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Collections of Bemisia tabaci from California desert regions have been shown to be a mixture of biotypes. These whitefly biotypes differ in a number of ways including their ability to induce silverleaf of squash. The physiological differences of the newly found whitefly biotype, including host preference, larval development, transmission of lettuce infectious yellows virus, and the induction of silverleaf symptoms, clearly distinguish it from the previously occurring biotype. Silverleaf of squash was induced by nymphal feeding activity-, however, the physiological condition of the host as influenced by light intensity, quality, and duration are important factors in silverleaf expression. Differences between the whitefly biotypes in induction of silverleaf are quantitative and qualitative. Double-stranded RNA bands were not detected from nymph-infested leaves or from silverleaf symptomatic tissue, suggesting that whitefly-induced silverleaf in California is similar to a systemic phytotoxemia.