that mean climate variables, as evaluated in this study, may not limit the expansion of BWA across the ranges of these host species, but may limit the damage. Future studies that have more detailed information (for instance, annual resolution of insect observations or more detailed snowpack data) and more locations in additional regions and climates will increase confidence in our results.

Characterizing Balsam Woolly Adelgid Infestations and Associated Tree Mortality in Idaho

Abstract Balsam woolly adelgid, Adelges piceae (BWA), is a nonnative insect that has invaded much of the true fir range in North America, reducing host tree growth and causing mortality. Here, we describe a field study to characterize BWA infestation and effects on host trees in Idaho from the onset of infestation within twenty-eight sites with repeated surveys in 2008, 2013, and 2018. Declining tree health was categorized into five BWA damage classes based on the percent of the tree crown–expressing symptoms caused by BWA. Between 2008 and 2018, BWA presence increased across most sites regardless of host species, with an average of 48% (0.1%–100%) host trees per acre being infested. Severe damage or mortality of true fir from BWA occurred across thirteen sampled forest habitat types, including those representing harsh, high elevations. Although a significantly greater proportion of subalpine fir died than grand fir by 2018, BWA caused grand fir mortality. All diameter size classes became infested with BWA, including seedlings, and BWA caused mortality regardless of tree diameter. This assessment provides baseline data for a deeper analysis of insect, host, stand, and environmental interactions that may elucidate factors driving severe stand mortality. Study Implications Balsam woolly adelgid (BWA) is a cryptic, nonnative, and chronically lethal insect of true firs that is frequently overlooked until tree health substantially declines. Therefore, it is often not considered a serious damage agent during forest management planning until after severe damage has occurred across a stand. The BWA damage classification, modified from Spiegel et al. (2013), may be applied during stand exams. This simple and effective five-tiered classification system can be used to categorize BWA damage within individual trees or stands based on the collective symptoms caused by the insect. Feeding by BWA can ultimately result in stand-level mortality of both over- and understory fir and may require artificial regeneration to restore forest function. Fir regeneration was abundant at most BWA-damaged sites; however, seedling-sized firs were often heavily gouted from BWA to levels that will prevent them from growing into seed-bearing trees. Reported levels of mortality caused by BWA within 10 years of infestation and forest habitat type associations can be referenced for making management decisions for not-yet or recently infested stands.

Species delimitation and invasion history of the balsam woolly adelgid, Adelges (Dreyfusia) piceae (Hemiptera: Aphidoidea: Adelgidae), species complex

AbstractThe Adelges (Dreyfusia) piceae (Ratzeburg) species complex is a taxonomically unstable group of six species. Three of the species are cyclically parthenogenetic [Ad. nordmannianae (Eckstein), Ad. prelli (Grossmann), and Ad. merkeri (Eichhorn)] and three are obligately asexual [Ad. piceae, Ad. schneideri (Börner), and Ad. nebrodensis (Binazzi & Covassi)]. Some species are high‐impact pests of fir (Abies) trees, so stable species names are needed to communicate effectively about management. Therefore, to refine species delimitation, guided by a reconstruction of their biogeographic history, we genotyped adelgids from Europe, North America, and the Caucasus Mountains region with 19 microsatellite loci, sequenced the COI DNA barcoding region, and compared morphology. Discriminant analysis of principal components of microsatellite genotypes revealed four distinct genetic clusters. Two clusters were morphologically consistent with Ad. nordmannianae. One of these clusters consisted of samples from the Caucasus Mountains and northern Turkey, and the other included samples from this region as well as from Europe and North America, where Ad. nordmannianae is invasive. A third cluster was morphologically consistent with Ad. piceae, and included individuals from Europe, where it is native, and North America, where it is invasive. In North America, the majority of Ad. piceae individuals were assigned to two geographically widespread clones, suggesting multiple introductions. The fourth cluster included individuals morphologically consistent with Ad. prelli or Ad. merkeri. However, based on genetic assignments, hybrid simulations, and approximate Bayesian computation, we find it likely that these are contemporary hybrids between Ad. nordmannianae and Ad. piceae that arose independently in Europe and North America, so we propose that Ad. prelli and Ad. merkeri are invalid. Finally, we synonymise Ad. schneideri (syn.n.) with Ad. nordmannianae and designate Ad. nebrodensis as subspecies Ad. piceae nebrodensis (stat.n.). Our revised taxonomy therefore recognises two species: Ad. nordmannianae and Ad. piceae, which we estimate to have diverged recently, during one of the last two interglacial periods. Finally, we comment on this species complex being in the midst of transition between sexual and asexual reproduction, a pattern that is probably common in Adelgidae.

