Of the 16 important spring and summer weed hosts of the beet leafhopper listed for areas 1, 2, and 3, only filaree and Russian-thistle are introduced species; in areas 4, 5, and 6, only green tansymustard is a native. The others are introduced plants that have become established on abandoned, waste, and deteriorated range lands.

Evidently the geographical distribution of the beet leafhopper was once confined to the warmer arid regions of the southwestern United States but has spread northward as its introduced weed hosts became established. It is of interest to note that the first curly top outbreak in California occurred after Russian-thistle had become established in the State. Later investigations have shown that the outbreaks are influenced by the acreage of Russian-thistle in the San Joaquin Valley of California, Snake River Plains of Idaho, the central Columbia River area of Oregon and Washington, and to a lesser degree in Arizona, Colorado, New Mexico, Texas, and Utah.

After fields are abandoned or range is overgrazed, a succession of plants takes over. First come annual weeds, which have little forage value, followed by some annual grasses. Then, if no further disturbance occurs, this condition is followed by native perennial grasses and shrubs. The order of these changes in southern Idaho is: First, Russian-thistle; second, mustards; and third, downy chess, an introduced annual grass that is not a host of the beet leafhopper. Our observations indicate that range fires generally have their origin in areas where downy chess forms the plant cover or where it has entered deteriorated sagebrush areas to such an extent that it will carry fires. During the fire season, it is the greatest range fire hazard in the Intermountain Region; it will burn like tinder. If it is burned under favorable conditions, it may reseed itself and again form the cover, but under unfavorable conditions, such as wind erosion and trampling by livestock, mustards, principally tumblemustard, and Russian-thistle may appear. The shift from mustards and Russian-thistle to downy chess and then back to these weeds may continue in an endless cycle. On burned areas, mustards are generally first and then downy chess, but with further disturbance either Russian-thistle appears or the ground may become bare. Mixed stands of Russian-thistle and mustards are the most important combination of weed hosts for leafhopper reproduction, since the leafhoppers can over-winter and reproduce their spring and summer generations in the same area.

THROUGHOUT THEIR GEOGRAPHICAL range, many leafhoppers are destroyed each year by parasites and predators, which attack the leafhopper in all its stages and are evidently a factor in reducing the population in the breeding grounds. Large numbers of eggs are destroyed by minute parasitic wasps that develop within them. The nymphs and adults of the beet leafhopper are attacked by three groups of internal parasites the big-eyed flies (Dorilaidae), the parasitic wasps (Dryinidae), and the twisted-winged parasites (Strepsiptera). The flies and wasps deposit their eggs in or on the leafhopper, and the resulting larvae develop within or partly within its body. Upon reaching maturity, the larvae work out of the leafhopper, causing its death. The twisted-winged parasites develop differently, as the female remains within the body of the leafhopper during its entire life and gives birth to living young. These tiny larvae crawl away, attach themselves to the first leafhopper they find, and bore into its body. They seldom become of any real importance, because the chances of their finding leafhoppers in which to develop are slight. The tendency of the beet leafhopper to move about from host to host during the season apparently reduces the effect of these parasites.

Several species of predacious bugs, one being a big-eyed bug (Geocoris pallens), destroy large numbers of leafhoppers by sucking out their body juices. Spiders, lizards, and birds reduce the number of the insects. The grazing of livestock, principally sheep, destroys many eggs in the plants which they eat.

Reducing curly top infection in susceptible crops by controlling the leafhopper with insecticides is a difficult problem, because continuous infection of the crops occurs by reinfestation during the susceptible period.

Applications of DDT will reduce beet leafhopper numbers and have a good residual toxicity, but it will not prevent the feeding of all leafhoppers that reinfest the fields. In instances where reinfestation occurs over a 2 to 3-week period, applications of DDT and other insecticides to tomato plants have not reduced the incidence of curly top. Double-hill planting of tomatoes has given limited protection against curly top under moderate to high infestations of the leafhopper in Utah. In other instances where reinfestations do not usually occur, such as in fields of sugar-beet seed, a single application of DDT in the fall has effectively reduced curly top.

Other control methods have been developed. First, the major host plants of this insect were determined and methods for their replacement by non-host perennial grasses were studied. The replacement of weed hosts by perennial grasses may best be accomplished by reseeding the abandoned and burned areas. If native perennial grasses are still present, protection against excessive grazing will accomplish the same purpose. Because the perennial grasses remain green until late in the season, they do not constitute a fire hazard when compared with downy chess.

The second method is the chemical control of the beet leafhopper in weed-host areas.

J. R. DOUGLASS, a South Carolinian, obtained his entomological training in Clemson Agricultural College, Kansas State Agricultural College, Cornell University, and Ohio State University. He has been with the Department of Agriculture since 1921. From 1923 to 1934 he was in charge of the Estancia, N. Mex., laboratory, investigating insects affecting beans in the Southwest, and since 1935 he has been in charge of the Twin Falls, Idaho, laboratory.

WILLIAM C. COOK is a graduate of Cornell University and has a doctor's degree from the University of Minnesota. He has been an entomologist in the Bureau of Entomology and Plant Quarantine since 1930. Before that time he was employed by the Montana Agricultural Experiment Station for work on cutworms, especially the pale western cutworm. He served from 1930 to 1943 in California, studying the beet leafhopper. Since 1943 he has been stationed at Walla Walla, Wash., studying wireworms and the pea aphid.