Insects and pathogens are a natural part of ecosystems and are essential to ecological balance in natural forests. Population dynamics of insects and pathogens are influenced by climate, management activities, natural tree defenses, and natural enemies. Trees can be weakend by a disease or an insect, and that weakened tree is then more vulnerable to insects, disease, and changes in temperature and precipitation. The effect on forested landscapes can be tree mortality and/or reduced tree vigor. Examining trends of individual insect and pathogen populations is useful in understanding their dynamic nature.
Over the last century, several forest types in the region have experienced significant declines due to infestations of insects and diseases.
Southern Pine Beetle
In the period 1984 to 2006, the volume of southern yellow pine trees decreased 57 percent in Western North Carolina. Eastern white pine was also attacked and killed. Southern pine beetle, an insect native to southern forests. attacks loblolly, shortleaf, Virginia, pitch, and table mountain pine. The insect periodically increases to epidemic proportions, causing severe mortality. Devastating outbreaks have been reported across the South since the early 1800’s. The most recent outbreak peaked in 2002. In the aftermath of large infestations, dead and downed trees provide abundant fuel for wildfires and pose additional threats to transportation corridors and public safety. The number of years of outbreak of southern pine beetle in Western North Carolina is shown from 1960 to 2005. Due to the preponderance of mixed pine forests in the southwestern part of the region, Graham, Cherokee, Swain, Macon, Jackson, and Buncombe counties had the highest occurrence.
Hemlock Woolly Adelgid
Although decreases in the volume are not yet reflected in the data, eastern and Carolina hemlocks are dying rapidly due to an infestation of hemlock wooly adelgid. The hemlock woolly adelgid is similar in biology to its relative, the balsam woolly adelgid. The insect feeds in fall, winter, and spring, depleting the tree of nutrients, leading to mortality in less than five years. The tiny adelgid is dispersed by wind, birds, and mammals.
The loss of eastern hemlock in coves and Carolina hemlock on steep south- or west-facing slopes across Western North Carolina will affect other biological values. Hemlock forests are used by many species of wildlife as a food source, a nesting site, a roosting site, and seasonal shelter. Hemlocks provide shade near creeks and streams and their disappearance is likely to increase water temperature, negatively impacting aquatic species.
Eradication of the hemlock woolly adelgid is not feasible; therefore, many regional groups, both public and private, are working together to protect highly valued stands of hemlock. Treatments include insecticides and the introduction of several beetle species that feed only on the hemlock woolly adelgid. These controls are the best long-term hope to save the hemlocks.
The gypsy moth was introduced into the U.S. in the mid-1800’s. The insect, now permanently established in 17 states, defoliates trees, making them vulnerable to other killing agents. Gypsy moths feed on a wide variety of trees, shrubs, and vines, but prefer all oak species, apple, beech, birch, basswood, and willow. Although gypsy moth is not yet permanently established in our counties, there have been numerous instances of human-mediated introductions of this pest over the past 10-15 years. A few of these introductions developed into small, isolated pockets of infestation that were subsequently eradicated. Additionally, the USDA Forest Service is implementing a program to slow the spread, which will delay the establishment of gypsy moth in Western North Carolina. The most effective strategy is continued efforts to delay permanent infestation combined with active forest management before the gypsy moth arrives. Pre-outbreak treatments focus on reducing the vulnerability of stands by removing the trees most likely to die and regenerating stands that are close to maturity or understocked.
Balsam Woolly Adelgid
Fraser fir stands, which grow in association with red spruce on scattered mountain tops such as Mount Mitchell in Yancey County, have declined significantly due to an infestation of the insect balsam woolly adelgid and air pollution. The adelgid, which appeared in North Carolina in the 1950’s, is a tiny, sucking insect that robs the tree of nutrients, leading to its decline and eventual death. The foliage in a dying tree turns yellow, then deep red, then falls to the ground. In our region, significant mortality of Fraser fir has occurred, followed by scattered regeneration. Although the species is still present in the ecosystem, red spruce is the dominant species in the spruce-fir forest type.