Nucleotide sequence comparisons suggest that curtoviruses and begomoviruses diverged after a recombination event altered insect vector specificity (Rybicki 1994). Vector Behavior and Disease Epidemiology Before the development of commercialized agriculture in the western United States, the native flora did not support large, economically damaging populations of the beet leafhopper. Conditions favoring the insect modified this dynamic with the changes in agriculture, including sporadic, intermittent farming and overgrazing of rangeland. This series of factors resulted in the disappearance of previously stable plant populations that were replaced by various winter and summer annual weed species. The weed hosts that succeeded the original plant species included mustards (Brassica spp.), Russian thistle (Salsola tragus), filaree (Erodium spp.), and Plantago spp. These widespread weeds became the preferred leafhopper hosts resulting in more frequent and severe epidemics. In the fall as crops senesce, the leafhopper vector moves to breeding areas in the foothills away from cultivated fields, overwintering on various biennials and perennials. Winter rainfall results in the germination and emergence of several annuals that serve as sites of leafhopper reproduction. The leafhopper moves to these annual plant species and lays eggs. In spring as plants in breeding areas dry, the leafhopper moves back to agricultural lands. The severity of infection depends on climatic factors affecting the presence of weed hosts, the prevalence and severity of the pathogen, and the reproductive capacity and migration of leafhoppers. Host Range Curly top virus, have a very wide host range within dicot plants, including over 300 species in 44 plant families (Strausbaugh et al., 2008). The most common infected hosts include sugar beets (for which the disease was first named), follow by tomatoes, peppers, beans, potatoes, spinach, cucurbits, cabbage, alfalfa, and many ornamentals. The virus seems to be restricted to trees with broad leaves, because no single-leafed plants were identified as the host of this virus. Transmission The usual agent is Beet Curly Top Virus (BCTV). BCTV is transmitted to from plant to plant by the beet leafhopper (Circulifer tenellus). Both the virus and the beet leafhopper have very wide host ranges. Once acquired by the leafhopper, BCTV is carried for the rest of the leafhopper's life, so long-distance spread is common. Infected plants will spread all over the field. The beet leafhopper acquires the virus from infected crop plants or weeds such as wild mustards and Russian thistle. Beet leafhopper only need few minutes in feeding period to