ABSTRACT
Most viruses are vector-borne and are thus spread through their vector’s dispersal, movement and migration. Th e movement of vectors is normally an active process, such as crawling, walking or fl ying, but can also be a passive process when they are carried by wind or moved on plants or in soil. Active movement is a part of the vector’s biology, but passive movement is an incidental process. Vectors have to move in search of food, a change of habitat, place for reproduction and to overcome adverse conditions. A few vectors may crawl in the fi eld, for example aphids (Alykhin and Sewell 2003), but many virus vectors have special adaptations to aid dispersal, the most obvious of which are wings and they undergo morphological and physiological changes to adapt themselves for fl ying. Th e fl ying process also depends upon the physiology of the host, climatic and meteorological conditions. Normally, dispersal of vectors is over relatively short distances, from plant to plant or fi eld-to-fi eld, and possibly habitat-to-habitat, and covers a distance of a few kilometers at the most and mainly within the surface boundary layers. However, movement may also be over much longer distances especially when the insect vectors are under the infl uence of atmospheric processes above the boundary layer. Vectors may migrate from one climatic region to another by their own fl ight activity and by wind fi eld systems of the upper atmosphere. Knowledge of these aspects of vector dispersal is essential in planning and developing strategic operational tactics in the fi elds to prevent or control the spread of vector-borne virus diseases. Flying is the most common method for dispersal. Insects can fl y at all times of the day and night depending upon the species. A few species only fl y during the day. Aphid fl ight is normally restricted to the daylight hours and by light and temperature thresholds. Many other small, wind-borne migrant pests are also day fl yers. Others are nocturnal and some small bugs are crepuscular. Flight activity measures the dispersal of the fl ying vector insects.
