ASSOCIATION BETWEEN BEMISIA-TABACI DENSITY AND REDUCED GROWTH, YELLOWING, AND STEM BLANCHING OF LETTUCE AND KAI CHOY

Colonization by high densities of sweetpotato whitefly, Bemisia tabaci, B-biotype, reduced growth of lettuce (Lactuca sativa) and kai choy (Brassica campestris). A 41% reduction in the weight of lettuce plants resulted when they were colonized by 200 adult whiteflies and their offspring for 3 wk. The weight of kai choy plants was reduced 14 and 21% when they were colonized by 100 and 200 adult whiteflies and their offspring, respectively, for 3 wk. In contrast, colonization by 10, 50, and 100 whiteflies and their nymphs per lettuce plant, or 10 and 50 whiteflies plus their nymphs per kai choy plant, did not reduce growth significantly. Exposure to 50 or more whiteflies plus nymphs caused leaf yellowing and distortion in lettuce, while exposure to 10 or more whiteflies plus nymphs caused stem and midvein blanching and leaf curling in kai choy. Weight loss increased as immature density increased on both plants. Exposure to 200 adults for 48 hr with no nymphs did not induce weight loss or symptom development in kai choy. If immature offspring were allowed to feed, however, symptoms developed in all cases. Removal of immatures from symptomatic plants with insecticide treatment resulted in nonsymptomatic new growth. The increase in symptoms with increasing numbers of whiteflies and the recovery of plants following whitefly removal support the hypothesis that these syndromes are phytotoxic disorders caused by a toxin or toxins and not by a pathogen. The importance of this information to growers and to sweetpotato whitefly management is discussed.