WEED control: applied botany 407 



thousands of acres. During this war sugar-beet growing developed rapidly in 

 California and the industry held great promise. Then suddenly, about 1925, 

 the curly top virus carried by the beet leafhopper Eutettix tenellus broke forth 

 in unprecedented violence and threatened to destroy the industry. Why did 

 this occur? 



Before the war, curly top had been a disease of little consequence, injuring 

 relatively few beets because natural circumstances provided an effective 

 control. After beet harvest the leafhoppers were subject to severe decimation 

 by hot dry weather and a lack of succulent feed. There was an obvious gap 

 in their food cycle. But after the war, with Russian thistle widely disseminated 

 throughout the major beet-growing areas, the hoppers migrated from beets 

 to thistle to green winter forage to beets. The cycle was completed, and leaf- 

 hopper populations increased by leaps and bounds; within a period of about 

 five years the beet-growing industry was almost eliminated. 



By a three-pronged attack of plant breeding for resistance, insect control, 

 and thistle control, the sugar-beet industry was able to make a recovery; 

 however, the same virus from the same sources has seriously affected tomato 

 growing in California. And direct crop damage from plant competition by 

 Russian thistle has caused no small loss to western agriculture. 



St. Johnswort is a rather insignificant member of the plant population of 

 Western European countries, including the British Isles. Early introduced 

 into the United States, it created no problem along the eastern seaboard. Its 

 introduction on the west coast is obscure, but in the early years of the present 

 century it began to take over large areas of excellent range land in the north- 

 coast counties of California. It also spread in the foothill ranges of Oregon, 

 Washington, and Idaho. Introduced free of its natural predators, it thrived 

 prodigiously and became dominant on many hundred thousand acres of 

 productive range land. 



Meanwhile, St. Johnswort had claimed similar large areas of range in 

 Australia, and, having successfully controlled prickly pear with insect preda- 

 tors, around 1940 the biological-control people introduced from France a 

 number of beetles known to have a taste for this weed. By 1943 these preda- 

 tors had become established in Australia (Wilson, 1943), and in the following 

 years importations were made into CaHfornia. Tests were completed, and the 

 beetles were first released in this state late in 1945. Additional releases were 

 made in 1946 and 1947. Within ten years, the major St. Johnswort-infested 

 areas of Humboldt County, California, have been released, infestations along 

 the Sierras are being suppressed, and excellent control is reported from the 

 other states in the Northwest. Meanwhile, work has been started toward 

 biological control of gorse, and insect predators are being sought for yellow 

 star and Italian thistles. 



Halogeton glomeratiis is an annual of the family Chenopodiaceae and 

 resembles Russian thistle in appearance. It was first found in this country 



