The balsam woolly adelgid (BWA) was introduced to New England in 1908 (Kotinsky, 1916) and the western states in 1928 (Annand, 1928) on nursery stock from Europe. The pest attacks all North American true fir species (Abies), with the possible exception of the divergent bristlecone fir (A. bracteata); impacts vary considerably (Flowers pers. comm.) Feeding adelgids inject a toxic saliva which causes thickening of the tree host’s vascular tissues which interferes with the transport of water and nutrients, often leading to premature needle loss and crown dieback (Pederson et al. no date)
Spread of the adelgid is by transport of the “crawler” stage by wind, animals, or people to new areas (Pederson et al. no date).
USFS scientists and managers developed a conservation priority-setting framework for forest tree species at risk from pest & pathogens and other threats. The Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation) uses FIA data and expert opinion to group tree species under threat by non-native pests into vulnerability classes and specify appropriate management and conservation strategies. The scientists prioritized 419 tree species native to the North American continent. The analysis identified 15 taxonomic groups requiring the most immediate conservation intervention because of the tree species’ exposure to an extrinsic threat, their sensitivity to the threat, and their ability to adapt to it. Each of these 15 most vulnerable species, and several additional species, should be the focus of both a comprehensive gene conservation program and a genetic resistance screening and development effort.
The greatest mortality has been in the southern Appalachians where balsam woolly adelgid kills mature specimens of Fraser fir (A. fraseri). Fraser fir is one of six species that face severe pest threats, are highly sensitive to that pest, but also have a high capacity to adapt (according to CAPTURE report). The project urges immediate conservation and the facilitation of resistance through breeding. Fraser fir is restricted to mountaintop environments in the southern Appalachians, where the species forms a unique forest type with red spruce (Picea rubens). Mortality is variable among mountaintops, with fewer than ten mature trees surviving on some mountains, although many immature trees also persist.
The adelgid was first noticed on a northern population of Fraser fir in 1957 (Johnson, 1980), and it subsequently spread to all populations (Johnson, 1980). Infested trees usually die within seven years (Johnson, 1980). A study on Mt. Guyot, Tennessee, in the Great Smoky Mountains National Park revealed that Fraser fir declined from 80% to 2.5% of living crown trees in the time period of 1967-1985 (Alsop & Laughlin, 1991). This demise resulted in a dramatic change in forest composition and dynamics on former Fraser fir sites. With the forest canopy removed, the understory changed from primarily blueberry (Vaccinium) and fir saplings to dense blackberry (Rubus), blueberry, and Viburnum populations. Increases occurred in the proportion of red spruce (P. rubens) and yellow birch (Betula alleghaniensis) in the forest canopy. The spruce-fir moss spider, Microhexura montivaga, and a narrowly endemic lichen, Gymnoderma lineare, that inhabit this unique habitat have been listed as a federally endangered species. Changes in avian species and populations also have been observed in other studies (cf. Rabenold et al., 1998).
Control of the adelgids with insecticides has not been successful or practical, particularly in Fraser fir populations residing in the Great Smoky Mountains National Park. Biological controls have been attempted (Schooley et al., 1984; Humble, 1994), but none has demonstrated satisfactory levels of success to date. A planting of genotypes from different mountaintops was established in the Great Smoky Mountains National Park during the early 1990s to conserve the genetic resources of this fir species (S.E. Schlarbaum personal knowledge).
In the West, BWA has caused significant damage and mortality to true firs since its appearance in 1928. It occurs primarily on subalpine fir (Abies lasiocarpa), Pacific silver fir (A. amabilis) and grand fir (A. grandis) in Oregon, Washington, Idaho and British Columbia. Subalpine fir and Pacific silver fir are infested in the mountainous areas and grand fir in the lowland valleys (Ragenovich and Mitchell 2006, FIDL#118).
Grand and Pacific silver firs with stem or bole infestations generally have needle loss, followed by a change in color to a grayish-green and, ultimately, death. These trees often die quickly, sometimes after only 2 or 3 years of heavy infestation (Ragenovich and Mitchell 2006, FIDL#118).
Subalpine fir is a pioneer species in harsh or disturbed environments, such as old lava flows, avalanche tracks and marshy meadows. Balsam woolly adelgid has removed the subalpine fir from some of these local areas (Ragenovich and Mitchell 2006, FIDL#118). According to Pedersen et al (no date), disappearance of high elevation subalpine fir stands may have the following adverse effects on the ecosystem :
Hydrological effects, including reduced ground water retention, increased rate of snow melt, and altered stream flow characteristics.
- Increased fuel loads and thus greater fire danger caused by dead trees.
- Wildlife may be affected by changes in stand composition.
- Loss of the tree canopy along streams can impact water quality and fish populations downstream (Stephenson 2010).
In the lower Willamette Valley in Oregon, and the Puget Sound Trough in Washington State, long-time infestations have deformed large old grand firs and virtually ended reproduction, so grand fir is slowly disappearing from these ecosystems (Ragenovich and Mitchell 2006, FIDL#118).
Aerial surveys indicate that more than 150,000 acres in Oregon show signs of currently being infested by BWA (Flowers pers. comm.).
Balsam woolly adelgid has been killing subalpine firs in Idaho since 1983 (Pederson et al.) When BWA populations reach high levels, they can kill firs in 3 years. Trees not killed by BWA often are killed by other insects or diseases (Pederson et al.) Aerial surveys are not adequate to monitor for BWA. Roadside surveys conducted in 2006-2007 sampled 1,016 plots; of them, 586 were found to be infested (Pederson et al.). 158 (27%) of the infested plots were in areas that had not been infested during the 1997-1998 survey (Pederson et al.) Between the 1997-1998 and 2006-2007 surveys, BWA advanced 91 miles north to the Canada border, 26 miles east to the Montana border, 10 miles west to the Washington border, and approximately 111 miles south (Pederson et al.). 73% of plots below, and 53% above 4,500 feet were infested (Pederson et al.).
In British Columbia, susceptible fir trees in coastal areas were killed in the 1960s and 1970s. The disease is now occurring at higher elevations and provincial officials are attempting to slow its spread by restricting areas in which fir can be grown in reforestation and by regulating movement of ornamental material (Leland Humble, Canadian Forest Service, pers. comm. June 2010).
Pedersen et al. (no date) expect climate change to exacerbate the effects of BWA in the West because
- Warmer temperatures, particularly at high elevations, will allow BWA to have more than 1-2 generations per season.
- Infestations will become more severe, resulting in faster tree decline.
- Expanding and more severe infestations will result in adverse effects on host type health and susceptibility to other insect and disease agents.
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Flowers, Rob. Entomologist, Oregon Department of Forestry. Pers. comm.. May 2010.
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