Notes on balsam woolly adelgid, Adelges piceae (Ratzeburg, 1844) (Hemiptera: Adelgidae), range expansion in Idaho, Montana and Utah

The earliest reports of balsam woolly adelgid, Adelges piceae (Ratzeburg, 1844), in North America occurred in Maine in 1908 and California in 1928 (Kotinsky 1916, Annand 1928). By the late 1960s, this non-native pest was common throughout the ranges of native true fi r species, Abies spp. (Pinaceae) in the eastern United States and along the Pacifi c Northwest coast. In the interior west, Livingston et al. (2000) detected balsam woolly adelgid in Idaho in 1983, then observed declining health and death of subalpine fir, A. lasiocarpa (Hooker) Nuttall, over the next 15 years, primarily in north-central Idaho. This seemingly slow invasion from the west coast to Idaho may be due, in part, to diffi culty detecting an insect less than one millimeter in size. Another factor may have been a geographic barrier, the Columbia Basin, slowing the west to eastward dispersal between viable host trees.

Predators collected from balsam woolly adelgid and Cooley spruce gall adelgid in western Oregon and Washington, U.S.A., with reference to biological control of hemlock woolly adelgid (Hemiptera: Adelgidae)

As part of a comprehensive study to survey predators associated with hemlock woolly adelgid, Adelges tsugae Annand, 1928 in the Pacific Northwest (PNW), U.S.A. (Kohler et al. 2008), predators of balsam woolly adelgid, Adelges piceae (Ratzeburg, 1844) and Cooley spruce gall adelgid, Adelges cooleyi (Gillette, 1907) (all Hemiptera: Adelgidae) were sampled on nine dates from June 2005 to October 2006 on a small number of trees. Samples were collected every 6–8 weeks. Predators were collected from two A. piceae infested grand fir, Abies grandis (Douglas ex David Don) Lindley, and one A. cooleyi infested Engelmann spruce, Piceae engelmanii Parry ex Engelmann, on the Oregon State University campus in Corvallis, Oregon, and two A. cooleyi infested Douglas-fir, Pseudotsuga menziesii (Mirbel) Franco (all Pinaceae). One Douglas-fir was an ornamental tree in Corvallis, Oregon and the other was in a seed orchard on Whidbey Island, Washington. Adelges cooleyi predators were collected by striking an infested branch four times over a plastic tub with an area of 0.16 m2. Adelges piceae predators were collected by brushing adelgid wool on the bole of infested trees with a 2-inch wide paintbrush over a beat sheet. Predators were collected from the tub and beat sheet with an aspirator. Adult insects were killed in a sealed 9-dram plastic vial containing a small piece of Hot Shot No-Pest Insecticide Strip (Spectrum Brands Inc., Atlanta, Georgia). Immature insects were killed in KAAD mixture (10 parts 95% ethanol, 1 part kerosene, 2 parts glacial acetic acid, and 1 part dioxane) and preserved in 70% ethanol (Borrer et al. 1989). Adult specimens were identifi ed either by taxonomic specialists or by comparison to previously identified museum specimens. Voucher specimens were deposited in the Oregon State Arthropod Collection, Department of Zoology, Oregon State University, Corvallis, Oregon.

Effect of different headspace concentrations of bornyl acetate on fecundity of green peach aphid and balsam woolly adelgid

ABSTRACTBalsam woolly adelgid (Adelges piceae) (Hemiptera: Adelgidae) (BWA) is an exotic pest introduced from Europe to North America in the early 1900s. Subsequent introductions and spread have enabled this pest to infest native Fraser fir stands in the Southern Appalachians and become a troublesome pest for the region’s Christmas tree industry. Means to study its fecundity and control it are consequently of high importance. Headspace solid phase micro-extraction coupled with gas chromatography and mass spectrometry were used to compare chemical differences in stem tissue between a resistant species, Veitch fir (Abies veitchii) and the susceptible Fraser fir (Abies fraseri). Comparisons demonstrated that bornyl acetate (BA), a terpenoid, was qualitatively more abundant in resistant Veitch fir than Fraser fir. Varying headspace concentrations of BA were tested to ascertain any biological impacts on egg eclosion of BWA, as well as fecundity of green peach aphid (Myzus persicae) (GPA), an insect serving as a proxy. Varying concentrations of BA and a known number of adelgid eggs did not indicate any impact of concentration on egg eclosion success. However, defoliated Veitch fir branches in treatment jars produced a significant negative impact on BWA eclosion success. Implications of these findings are discussed.

