Circulative and Propagative Virus Transmission by Aphids

The study of the transmission of aphid-borne plant viruses that are circula­ tive in their vectors began with a report by McClintock & Smith in 1 918 on spinach blight ( 113), the causative virus of which is still unknown. Since these early beginnings, the study of aphid vector-virus relationships has continued unabated. Among all kinds of organisms, varying from fungal zoospores, nematodes and mites to both mandibulate and suctorial insects, aphids have the great­ est range of nontrivial vector-virus relationships. A brief survey of the four main types of relationships that dominate the present thinking of research workers in the vector field is as follows. The more transient of the relationships is the nonpersistent, stylet­ associated (some believe regurgitative) pattern where transmission can be effected in less than a minute. Full inoculative capacity, although fully realized in a single naturally interrupted probe of only 15-20 sec in dura­ tion, is rapidly lost. The rate of loss is greater during feeding than during fasting. Using the nonpersistent relationship as a model, the other relation­ ships can be characterized, in part, by an increase in the time needed to acquire virus and to develop full transmission efficiency, and an increase in the time that the aphids will continue to transmit without again feeding on a virus source. In the semi persistent relationship, maximum inoculative capacity occurs at the end of the acquisition access period. The rate of transmission de­ creases with time, independent of whether the aphid is feeding or fasting.