Weekly spraying for aphid control reduced natural infection by yellows viruses from 50% to 14% in sugar beets planted April 10 and increased sugar yield 30% by mid-October. While spraying this often is not an economical means of control for virus yellows, the experiment showed that a considerable reduction in yields can be caused by naturally occurring strains of yellows viruses. However, sugar beets planted May 7 remained essentially virus free and by October 22 yielded 10% more sugar than beets planted in April, half of which became diseased.