Recovery trends and predictions of Fraser fir (Abies fraseri) dynamics in the Southern Appalachian Mountains

The endemic Fraser fir (Abies fraseri (Pursh) Poir.) is found in only seven montane regions in the southern Appalachians above ca. 1500 m elevation. Due to widespread insect-caused mortality from the invasive balsam woolly adelgid (Adelges piceae Ratzeburg), as well as possible impacts from climate change and atmospheric pollution, the future of Fraser fir populations remains uncertain. Long-term monitoring programs have been in place since the 1980s, and here we present the first predictive population models for endemic Fraser fir populations using the inventory data from Great Smoky Mountains National Park, which contains 74% of extant Fraser fir forests. Using two kinds of population data (understory density counts and overstory census data), we model Fraser fir population dynamics on five different mountaintops as a stage-structured matrix model with transition parameters estimated using hierarchical Bayesian inference. We predict robust recovery over the next several decades for some Fraser fir populations, particularly where mature overstory fir has persisted throughout the last two decades, and continued decline for populations at the lowest elevations. Fraser fir densities are already low at these lower elevations, suggesting that this population is vulnerable to local extirpation.

Climate Risk Modelling of Balsam Woolly Adelgid Damage Severity in Subalpine Fir Stands of Western North America

The balsam woolly adelgid (Adelges piceae (Ratzeburg) (Homoptera: Adelgidae)) (BWA) is a nonnative, invasive insect that threatens Abies species throughout North America. It is well established in the Pacific Northwest, but continues to move eastward through Idaho and into Montana and potentially threatens subalpine fir to the south in the central and southern Rocky Mountains. We developed a climatic risk model and map that predicts BWA impacts to subalpine fir using a two-step process. Using 30-year monthly climate normals from sites with quantitatively derived BWA damage severity index values, we built a regression model that significantly explained insect damage. The sites were grouped into two distinct damage categories (high damage and mortality versus little or no mortality and low damage) and the model estimates for each group were used to designate distinct value ranges for four climatic risk categories: minimal, low, moderate, and high. We then calculated model estimates for each cell of a 4-kilometer resolution climate raster and mapped the risk categories over the entire range of subalpine fir in the western United States. The spatial variation of risk classes indicates a gradient of climatic susceptibility generally decreasing from the Olympic Peninsula in Washington and the Cascade Range in Oregon and Washington moving eastward, with the exception of some high risk areas in northern Idaho and western Montana. There is also a pattern of decreasing climatic susceptibility from north to south in the Rocky Mountains. Our study provides an initial step for modeling the relationship between climate and BWA damage severity across the range of subalpine fir. We showed that September minimum temperature and a metric calculated as the maximum May temperature divided by total May precipitation were the best climatic predictors of BWA severity. Although winter cold temperatures and summer heat have been shown to influence BWA impacts in other locations, these variables were not as predictive as spring and fall conditions in the Pacific Northwest.

The Speed of Invasion: Rates of Spread for Thirteen Exotic Forest Insects and Diseases

Invasive, exotic insects and diseases have a devastating effect on North American forests. The rate of spread, or range expansion, is one of the main determinants of an invasive organism’s impact, and can play a major role in structuring management response options. To better understand how exotic organisms have spread through our forests, this study employs a consistent, rigorous analytical framework to analyze a comprehensive geospatial database for the spread of seven exotic insects and six diseases. This study includes new data for six insects and two diseases in combination with five invasive species previously analyzed using the same technique. The quantile regression analysis of over 3000 records of infestation over the preceding century show that the rate of spread of invasive forest insects and diseases ranges from 4.2 km·year−1 to 57.0 km·year−1. The slowest disease spread was white pine blister rust (Cronartium ribicola) at 7.4 km·year−1 while the most rapid disease spread was chestnut blight (Cryphonectria parasitica) at 31.3 km·year−1. The slowest insect spread was balsam woolly adelgid (Adelges piceae) (4.2 km·year−1) while the fastest was emerald ash borer (Agrilus planipennis) at 57.0 km·year−1. Species that can fly long distances or are vectored by flying insects have spread faster than those that are passively dispersed. This analysis highlights the difficulty of estimating spread rates from studies of individual dispersal or flight distances, but the estimated spread rates in this study are generally in line with previous estimates.

