Invasive and Exotic Species

Introduction

Biological Control of Hemlock Woolly Adelgid

From: Cheah, C., M. E. Montgomery, S. Salom, B. L. Parker, S. Costa, and M. Skinner, 2004. Biological control of hemlock woolly adelgid. USDA For. Serv. FHTET-2004-04, Reardon, R. and B. Onken (Tech. Coordinators), 22pp

Overview

The hemlock woolly adelgid (HWA), Adelges tsugae Annand (Homoptera: Adelgidae), is a non-native pest and has become the single greatest threat to the health and sustainability of hemlocks (Tsuga species) in eastern North America. The defoliation and mortality of hemlock forests equates to the loss of distinctive habitat and microclimates, and degradation of biodiversity. Also, it poses a significant threat to ornamental hemlocks, which are widely used as landscape trees and provide valuable biodiversity to the urban environment.

Figure 1a Infestation, Ovisacs on the underside of a branch Photo by Michael Montgomery, USDA Forest Service

Figure 1b Life Cycle, Adult and eggs inside an ovisac Photo by Michael Montgomery, USDA Forest Service

The HWA is an insect 1 to 2 mm long (about the size of this letter “o”) with piercing/sucking mouthparts, which it inserts at the base of hemlock needles to feed on nutrients stored in young twigs (McClure et al 2001). The tree’s vitality depends on these nutrients; depleting the nutrient reserves causes needle loss and a reduction in shoot growth, and can seriously impair tree health. As the adelgid matures it produces increasing amounts of white woolly wax, with which it protects itself and its eggs (Fig. 1). Populations of HWA in eastern North America fluctuate greatly in response to various factors, such as cold winter temperatures, declining tree health, and drought. Controlling HWA on ornamental hemlocks is relatively easy compared to controlling it in forested and woodland settings.

Figure 1c Adult(s), With wax removed Photo by Michael Montgomery, USDA Forest Service

Individual trees can be treated either by drenching foliage with insecticidal soaps or horticultural oils, or by injecting the trunk or soil with chemical insecticides (e.g., imidacloprid). Regardless of the application method, care must be taken, because insecticide applications can unintentionally lead to an increase in secondary pests, such as spider mites and scales. In the forest, and especially in those situations in which insecticide use is not appropriate, biological control with predators and pathogens is the preferred way to manage HWA populations.

Distribution

HWA is native to Asia (Japan, India, Nepal, southwestern China, and Taiwan), where it is a common but harmless inhabitant of several hemlock (Tsuga) species. It was first observed in Oregon and British Columbia in the 1920s on western hemlock (T. heterophylla [Ref.] Sargent) and mountain hemlock (T. mertansiana [Bong.]) Carr. It can now be found from northern California to southeastern Alaska, where it is relatively harmless. Natural enemies account for some HWA mortality, but apparently western and mountain hemlock, as well as hemlock species found in Asia, have host defenses against HWA.

Hemlock woolly adelgid was first collected from hemlock in the eastern United States in 1951 near Richmond, Virginia. The pathway and source of the introduction are undetermined but thought to be nursery stock from Asia. It quickly established itself in natural stands and spread rapidly. By the 1980s it was causing widespread mortality. It has since spread into 16 states on the eastern seaboard, from

Figure 2. Native range of hemlock (green) and range of hemlock woolly adelgid (brown) in 2003.

northeastern Georgia to southeastern Maine, where it is a serious pest of eastern hemlock (T. canadensis [L.] Carr.) and Carolina hemlock (T. caroliniana Engelm.). Since 1993, the main front of the HWA infestation has been advancing at a rate of approximately 17 kilometers (10 miles) per year (Fig. 2), and isolated infestations have been discovered far ahead of the main front.

Hemlock woolly adelgid spreads mainly as eggs or “crawlers,” the mobile first instar nymphs of the insects that hatch from the eggs, which are carried by wind, birds, other forest animals, or people. In the eastern United States, HWA is established in much of cold hardiness zone 6 (mean annual minimum temperature -25° to 18°C [-13° to 0°F]) and parts of zone 5 (-30° to -23°C [-20° to -10°F]). It is likely to continue to spread and threaten eastern and Carolina hemlocks across much of their natural ranges.

Biology and Damage

In North America, hemlock woolly adelgid is parthenogenetic (only females occur, reproducing without males) and produces two generations a 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 (Fig. 3). However, a suitable species of spruce is not present in North America, so this portion of the populatin dies before sexual reproduction occurs.

The lifecycle of HWA promotes a rapid increase in population (Fig. 3). Shortly after the sistens eggs hatch, the first instar nymphs relocate to the bases of needles and immediately become dormant (aestivation). As temperatures cool in the fall, the sistens nymphs break dormancy and begin to feed and develop throughout the winter when temperatures are moderate. Adults begin to lay eggs in March – earlier in southern states or during mild winters. During late fall and early spring, few natural enemies are active and hemlocks produce abundant quantities of sugar and amino acids, which provide good nutrition in the twigs where adelgids feed. This results in a high level of egg production. A single sistens female typically lays between 50 and 175 eggs (as many as 300

Figure 3. Hemlock woolly adelgid annual life cycle on hemlock in North America.

have been observed) (McClure et al. 2001). The progrediens generation lays fewer eggs, typically between 25 and 125, but offspring mature rapidly after hatching. Egg production in early spring and again in early summer has a multiplier effect on the population, which if unchecked by natural enemies or other factors results in exponential population growth.

Hemlock woolly adelgid feeding on eastern and Carolina hemlocks cause the needles to desiccate (dry up) and the buds to stop growing. Within a few months of heavy infestation, the tree looks grayish green, needles begin to drop off, and little or no new foliage is produced. Foliage loss and dieback of major limbs become visible in 2 to 4 years. An infested hemlock may survive for many years, but its foliage is usually sparse at the branch tips and very top of the crown (Fig. 4). Weakened trees often succumb to diseases and attacks from other insects, such as hemlock borer Melanophila fulvoguttata (Harris), elongate hemlock scale Fiorinia externa Ferris, and are easily broken and thrown by wind.

Figure 4a Damage, Recently infested Photo by Michael Montgomery, USDA Forest Service

Figure 4b Control, Tree removal Photo by Michael Montgomery, USDA Forest Service

Focus on Biological Control

Biological control uses natural enemies (predators, parasitoids, or pathogens) to lower pest populations. There are no known parasitoids of HWA, worldwide, and there are only a few predators of HWA in the eastern United States. Most of the predators tend to be generalists (McClure 1987, Montgomery and Lyon 1996, and Wallace and Hain 2000). Some non-host-specific, native fungi have been found to infect HWA, but naturally occurring epizootics have not been observed (Reid et al 2002).

Native predators and pathogens in North America cannot maintain HWA populations at low enough densities to prevent them from damaging hemlock forests, so in 1992, studies began on biological control agents imported to the United States from Japan, China, and Canada (Fig. 5).

Figure 5. Worldwide occurrence of Hemlock (red) with arrows showing where biological control agents were collected for importation to the United States.

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