Balsam woolly adelgids (Adelges piceae) are small wingless insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s that infest and kill fir

Fir

Firs are a genus of 48–55 species of evergreen conifers in the family Pinaceae. They are found through much of North and Central America, Europe, Asia, and North Africa, occurring in mountains over most of the range...

s, especially Balsam Fir

Balsam Fir

The balsam fir is a North American fir, native to most of eastern and central Canada and the northeastern United States .-Growth:It is a small to medium-size evergreen tree typically tall, rarely to tall, with a narrow conic crown...

 and Fraser Fir

Fraser Fir

Abies fraseri is a species of fir native to the mountains of the eastern United States. It is closely related to Abies balsamea , of which it has occasionally been treated as a subspecies or a variety Abies fraseri (Fraser fir) is a species of fir native to the mountains of the eastern United...

. They are an invasive species

Invasive species

"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....

 from Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 introduced to the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 around 1900.

Because this predator is not native, the Fraser fir has not evolved any type of defense against it. These insects typically lay about one hundred eggs and have three generations per year. The adelgid attacks the tree by feeding in fissure

Fissure

In anatomy, a fissure is a groove, natural division, deep furrow, elongated cleft, or tear in various parts of the body.-Brain:...

s within the bark of trees larger than about four centimeters in diameter at breast height. As it feeds, it releases toxin

Toxin

A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...

s contained within its saliva

Saliva

Saliva , referred to in various contexts as spit, spittle, drivel, drool, or slobber, is the watery substance produced in the mouths of humans and most other animals. Saliva is a component of oral fluid. In mammals, saliva is produced in and secreted from the three pairs of major salivary glands,...

. These toxins reduce the conductance of sapwood

Sapwood

Sapwood may refer to:* Sapwood, the part of living wood where sap flows, as distinct from the heartwood, where it doesn't* SS-6 Sapwood, the NATO reporting name for the R-7 Semyorka intercontinental ballistic missile...

 being built, which causes water stress and kills the trees.

The Great Smoky Mountains National Park

Great Smoky Mountains National Park

Great Smoky Mountains National Park is a United States National Park and UNESCO World Heritage Site that straddles the ridgeline of the Great Smoky Mountains, part of the Blue Ridge Mountains, which are a division of the larger Appalachian Mountain chain. The border between Tennessee and North...

, in North Carolina and Tennessee

Tennessee

Tennessee is a U.S. state located in the Southeastern United States. It has a population of 6,346,105, making it the nation's 17th-largest state by population, and covers , making it the 36th-largest by total land area...

 contains about 75% of all southern spruce-fir ecosystem

Southern Appalachian spruce-fir forest

The Southern Appalachian spruce-fir forest is a type of montane coniferous forest that grows in the highest elevations in the southern Appalachian Mountains of the eastern United States...

s. These ecosystems covered vast portions of the Southeast during the last ice age

Ice age

An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

, when the climate was cool and moist. Since the retreat of the glacier

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

s, this ecosystem has been confined to the higher elevations in the mountains of the Southeastern United States

Southeastern United States

The Southeastern United States, colloquially referred to as the Southeast, is the eastern portion of the Southern United States. It is one of the most populous regions in the United States of America....

. The spruce-fir forest has evolved as an island, away from any other ecosystems of its type. This has presented valuable research and a unique habitat. These forests have a very dense canopy

Canopy (forest)

In biology, the canopy is the aboveground portion of a plant community or crop, formed by plant crowns.For forests, canopy also refers to the upper layer or habitat zone, formed by mature tree crowns and including other biological organisms .Sometimes the term canopy is used to refer to the extent...

 and a moist understory

Understory

Understory is the term for the area of a forest which grows at the lowest height level below the forest canopy. Plants in the understory consist of a mixture of seedlings and saplings of canopy trees together with understory shrubs and herbs...

.

Since the invasion of the balsam woolly adelgid, discovered in 1957, Fraser fir mortality rates have been 90-99%. Although some areas are being regenerated by young firs, there is much change in understory composition, including invasion by both woody and herbaceous

Herbaceous

A herbaceous plant is a plant that has leaves and stems that die down at the end of the growing season to the soil level. They have no persistent woody stem above ground...

 species. Red Spruce

Red Spruce

Picea rubens is a species of spruce native to eastern North America, ranging from eastern Quebec to Nova Scotia, and from New England south in the Adirondack Mountains and Appalachians to western North Carolina.-Physical description:...

, the spruce component of the spruce fir ecosystem, has also been suffering declines. Some researchers attribute these declines to damage from wind, which is usually blocked by the firs. Balsam woolly adelgids have destroyed about 95% of the Fraser firs in the Great Smoky Mountains National Park, creating "ghost forests". One concern is that acid rain

Acid rain

Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...

 and the deterioration of the ozone

Ozone

Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

, among other pollutant

Pollutant

A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...

s are contributing to a weak immune system for the Fraser firs and making them more susceptible to the balsam woolly adelgids. The Spruce-fir moss spider

Spruce-fir moss spider

The Spruce-fir moss spider, Microhexura montivaga, is an endangered species of spider found at high elevations in the southern Appalachian Mountains. First identified in 1923, they inhabit moss that grows on rocks underneath the forest canopy....

, which lives on moss mats below the forest canopy, is considered endangered due to the decline of the Fraser fir.

The Balsam woolly adelgid also has a significant impact on Pacific Northwest forests. Specifically, grand fir

Grand Fir

Abies grandis is a fir native to the Pacific Northwest and Northern California of North America, occurring at altitudes of sea level to 1,800 m...

, silver fir

Silver Fir

Abies alba, commonly known as the European silver fir, is a fir native to the mountains of Europe, from the Pyrenees north to Normandy, east to the Alps and the Carpathians, and south to southern Italy and northern Serbia.-Description:...

 and subalpine fir

Subalpine Fir

The Subalpine Fir or Rocky Mountain Fir is a western North American fir, native to the mountains of Yukon, British Columbia and western Alberta in Canada; southeastern Alaska, Washington, Oregon, Idaho, western Montana, Wyoming, Utah, Colorado, New Mexico, Arizona, northeastern Nevada, and the...

 in Washington and Oregon

Oregon

Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...

 all can serve as a host. Extensive mortality due to this pest in the Cascade Mountain

Cascade Range

The Cascade Range is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, and the notable volcanoes known as the High Cascades...

range was recorded during the 1950s and 1960s. Land area affected by the adelgid in the Pacific Northwest has increased from 83,325 acres (337 km²) in 2004 to 108,128 acres (438 km²) in 2005.