A Severity Rating System for Evaluating Stand-Level Balsam Woolly Adelgid (Hemiptera: Adelgidae) Damage in Two Abies Species in Western North America

Severity rating systems are fundamental to understanding the impacts of disturbance agents in forest stands. The balsam woolly adelgid (BWA), Adelges piceae (Ratzeburg) (Hemiptera: Adelgidae), is an invasive forest pest in North America that infests and causes mortality in true fir, Abies spp. There is currently no single system for evaluating damage caused by BWA in the western United States. Because range expansion through favored habitat is inevitable, it is imperative to begin long-term monitoring using a unified approach to evaluate changing conditions and hasten management opportunities. We developed a new rating system for two western host species: grand fir and subalpine fir. Unlike other severity scales, our index describes stand-level damage rather than impacts on individual host trees alone. We sampled 57 sites across the current range of BWA in the western United States and compiled severity indices using 10 metrics of overstory and understory damage. We used analyses of variance to identify five discrete severity classes and translated the results into a descriptive table of damage characteristics for each class. This index is proposed as an improvement over existing rating systems for western North America because of its broader scope, demonstrated ability to distinguish between classes, and identification of the predominant indicators that will improve the efficiency and efficacy of field assessments. The adoption of this system will facilitate long-term monitoring through site resurveys that will be directly comparable over time, also allowing future studies to conduct risk assessments and target stands that face the greatest threat to forest health.

Physico-Chemical Responses of Fraser Fir Induced by Balsam Woolly Adelgid (Homoptera: Adelgidae) Infestation

The balsam woolly adelgid, Adelges piceae (Ratzeburg) (Homoptera: Adelgidae), is a major pest in Fraser fir, Abies fraseri (Pursh) Poir, Christmas tree plantations in the southern Appalachian Mountains. Shortly after eclosion, A. piceae larvae insert their stylets into intercellular bark tissue to ensure a constant feeding source from cortical parenchyma cells (Balch 1952, Can. Dept. Agric. Publ. No. 867, 76 pp.). Thereafter, the developing adelgid is attached at that site for the remainder of its life (Balch and Carroll 1956, Can. Dept. Agric. Publ. No. 977, 7 pp.). Insects that use a piercing-sucking mode of feeding, such as adelgids, elicit a response similar to that of fungal and bacterial plant pathogens (Walling 2000, J. Plant Growth Regul. 19: 195–216; Bernards and Spohr 2008, Pp. 189–211, In Schaller, A. (ed.), Induced Plant Resistance to Herbivory, Springer Science, Stuttgart, Germany). The synthesis of new compounds or structural barriers after herbivore-induced damage occurs, including the accumulation of proteins and secondary metabolites, are characteristics of induced defenses (Bernards and Spohr 2008). For example, the formation of cell wall modifications can be induced by wounding and herbivory (Ginzberg 2008, Pp. 131–146, In Schaller, A. (ed.), Induced Plant Resistance to Herbivory, Springer Science, Stuttgart, Germany). Host resistance mechanisms include rapid accumulation of monoterpene and juvabione-related compounds, the production of secondary periderm (Mullick 1975, Can. J. Bot. 53: 2443–2457; Hain et al. 1991, Pp. 271–287, In Baranchikov, Y.N., Mattson, W.J., Hain, F.P., Payne, T.L. (eds.), Forest Insect Guilds: Patterns of

White-Tailed Deer and Balsam Woolly Adelgid Effects on Balsam Fir in Canaan Valley

