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Disease Cycle and Epidemiology

Thrips deposit their eggs into plant tissue and the eggs hatch after 2-3 days, depending on the temperature and the plant host species (Figure 12). There are two feeding larval stages that are followed by two non-feeding pupal stages. The life cycle takes about 20-30 days from egg to adult, again depending on the temperature. Thrips disperse over long distances by wind. TSWV must be acquired by thrips during the larval stage of their development to be transmitted. Thus, only immature thrips that acquire TSWV, or adults derived from such immatures, transmit the virus. The ability of thrips to acquire TSWV decreases as the thrips age. Although the time in development that thrips can acquire the virus is limited, the wide host range for both virus and thrips facilitates development of epidemics. Once acquired by the larvae, the virus is passed transtadially, i.e. TSWV persists through insect molts from larval to adult stages. The virus replicates in thrips, and the thrips can transmit the virus during their entire life. There are at least ten species of thrips that transmit tospoviruses. Frankliniella occidentalis, the western flower thrips (WFT) (Figure 13), is considered to be the most important vector species because it is globally distributed and can transmit most tospoviruses.

Figure 12

Figure 13

The acquisition of TSWV by thrips larvae is an area of continuing study. Some evidence indicates that the viral glycoproteins (GPs) bind to the midgut epithelium and have a role in the process of virus uptake in the midgut (Figure 14). The virus then moves to other cells and organs, and becomes well established in the muscle cells (Figure 15). Another perspective is that the temporary association between the midgut, visceral muscle and salivary gland complex in the larval stage provides the avenue for the virus to become systemically established in the thrips. Eventually, the virus enters the salivary glands. Although the route to the salivary glands still needs to be determined, virions are excreted with the saliva (Figure 16) into host plants during thrips feeding.

Figure 14	Figure 15
Figure 16

The epidemiology of the diseases caused by TSWV remains poorly understood. It was only in 1993 that it was confirmed that TSWV multiplies in its vector. The wide host range of TSWV and the occurrence of at least ten species of thrips that transmit TSWV make eliminating the sources of primary inoculum of the virus impractical. Additionally, viruliferous thrips may overwinter in the field. The population of viruliferous thrips and the stage of crop development appear to be crucial in disease development. For lettuce grown in Hawaii, it was found that the earlier the crop is infected with the virus, the more severe the loss. An elegant model developed to demonstrate the potential economic impact of TSWV in lettuce helped growers determine if they should continue to grow the crop once virus infection was confirmed.