The spread of tomato yellow leaf curl virus (TYLCV) is significantly correlated with the population size of its vector,Bemisia tabaci Genn. The perennial weedCynanchum acutum L. and the annual weedMalva parviflora L. were found to be natural hosts of TYLCV in the Jordan Valley.C. acutum is not a preferred host forB. tabaci, but the whitefly feeds on it sufficiently long to acquire the virus. Whiteflies marked with fluorescent dust while feeding naturally onC. acutum along the banks of the Jordan River, were subsequently trapped within the main tomato-production area 7 km away. An increase in theB. tabaci population and in TYLCV infectivity was found in plots surrounded by windbreaks. The epidemiological cycle of TYLCV is described and cultural control measures are suggested.