Abies balsamea (Balsam Fir) was historically abundant in parts of West Virginia, but it was drastically reduced by extensive logging around the turn of the 19th–20th centuries. More recently, intense herbivory by Odocoileus virginianus (White-tailed Deer) and an infestation of Adelges piceae (Balsam Woolly Adelgid), among other possible factors, have caused Balsam Fir to decline in the region. Understanding the impacts of White-tailed Deer and Balsam Woolly Adelgid is important for restoring Balsam Fir in Canaan Valley. On the Freeland Tract of the Canaan Valley National Wildlife Refuge, we surveyed the intensity of browsing on Balsam Fir and estimated deer population levels, while Refuge personnel assessed the intensity of the adelgid infestation. Even though estimates of deer density were significantly greater during 1999–2000 than in 2001–2002, the percent of buds browsed was similar during the two periods. During 2001–2002, browsing was more intense on Balsam Fir trees that were outside than on trees inside an exclosure. Deer had minimal to moderate impact on mature trees, but in many cases, they completely browsed all of the buds on trees <3.3 ft (1 m) tall. The proportion of trees infested by Balsam Woolly Adelgid rose by 17.4%—from 51.3% in 2000 to 68.7% in 2002. The average diameter at breast height of Balsam Firs heavily infested by adelgids was significantly larger than of those trees classified as moderate, light, or not infested. The Balsam Woolly Adelgid is of greater conservation concern than White-tailed Deer because some of the trees tallied in the first phase of our survey were already dead. Restoration efforts that involve the planting of seedlings should consider the impacts of deer on seedlings and the impacts of adelgids on mature trees.

Assessment of Environmental Conditions and Preferences for Rearing the Balsam Woolly Adelgid (Hemiptera: Adelgidae) on Host Material

Abstract The optimal environmental conditions for rearing the balsam woolly adelgid, Adelges piceae (Ratzeburg) (Hemiptera: Adelgidae), on its natural host, Fraser fir, Abies fraseri (Pursh) Poir, was determined by varying temperature, photoperiod, and humidity as well as by evaluating texture and geotaxic preferences. Five locations with a set temperature and relative humidity (RH) were chosen to test for optimal environmental conditions: three environmental chambers maintained at 17, 20, and 25°C with 75% RH; an insectary maintained at 21.1°C and 72% RH; and a greenhouse with varying temperature and RH levels. Overall, 20°C appeared to be the optimal temperature for development, and 25°C was the least favorable of the temperatures tested. Comparing Fraser fir cuttings to seedlings, seedlings (15.51/cm) were five times more infested than cuttings (3.02/cm), and the percentage of adult survival was higher on seedlings (67%) than on cuttings (17%). The highest percentage of adelgids reaching full developme...

Indirect host-mediated effects of an exotic phloem-sap feeder on a native defoliator of balsam fir

Since its introduction from Europe, balsam woolly adelgid [Adelges piceae (Ratzeburg) (Hemiptera: Adelgidae)], a phloem-sap feeder, has spread throughout the balsam fir [Abies balsamea L. (Mill.)] forests of eastern Canada. Trees under A. piceae attack develop “gout” and differ from unattacked trees in physiology, morphology, growth, and chemistry. The native and eruptive eastern spruce budworm [Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae)] also attacks fir, causing severe defoliation during outbreaks. While balsam woolly adelgid and budworm feed at different times and on different host tissues, such spatiotemporally separated herbivores may still interact via host-mediated indirect effects. We examined the relationship between gout and the performance of developing budworm larvae in balsam fir dominated stands in western Newfoundland. We tested for adelgid–budworm interactions in unthinned and precommercially-thinned (∼20years past) stands, because the host’s growing condition can affect foliage composition and herbivore performance.Budworm attained lower pupal weight when reared on trees with high levels of gout. In unthinned stands moderate gout reduced budworm survivorship, but there was no such effect in thinned stands. Gout did not affect budworm sex ratio. Although our data are consistent with interactions mediated by foliage quality (rather than quantity), and although budworm survivorship was associated with several aspects of foliage chemistry (Mg and N, positive; monoterpenes, negative), we were unable to identify specific host quality changes underlying the adelgid–budworm interaction. Our study demonstrates that A. piceae attack is an important factor influencing budworm performance, and it should be considered when analyzing budworm population dynamics and when developing management protocols for forests impacted by A. piceae attack.

