Laurelmountainboro's Weblog

November 3, 2007

HEMLOCK TREE INFESTATION: HWA

To see photos of the hemlock wooly adelgid click: http://www.flickr.com/photos/lmborolmpark/ .
Information in this article is taken from the PA. Department of Conservation and Natural Resources** and the USDA Forest Service, Northeastern Area State and Private Forestry***.
Is Laurel Mountain Borough going to have to sacrifice its bird feeders to save its hemlock trees?
The feeders not only encourage bears to invade resident’s yards, but they encourage birds, which can spread the insect, Adelges tsugae, from infested eastern hemlock trees. The presence of this insect can bring death to the trees it feeds on.
The invasive insect, aka hemlock woolly adelgid and HWA, is native to Asia. It infects all species of hemlock, but only the eastern hemlock (Tsuga canadensis) and the Carolina hemlock are vulnerable when attacked (the latter doesn’t grow this far north).
Eastern hemlock is commonly planted as a tree, shrub, or hedge in ornamental landscapes. There are at least 274 known cultivators of eastern hemlock.
The war against the insect has reached our community, which is characterized by its many hemlock trees of majestic beauty, height and girth. The high number of these stately trees in LMB’s landscape can’t help but enable the insect infestation to spread.
The war against HWA infestation began about 1951 when it was first reported in Virginia. By 2005 it covered about half the range of hemlocks in parts of 16 States from Maine to Georgia. Its impact has been most severe in some areas of Virginia, New Jersey, Pennsylvania, and Connecticut.
The insects migrated to eastern Pennsylvania in the late 1960s, when their militia gradually marched over the Laurel Ridge. Battles to protect the hemlocks have been ongoing in
The hemlock woolly adelgid is tiny, less than 1/16-inch (1.5-mm) long, and varies from dark reddish-brown to purplish-black in color. As it matures, it produces a covering of wool-like wax filaments to protect itself and its eggs from natural enemies and prevent them from drying out. This “wool” (ovisac) is most conspicuous when the adelgid is mature and laying eggs. Ovisacs can be readily observed from late fall to early summer on the underside of the outermost branch tips of hemlock trees.
John Miller, LMB counsel member, raised the issue at LMB’s July borough counsel meeting, officially bringing LMB into the ongoing war being fought in Cook, Fairfield and Ligonier townships.
John showed counsel members and residents samples of branches infected with [the small, aphid-like insect that threatens the health and sustainability of eastern hemlock 2]. He cut them from a row of trees bordering his yard, but said he’s seen the insects on other LMB hemlocks.
All HWA individuals are female and have asexual reproduction. Their life cycle is complex because there are two complete, overlapping generations a year. The overlap occurs mid- to late-spring.
The spring generation (progrediens) ovisacs contain 20-75 eggs. It develops from spring (March) of its first year to early summer of the following year (June).
The winter generation (sistens) ovisacs contain up to 300 eggs. It develops between early summer (June) its the first year and mid-spring of the following year (March).
HWA is unusual because it enters a period of dormancy during the hot summer months. The nymphs during this time period have a tiny halo of woolly wax surrounding their bodies. (This is what John spotted on his`hemlocks.) When cooler weather returns, usually in October, they begin to feed, and continue to do so throughout the winter months.
Each generation has six stages of development: the egg, four nymphal instars, and the adult.
The first instar nymphs, called crawlers, seek suitable feeding sites on the twigs at the base of hemlock needles. This early spring larvae stage is the most harmful for hemlocks because it gives them a “triple whammy.”
First, they feed on the stored starches by sucking fluid from the base of the needles (starches are more critical to the hemlock’s long-term survival than is its sap). Second, its spread is enabled both by winds and birds, deer, and other forest-dwelling mammals contacting the sticky ovisacs. However, isolated infestations and long-distance movement of HWA most often occurs by people transporting infested nursery stock.
Lastly, it’s suspected that HWAs inject a toxin into the trees while they feed—a toxin that accelerates needle drop and branch dieback.
The nymphs settle in the hemlock needles, where they remain for the remainder of their development, feeding on young twig tissue.
In the northern range, the hemlock decline and mortality typically occurs within 4 to 10 years of HWA infestation. If other hemlock stressors* are present, the rate and extent of hemlock mortality is accelerated.
Cultural, regulatory, chemical, and biological controls can reduce the hemlock woolly adelgid’s rate of spread and protect individual trees, according to the USDA Forest Service, Northeastern Area State and Private Forestry web site (www.na.fs.fed.us)
Moving bird feeders away from hemlocks and removing isolated infested trees from a woodlot can help prevent further infestations.
State quarantines help prevent the movement of infested materials into noninfested areas.
Chemical control options, such as foliar sprays using horticultural oils and insecticidal soaps, are effective when trees can be saturated to ensure that the insecticide comes in contact with the adelgid.
Several systemic insecticides have also proven effective on large trees when applied to the soil around the base of the tree or injected directly into the stem. Chemical control is limited to individual tree treatments in readily accessible, non-environmentally sensitive areas; it’s not feasible in forests, particularly when large numbers of trees are infested. Chemical treatments offer a short-term solution, and applications may need to be repeated in subsequent years.
The best option for managing hemlock woolly adelgid in forests is biological control. Although there are natural enemies native to Eastern North America that feed on hemlock woolly adelgid, they aren’t effective at reducing populations enough to prevent tree mortality. Therefore, biological control opportunities using natural enemies (predators and pathogens) from the adelgid’s native environment are currently being investigated. Several predators known to feed exclusively on adelgids have been imported from China, Japan, and Western North America and are slowly becoming established throughout the infested region.
It will likely take a complex of natural enemies to maintain hemlock woolly adelgid populations below damaging levels. Efforts to locate, evaluate, and establish other natural enemies continue.
*{including drought, poor site conditions, and the presence of other insect and disease pests such as elongate hemlock scale (Fiorinia externa), hemlock looper (Lambdina fiscellaria fiscellaria), spruce spider mite (Oligonychus ununguis), hemlock borer (Melanophila fulvogutta), root rot disease (Armillaria mellea), and needlerust (Melampsora parlowii)}
** PA. Department of Conservation and Natural Resources:
http://www.dcnr.state.pa.us/forestry/woollyadelgid/index.aspx
*** USDA Forest Service, Northeastern Area State and Private Forestry
11 Campus Boulevard, Suite 200
Newtown Square, PA 19073
http://www.na.fs.fed.us/fhp/hwa
Also view: http://www.invasivespeciesinfo.gov/animals/hwa.shtml
Picture files: http://www.forestryimages.org/browse/subimages.cfm?sub=289
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