The Effect of the Hemlock Woolly Adelgid
The Hemlock Woolly Adelgid is an insect that has invaded much of eastern North America. It is a native of Japan or possibly China; however the way in which this species was introduced to the United States is not clear (McClure et al, 2001). In the northeastern United States, this insect feeds mostly on the eastern hemlock (T. Canadensis) tree. These insects feed at the base of the needles of the hemlock and they are able to penetrate into the parenchyma cells of the xylem rays. Parenchyma cells are cells that transfer and store nutrients for the plant (McClure et al, 2001). This type of feeding causes the needles to desiccate very quickly, sometimes within the first few months of infestation. Additionally, many of the needle buds are destroyed as well, which prevents any new tree growth. In heavy infestations of the hemlock woolly adelgid, hemlock trees may die after only four years of infestation. In smaller infestations, however, sometimes the tree may survive for much longer and could even recover (McClure et al, 2001). |
This species of tree is very important to its ecosystems because it provides cover for many land animals such as deer, turkeys, and rabbits (McClure et al, 2001). Additionally, many birds use the eastern hemlock as a food source. For instance, in Connecticut, 90 different species of birds use the hemlock as a food source (McClure et al, 2001). Eastern hemlocks are not only important land animals, but fish as well. For instance, brook trout are found frequently in streams that are shaded by hemlocks (McClure et al, 2001). Hemlocks have also been a valuable resource for humans as well. Although it is not generally used for timber, it has frequently been used for building small structures such as barns and sheds (McClure et al, 2001). |
(http://www.arboretum.harvard.edu/research/images/adelgid.jpg)
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Hemlock Woolly Adelgid infestations have been hard to prevent, mostly because there are no natural predators of this insect in North America. However, since 1992, there has been a push to import natural predators of this insect. For instance, the lady beetle (Pseudoscymnus tsugae Sasaji) is one of the natural predators of the hemlock woolly adelgid in Japan. In recent years, it has been released into these environments and has shown promise in curbing some of these infestations (McClure et al, 2001). However, no one can be sure that the introduction of the lady beetle will be a long term solution for the problem. Additionally, no one can be sure that the lady beetles will not also affect the balance of these ecosystems. |
