The toxicity of neem extracts that induce abnormality and lethality is strongly enhanced by the ingestion rather than surface contact. Oral toxicity of neem extracts was analyzed using a plant sapping pest Bemisia tabaci. When adults were allowed to ingest various doses (0, 5, 10, and 50 ppm) of azadirachtin diluted in 20% sugar solution in a double-layered parafilm chamber for 72 h, mortality was 12.5, 77.5, 97.5, 100%, respectively. When a cucumber leaf which has many nymphs were submerged into the solution of neem extract, adult eclosion was decreased to 87.0, 16.6, 16.1, and 6.6%, respectively. Further, when neem extracts were irrigated into the tomato cultivar the average number of nymphs per leaf was decreased to 60.3% in comparison with the untreated control. These result suggest that systemic treatment of neem extracts was effective for the control of whiteflies as well as other sucking insect pests.