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Western Poison-ivy (Toxicodendron rydbergii) is widespread in sunny, open habitats in the western United States (but apparently is not known from California) and its range extends eastward in the northern part of the United States and adjacent Canada south to New York (and, at higher elevations, to Virginia and West Virginia). Poison-ivy and its close relatives are well-known for possessing skin-irritating oil (urushiol), which can cause severe allergic reactions in humans.
The taxonomy and nomenclature of North American Toxicodendron has been in flux for over a century, largely due to confusing within-species variation in growth form, leaf and leaflet shape, and other features (e.g., Gillis 1971; Gartner 1991). This has resulted in an abundance of synonyms, but five species are now generally recognized: Common Poison-ivy (T. radicans), Western Poison-ivy (T. rydbergii), Eastern Poison-oak (T. pubescens), Western Poison-oak (T. diversilobum), and Poison-sumac (T. vernix) (Senchina 2006). Unlike Eastern Poison-ivy, Western Poison-ivy does not climb, instead growing as a suberect shrub typically less than 1 m tall (although Eastern and Western Poison-ivy may intergrade to some extent, making identification less clear in some areas) (Gleason and Cronquist 1991). The fruits and the petioles (leaf stalks) of the 3-leafleted leaves are glabrous (smooth, without hairs or glands) and leaflets and teeth are more or less pointed. (Gleason and Cronquist 1991).
Senchina (2008) reviewed the literature on animal and fungal associates of Toxicodendron in North America with a particular eye toward identifying potential biological control agents. Interest in finding new ways to control poison-ivy and its relatives may increase in coming years given data suggesting that these plants may become more abundant and more ‘‘toxic’’ in the future, potentially affecting global forest dynamics and human health (Mohan et al. 2006).