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Hemlock Woolly Adelgid and Its Effect on Campus Trees by Bill Belden
Tsuga Canadensis
    Hemlock woolly adelgid, Adelges tsugae, has become a problem insect on campus, attacking several species of Tsuga, particularly T. canadensis. The woolly adelgid, a native of Asia, was introduced in North America in the 1920s. From northern California to southeastern Alaska it seems to be relatively innocuous on western hemlock, T. heterophylla, and mountain hemlock, T. mertensiana. In contrast, eastern North America has seen significant damage to forests and plantings of eastern hemlock and Carolina hemlock from Virginia (where the pest was first discovered 40 years ago) to New England.
    We have undertaken an aggressive approach to maintaining control of this aphidlike insect. Not only can we act as a regional model on how to maintain control of this pest, but we have a particular responsibility because of our proximity to native stands of T. canadensis, where there is currently no effective method of control. The hemlock woolly adelgid is hardly visible to the naked eye, "about the size of a period on this printed page," according to Dr. Mark S. McClure of the Connecticut Agricultural Experiment Station. One of the signs of infestation is the presence of a dry, white woolly substance, most noticeably in the axils of needles on young twigs. According to McClure, "an egg mass resembles the tip of a cotton swab, although somewhat smaller." Keep that magnifying lens handy!
    Several methods to consider for prevention and/or control of the adelgid include improving tree health, mechanically removing adelgids, planting resistant species, and using pesticides. Watering a hemlock tree in periods of drought; pruning infested, dead, and dying branches; and "intentionally dislodging eggs and crawlers by directing a strong stream of water at infested branches," according to McClure, may be of some value in helping to preserve a specimen. Generally, however, pesticide use is required, and there are two methods of control: horticultural oils and
  systemic insecticides. Horticultural oils are sprayed on the plant to suffocate the insect. Systemic insecticides are taken up by the tree and ingested by the adelgid, poisoning the insect but not the tree. In deciding to spray oils, the size and location of the specimen must be considered, as 100% coverage of the foliage and stems is necessary. Whatever the program, treatment must be consistent and continue until the danger has passed.
    In the fall of 1997, Kim Tripp and I recorded the condition of all the hemlocks on campus. We looked at the importance of the plant to our collection, if it was infected, if it was of unusual size, and its location. This information helped us decide on a treatment program for each plant. We grouped all plants into eight categories: (1) irreparably infested/remove (including all hemlock hedges), (2) severely infested unimportant specimen/remove, (3) severely infested important specimen/treat systemically, (4) moderately infested unimportant specimen/remove, (5) moderately infested important specimen/treat with horticultural oil, (6) lightly infested important specimen/treat with horticultural oil, (7) lightly infested unimportant specimen/remove, and (8) uninfested/treat with horticultural oil, monitor twice annually and reassess treatment.
    Into the first year of this program, we are looking for ways to improve it. Ideally all infested tissue should be burned following removal, but this is rarely possible. As a result, we are investigating alternative methods of dealing with infested chips and brush such as taking all foliage and chips to be immediately and deeply buried in the landfill. We have sprayed with horticultural oil and continue to monitor to see if we have to make a second application using summer oil, as there are two generations per year of the woolly adelgid.
    In developing our program, we found the pest on the T. caroliniana next to Park House and the T. canadensis 'Pendula' in front of Wilson House. Because of their proximity to the dorms, these were not good candidates for oil spraying, and we decided on injecting these plants using the Mauget program with the systemic insecticide Imicide (Merit). For those of you unfamiliar with the Mauget technique, think of it as giving medicine to the tree intravenously. This program is environmentally friendly because the tree takes up all the pesticide. A small hole is drilled in the top of the root flare (ideally, holes are drilled every six inches), an injector tube is inserted, and the capsule installed on the tube. Uptake is quick; sometimes the first capsule is emptied by the time you finish installing the last capsule. To date, injected trees show good control of the adelgid and will be monitored very carefully. With luck we will be able to get a year's control from this one injection.
    Some of the trees slated for removal were taken down this past summer. Many people in the industry see the devastation caused by this pest as analogous to that of Dutch elm disease. We will continue to do our part to help preserve the hemlocks at the Botanic Garden so that future generations can enjoy these plants. decoration
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