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

The causes of plant disorders such as squash silverleaf and tomato irr egular ripening associated with feeding of the whitefly, Bemisia tabac i (B-biotype), are not well understood. Whether immatures, adults, or both cause symptoms is important to management because it determines t he target of control and selection of appropriate management strategie s. We measured the relative ability of adult and immature whiteflies t o induce symptoms of squash silverleaf in zucchini. Plants exposed to 20 adult whiteflies for 48 h and subsequently dipped in insecticidal s oap 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 sig nificant regression was found (SYMPTOM = 1.37 + 0.064 IMMATURE, R2 = 0 .68) indicating a positive relationship between the severity of silver leaf and the number of immatures present on a plant. In no case were s ilverleaf symptoms present unless immatures were also present on the p lants. When this experiment was repeated under more stringent conditio ns in which plants were exposed to 20 adults per day for each of 10 co nsecutive days, but not to immatures, plants did not develop silverlea f symptoms. These findings support previous hypotheses that adult B. t abaci do not contribute significantly to induction of squash silverlea f and directly contrast recent suggestions that adult B. tabaci at rel atively low levels can induce silverleaf symptoms.