418 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 15 



occurred. When the pasture vegetation became dry on the hilltops 

 and floor of Little Panoche Valley, the nymphs and adults assembled on 

 Filaree growing in the drainage furrows and when this food supply 

 became dry the hoppers congregated on annual and perennial plants. 

 Mountain Passes.— An examination was made of other mountain 

 passes but up to the present time no enormous assemblage of beet 

 leafhoppers has been found. In Pacheco Pass, about 28 miles north of 

 Little Panoche Valley, the Red Stem Filaree was tall and dense. Spring 

 brood leafhoppers are rarely taken in tall pasture vegetation. In the 

 Coalinga-King City Pass, situated about 50 miles south of Little Panoche 

 Valley the Red Stem Filaree was short and resembled somewhat the 

 condition on the Panoche Hills, but no large congregation of hoppers 

 were found. In the Altamont Pass, about 80 miles north of Little 

 Panoche Valley, the spring brood adults were rarely taken. 



It is evident that in canyons and mountain passes of the northern 

 part of the San Joaquin Valley, there are limiting factors which check the 

 multiplication of the beet leafhopper. We have no evidence to show 

 whether the eggs failed to hatch or the recently hatched nymphs suc- 

 cumbed on the foothills. At Manteca eggs deposited in the foliage of 

 sugar beets from November 1 to January 15, 1919 failed to hatch out- 

 of-doors. During the winter a high mortality of the nymphs occurred 

 which hatched frcm eggs deposited during September and October. 



The character of the Red Stem Filaree may be an indicator of favor- 

 able Eutettix foothill breeding grounds, but nevertheless, there may be 

 composite controlling factors which hold this insect in check in some of 

 the canyons and mountain passes of the northern part of the San 

 Joaquin Valley. Cold winds and fogs sweeping from San Francisco 

 Bay through the Altamont Pass, extending east and west through the 

 Coast Range may be a critical factor in reducing the nwcnber of recently 

 hatched nymphs. Cloudiness, rainfall, wet soil and dense pasture 

 vegetation may be other related factors affecting the recently hatched 

 nymphs. In years with an abundance of rainfall, fungus diseases may 

 reduce the number of dark overwintering forms. 



Let us now compare some of the apparently favorable factors of the 

 beet leafhopper on the Panoche Hills. Little Panoche Creek flows 

 south for about six miles from the simimit of the Coast Range and then 

 meanders southwest for another six miles to the entrance of the mount- 

 tain pass. It is evident that the slope exposure in Little Panoche 

 Valley is far more favorable than in a motintain pass extending east and 

 west through the Coast Range and with a break in the mountain range 

 at the coast. Hall (1, 2) has determined mathematically that "the 



