SQUASH SILVERLEAF SYMPTOMS INDUCED BY IMMATURE, BUT NOT ADULT, BEMISIA-TABACI

The causes of plant disorders such as squash silverleaf and tomato irregular ripening associated with feeding of the whitefly, Bemisia tabaci (B-biotype), are not well understood. Whether immatures, adults, or both cause symptoms is important to management because it determines the target of control and selection of appropriate management strategies. We measured the relative ability of adult and immature whiteflies to induce symptoms of squash silverleaf in zucchini. Plants exposed to 20 adult whiteflies for 48 h and subsequently dipped in insecticidal soap to remove immature offspring did not develop silverleaf symptoms. In contrast, all plants exposed to adults, but on which immatures were allowed to develop, showed symptoms of silverleaf. Furthermore, a significant regression was found (SYMPTOM = 1.37 + 0.064 IMMATURE, R2 = 0.68) indicating a positive relationship between the severity of silverleaf and the number of immatures present on a plant. In no case were silverleaf symptoms present unless immatures were also present on the plants. When this experiment was repeated under more stringent conditions in which plants were exposed to 20 adults per day for each of 10 consecutive days, but not to immatures, plants did not develop silverleaf symptoms. These findings support previous hypotheses that adult B. tabaci do not contribute significantly to induction of squash silverleaf and directly contrast recent suggestions that adult B. tabaci at relatively low levels can induce silverleaf symptoms.