About 5000 species of aphid have been described and of these, some 450 species have colonised food and fibre crops. As direct feeders on plant sap, they damage crops and reduce yields, but they have a greater impact by being vectors of plant viruses.

The transmission of these viruses depends on the movements of aphids between different parts of a plant, between nearby plants and further afield. In this respect, the probing behaviour of an aphid tasting a host is more damaging than lengthy aphid feeding and reproduction by stay-put individuals. The movement of aphids influences the timing of virus epidemics.

More Info: en.wikipedia.org