Hemlock woolly adelgid

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The hemlock woolly adelgid Adelges tsugae is a small aphid-like insect, less than 1 mm long, wrapped up in a white, woolly mass when laying eggs (Fig. 5.8a), that as the name suggests, infests hemlock (Tsuga) trees. Native to Asia, it was first found in western North America in 1924 and then spread through the continent, probably carried on infected hemlock trees. The hemlock woolly adelgid became a problem when it reached the east coast in the early 1990s because the two hemlock species here - eastern hemlock T. canadensis and Carolina hemlock T. caroliniana - proved much more susceptible to the pest than western or Asiatic hemlocks. The hemlock woolly adelgid is now seen as the single biggest threat to these hemlocks with potential impacts every bit as devastating as Dutch elm disease (Section 5.4.5) and chestnut blight (Section 5.4.6).

The small insects cluster on young branches and inject a toxic saliva as they suck the sap. Within a few months of infection, the needles and most buds die leading to the loss of major branches and often eventual death of the tree within as little as 4 years of infection, aided by other insect and fungal attacks on the weakened trees. Mortality in an area is high (50-100% of trees) and, since hemlock doesn't sprout from the base nor are its seeds stored in the soil (Brooks, 2004), death of the tree leads to the loss of the hemlock and conversion of the forest from a dense and dark conifer-dominated forest to hardwoods dominated by black birch Betula lenta with red maple Acer rubrum and oaks - a radical change with many ecological consequences (Kizlinski et al, 2002). As with Dutch elm disease, hemlocks were devastated around 5000 years ago, taking 1000-1500 years to recover.

The hemlock woolly adelgid is currently found in 15 states along the eastern seaboard (Fig. 5.8b), covering around half of the native range of the hemlocks. The pest is spreading at the rate of 12-20 miles (20-30 km) each year, blown by the wind and easily carried by birds and animals. Added to this it has two

200 miles 200 km

Year of first infestation 1968-1984 1985-1990 1991-2002 Not infested

First infected 1951

Figure 5.8 (a) Masses of the hemlock woolly adelgid (HWA) on the underside of an eastern hemlock shoot, (b) infestations of the HWA in eastern USA between 1951 and 2002 (a: Photograph by Dr Mark McClure of the Connecticut Agricultural Experiment Station [Retired]; b: redrawn from data of USDA Forest Service, Northeastern Research Station.)

200 miles 200 km

Year of first infestation 1968-1984 1985-1990 1991-2002 Not infested

Native range of hemlock

First infected 1951

Figure 5.8 (a) Masses of the hemlock woolly adelgid (HWA) on the underside of an eastern hemlock shoot, (b) infestations of the HWA in eastern USA between 1951 and 2002 (a: Photograph by Dr Mark McClure of the Connecticut Agricultural Experiment Station [Retired]; b: redrawn from data of USDA Forest Service, Northeastern Research Station.)

generations a year and they reproduce parthenogenetically (without the need for males), so every new individual is female and capable of laying up to 300 eggs twice a year. In the short term, control is problematical. It is difficult to spot in the tree until it is well established; insecticides and other forms of sanitary control can be used on individual trees but are not practical on whole forest stands. Trees in wet areas usually survive for longer so irrigation during dry spells may help, but again this is difficult on a large scale. The main hope is biological control; a number of insects are possible contenders, the most hopeful being a ladybird (ladybug) beetle native to Japan, Pseudoscymnus tsugae. This feeds almost exclusively on adelgids and millions of these beetles have already been released in 15 eastern states in an effort to control the hemlock woolly adelgid.

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