Populations of the wingless form of Myzus persicae (Sulz.), infected with severe yellows (SBYV), mild yellows (SBMYV), and both viruses, were introduced to single sugar-beet plants which were growing in a normal crop.

The aphids and their progeny walked readily from plant to plant, both along and across the rows, during the following eight weeks. Although they were heavily attacked by ladybirds, their mobility saved the population from total destruction. They carried and transmitted the viruses; within a month patches of sugar beet more than 11 feet in diameter had become infected, and within two months the diameter had extended to about 20 feet.

SBMYV spread to more than twice as many plants as SBYV. When introduced alone, either virus spread to twice as many plants as when the two viruses were introduced together. The rate of increase of the infection fell off during the season; it was estimated as eight- to sixteen-fold per month in June—July, about three-fold in August, and 1·5- to 1·9-fold in September. Natural infections-appear to behave similarly.

Apterae of M. persicae are considered to be predominantly responsible for yellows virus transmission within the crop, and their mobility, in contrast to the immobility of those of A. fabae Scop., is considered to be the principal reason why M. persicae is a much more important vector than A. fabae.

The extent of spread of virus by apterae shown to occur in the root crop must inevitably be much greater in the seed crop, in which aphid populations are much higher. This consideration serves to explain why existing control measures in the seed crop do not protect the root crop from infection. The risk from the seed crop, which is the reservoir of the disease, would be countered if seed crops were grown only in alternate years.