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In 1991, a whitefly infestation of unprecedented severity resulted in serious losses of a wide range of food and fiber crops across the southern United States (Perring et al. 1991). Initial indications were that the outbreak consisted of the “type B” or poinsettia strain of the sweetpotato whitefly, Bemisia tabaci (Gennadius). In Canada, B. tabaci was first collected from greenhouse poinsettias in 1988 (Broadbent et al. 1989) and was subsequently found in 18 of 3 1 Ontario greenhouses sampled.