Hemlock Woolly Adelgid

Identification

Hemlock wooly adelgids (Adelges tsugae) are small insects (1 to 2 mm long). The adelgid itself is not visible to the naked eye, but the white, woolly secretion that protects the adelgid and its eggs is visible and indicates an infestation. The adelgids feed on the new shoot growth and the white woolly masses are visible on the underside of young twigs.

Hosts

Within the southern Appalachians, hemlock woolly adelgids is a serious pest of the two species of hemlock native to the eastern United States, eastern hemlock (Tsuga canadensis) and Carolina hemlock (T. caroliniana). In the western United States Hemlock woolly adelgid is a minor pest of western hemlock (T. heterophylla) and mountain hemlock (T. mertasiana).

Impacts

The adelgids feed on young twigs by inserting their stylet-like mouth parts through the bark near the base of needles. Fluid from the tissues of the inner bark and phloem are extracted. Once the hemlocks are infested, their foliage is discolored, to a gray-green color. The needles dessicate and fall prematurely. Regeneration of needles and buds is inhibited and the crown thins, becoming increasingly more transparent (Rhea 1996). Foliage loss and dieback of major limbs become visible in 2 to 4 years (Cheah et al. 2004). Complete defoliation and tree mortality occurs generally within 5 years of initial infestation.

Eastern hemlock is an ecologically important tree in the southern Appalachians. Hemlock forests are a unique ecosystem, provide habitat for a wealth of rare plants and animals. Eastern hemlocks are late succesional trees that, if left undisturbed will dominate stands (McClure et al. 2001).

Hemlock woolly adelgid seriously threatens the existence of these two native species of hemlocks in the southern Appalachians. The potential negative impacts are great and include loss of an important ecosystem, decrease in habitat for many rare plants and animals, increased fire risks, and loss of tourism revenues.

According to Rhea (1996), the southern Appalachians hemlocks are the preferred habitats for many animals such as:

Salamanders

• Cow Knob salamander (Plethodon punctatus)
• Shenandoah salamander (Plethodon shenandoah)

Birds

• Blackburnian warbler (Dendrocia fusca)
• Veery (Catharus fuscesens)
• Black-throated blue warbler (Dendrocia caerulescens)
• Black-throated green warbler (Dendrocia virens)
• Dark-eyed junco (Junco hyemalis)

Mammals

• Southern red-backed vole (Clethrionomys gapperi)
• Woodland jumping mouse (Napaeozapus insignis)
• Smoky shrew (Sorex fumeus)
• Red Squirrel (Tamiasciurus hudsonicus)

In addition, shade produced from hemlock forests maintain the cool temperatures necessary for the native brook trout (Salvelinus fontinalis) to survive in southern Appalachian streams. Hemlocks are very important aesthetically to the southern Appalachians as well. Tourism is a major source of income within the southern Appalachians and hemlocks provide beautiful settingsfor picnic areas, hiking trails, andaesthetic views.

History and Distribution

Hemlock woolly adelgid is a minor pest in its native range of Asia (Japan, India, Nepal, southwestern China, and Taiwan). Within its native range, it feeds upon several species of hemlocks which are not seriously affected by the feeding (The western United States species of hemlock (T. heterophylla and T. mertasiana also display this resistance). Hemlock woolly adelgid was introduced into the western United States in the 1920s. It was first found in the eastern United States in 1951 near Richmond, Virginia (Rhea 1994). Infested nursery stock from Asia is believed to be the avenue of introduction. It quickly established itself in the wild where it began to slowly spread. Currently hemlock woolly adelgid can be found in Hemlock forests from Maine to Georgia. In the southern Appalachians, it can be found fromnortheast Georgia, north along the border of Tennessee and North Carolina into central Virginia. It is believed to be spreading at a rate of approximately 10 miles per year (Cheah et al. 2004). The eggs and “crawler” stage adelgids are spread wind, birds, mammals, as well as humans. Hemlock wooly adelgid is likely to spread to and infest the entire range of eastern and Carolina hemlock.

Biology

In North America, hemlock woolly adelgid reproduction is by females only (parthenogenetic). Two generations are produced each year. One, the sistens, is wingless, hatches in late spring, overwinters, and survives about nine months. The other, the progrediens, hatches in early spring, is comprised of both wingless and winged (sexupara) offspring, and survives for about three months. Sexupara fly from hemlock in search of a species of spruce (Picea) on which to deposit eggs. However, a suitable species of spruce is not present in North America, so this portion of the population dies before sexual reproduction occurs (Cheah et al. 2004).

Control

Individual trees can be protected from infestations of hemlock woolly adelgid either by treating them with an insecticidal soap or by injecting the trunk or soil with chemical insecticides. This is easily done in an ornamental setting but treating stands of hemlocks rapidly becomes too costly and time-consuming for practical use. Recently, a biological control program for hemlock woolly adelgid was initiated. In 1992 explorations for natural enemies within hemlock woolly adelgid’s native range began (Cheah et al. 2004). Several species were found that potentially could be used for biological control. The most promising of these was Sasajiscymnus tsugae, a beetle from Japan. In 1995 S. tsugae was released into infested forests in Connecticut. In 1999 releases were initiated in the southern Appalachians as well as other portions of hemlock woolly adelgid’s range. S. tsugae hasa shown great potential to affect adelgid populations but with varying results depending upon adelgid population size and climatic conditions (Cheah et al. 2004). Several other insects are currently being evaluated for potential use as biological control agents.