Relationship between an exotic phloem feeder and balsam fir (Abies balsamea L. (Mill.)) foliar chemistry: implications for two native defoliators

The balsam woolly adelgid (Adelges piceae) is a gout-inducing hemipteran native to the silver fir forests of Europe. Introduced to eastern North America approximately 100 years ago, it is now found in most balsam fir forests in Atlantic Canada. When A. piceae feed, they trigger a reaction in the host branch that alters both xylem and phloem morphology. We conducted a field survey to examine the relationship between A. piceae gout density and balsam fir foliar chemistry and shoot growth in naturally unthinned and precommercially thinned stands. A. piceae gout density negatively affected branch growth and was related to changes in the chemistry of older, but not current-year foliage. Older foliage experienced decreases in camphene and bornyl acetate, while foliar concentrations of camphene, myrcene, phenolics, potassium and water differed between thinned and unthinned stands. Foliar chemistry was also influenced by interactions between thinning and A. piceae gout density in old foliage. This study suggests that changes in balsam fir associated with A. piceae gout density may force native defoliators that feed in highly gouted trees to adapt to diets of different chemical compositions and that thinning may alter these interactions.

Forest Response to the US 1990 Clean Air Act: The Southern Spruce-Fir Ecosystem

The history of the Black Mountains in North Carolina and the southern Spruce-Fir ecosystem has been fraught with widespread forest decline since the mid 1960’s. Balsam Woolly Adelgid attacks and acidic deposition were two of the most recognized causes of decline. Uncertainty arose about the future of these forests, and projections were made regarding the endangerment or extinction of the endemic Fraser fir ([Pursh] Poiret). This study analyzed data sets from a permanent plot network in the Black Mountains dating 1985, 2002, and 2012. Indications that the Fraser fir population is stabilizing from a “boom-bust” cycle of population growth and has entered the stem exclusion stage of forest stand development are evident. Fir live stem density increased more than 250% from 1985 to 2002, and then declined 40% by 2012 at the highest elevations in the forest. Overall, fir appeared to be more impacted on western facing slopes than eastern ones. The population of red spruce experienced a steady decrease in live stem counts, but an increase in live basal area through all years, and at all elevation classes (1675 m, 1830 m, and 1980 m), indicating a normal progression through stand development. Red spruce was also most negatively impacted on western facing slopes. Live stem density was significantly higher (P 0.001) than eastern plots, but live basal area was similar between the two aspects. Atmospheric deposition concentrations of the four main acidic molecules at Mt. Mitchell all peaked in 1998, but decreased by 2012. These reductions, occurring shortly after tightened regulations in the 1990 amendments to the Clean Air Act may have potential implications for increased forest resilience.

Regeneration responses to exogenous disturbance gradients in southern Appalachian Picea-Abies forests

Because of the devastation caused by the combined impacts of the balsam woolly adelgid (Adelges piceae; BWA, a non-native insect) and chronic acid deposition, Picea-Abies (spruce-fir) forests are one of the most threatened vegetation communities in North America. Endemic Abies fraseri (Fraser fir), the dominant overstory species in these forests, has experienced near complete overstory mortality as result of the adelgid. Observed forest regeneration patterns suggest high spatial variability, with dense patches of Rubus spp. (blackberry), A. fraseri, and deciduous regeneration repeating across the landscape. To quantify the spatial variability and density of A. fraseri, Picea rubens (red spruce), and deciduous regeneration in these forests, we sampled 60 randomly selected plots within Picea-Abies forests of Great Smoky Mountains National Park (GSMNP). As a measure of local variability in regeneration, we used regeneration density within 30 1×1m subplots per plot to calculate a coefficient of within-plot variation for each species. Despite the impacts of the BWA and chronic acid deposition, A. fraseri remained an important component of southern Appalachian Picea-Abies forests at elevations above 1750m. Density of A. fraseri in all size classes was associated with time since disturbance by BWA infestation. On plots where A. fraseri regeneration occurred, its local variability (among 30 subplots) was significantly greater than that of P. rubens or deciduous species. Regression models attributed this local-scale variability in A. fraseri regeneration to the influence of elevation, Rubus spp. cover, B-horizon nitrogen concentration, and O-horizon calcium to aluminum ratio. We propose that co-occurring gradients of BWA-induced mortality and acid deposition have created patches of increased light and nitrogen availability, which have increased competition from ruderal species such as Rubus spp. Additionally, our results suggest that high soil aluminum content relative to calcium may exclude A. fraseri from certain locations, further contributing to the local variability of A. fraseri regeneration.