Early fall migrations of beet leafhoppers into the fields of seed beets cause greater damage than later migrations, because smaller plants are more susceptible to the disease than larger plants and fields of small plants are more attractive to the insects.

The beet leafhopper prefers an open, sunny environment, and as the soil surface becomes shaded with the beet foliage, the field is less attractive to the insects. Usually fewer than 10 percent of the leafhoppers from the southern Arizona breeding areas are curly top infective, but if they stay in the field the virus is rapidly passed through the beet plants to the non-infective insects, and the disease spreads rapidly.

Since fall migrations of beet leafhoppers are of comparatively short duration and growth of the plants is rapid, control is necessary only for 3 or 4 weeks. DDT dusts or sprays are effective, but because there is too little foliage to hold the insecticide, control is sometimes difficult on young plants, which are most susceptible to curly top.

Here the new systemic insecticides, such as phorate or Di-Syston, which are translocated within the plant, are of special value. By treating the seed with them, the cotyledons and first pair of leaves of the sugarbeet plants become so toxic to the leafhoppers that one feeding will kill them and protection is afforded until the plants reach the four-leaf stage, when there is enough foliage to hold a DDT dust or spray.

VIRUS YELLOWS, or beet yellows, is caused by the aphid borne beet yellows virus. It generally is considered non-persistent. The ability to transmit usually does not last more than 1 or 2 days.

The disease has been recognized for many years in Europe, but it was not reported in the United States until 1951, and was first discovered in sugarbeets grown for seed in Arizona in 1955.

The appearance of the disease was accompanied by losses in yield of seed. Studies to determine more definitely its effect on the crop and possibilities of treatment were commenced in 1956.

The green peach aphid (Myzus persicae) was known to be an efficient vector of beet yellows and was also recognized as a pest of the sugarbeet seed crop in Arizona. Before the appearance of virus yellows, feeding damage alone was negligible unless the insect became exceedingly numerous. Flight trap records as well as population studies and observations on known host plants showed that few of the insects survived the southern Arizona summers. The first winged forms appear in late September and early October. Flights continue in low numbers throughout the winter, but wingless forms constantly increase in the beet fields. Many winged forms develop in the fields by March, and large numbers are flying in March and April.

Knowledge of reservoirs of the beet yellows virus is incomplete, but because the aphids do not usually retain the virus more than 48 hours, the source must be within a 2-day flight range of the insect and probably the more important reservoirs are local. Beets that have escaped cultivation or fields not plowed up after seed harvest are likely sources. The virus also has been recovered in a low percentage of trials from Australian saltbush along ditch banks within the cultivated area.

It appears that a low initial infection occurs with the influx of aphids in October, November, and December. Virus is spread from them by the large numbers of winged forms during late February, March, and April. The buildup of wingless forms in winter might be relatively unimportant if they are controlled before the winged forms appear in early spring.

Experiments were made in which plots were inoculated with virus yellows by artificial infestations of infective green peach aphids on different dates. They showed that the greatest reductions in yield resulted from early inoculations. Later inoculations gave progressively less damage. Infestations with infective aphids when the plants were blooming also resulted in a lower percentage of germinating seed.

MANY CEREAL crops are affected by virus diseases.

A leafhopper (Endria inimica) transmits a persistent virus that causes a disease of wheat called striate mosaic because of the chlorotic dashes or streaks that occur along veins of infected leaves in the early stages.

Wheat striate mosaic virus was first found on wheat in South Dakota in 1950.

No serious losses are known to be a result of striate mosaic, but the disease is severe on some wheat varieties and is considered to be of potential importance. The virus has been transmitted by means of the leafhopper to oats, barley, and some native grasses.

Wheat striate mosaic virus has a latent period in its vector. Ten to 14 days elapse between the time the leafhopper begins to feed on an infected plant and the time it can transmit virus to a healthy plant.

Leafhoppers then may continue to transmit the virus for a month or two. This long period of persistence of the virus in its vector means that leafhoppers could acquire virus from infected plants in the summer and carry the virus over directly to fall-sown wheat. Since infected plants can survive the winter, the disease cycle can be continued during the next growing season. Another virus disease of wheat, wheat streak mosaic, is transmitted not by an insect but by the wheat curl mite (Aceria tulipae). This disease was known for some time. The vector was discovered in 1953. It was an important discovery, as mites were not generally recognized as vectors.

The wheat streak mosaic disease is particularly severe in Kansas and other Midwestern States, although the virus exists in other localities in the United States and Canada. Leaves of infected wheat plants develop yellow streaks, the plants wilt quickly, and the yield is much less than normal.

Conditions that existed in Kansas in 1958 illustrate the factors that are important in a severe outbreak of the disease. The cool, wet summer of that year encouraged growth of volunteer grain and also favored multiplication of the mites that live on volunteer grain as well as on native grasses. Because many of the grain plants and the grasses were infected with the wheat streak mosaic virus, it was picked up by great numbers of the mites. When the winter wheat began to grow early in the fall, the mites, which were blown about by the wind, transmitted virus to many of the young wheat plants. So many plants were weakened by infection that the wheat crop in Kansas in 1958-1959 was reduced by more than an estimated 46 million bushels.