410 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 15 



field, in the southern part of the valley has 5.39 inches. Table 4, gives 

 the annual rainfall of towns on the western and eastern sides of the 



valley : 



Table 4. Average Rainfall from South to North in San Joaquin Valley 



Fog. — Fog is common during the winter months, but decreases in 

 density and frequency of occurrence northward in the Sacramento 

 Valley. In the southern part of the valley, fog is dense during the 

 night and morning, but frequently disappears or lifts during the day, 

 though sometimes continuing as a high fog for several days. The 

 lower lying parts of the valley are sometimes subject to light fogs in the 

 autumn and spring, when other portions are free from it. 



Fog probably delays the spring migration of the beet leafhopper into 

 the Sacramento Valley. During foggy days the bugs are sluggish and 

 inactive, and when fog occurs before sunset no activity is displayed by 

 the adults. 



Dew. — An unfavorable factor to the overwintering beet leafhopper 

 may be the heavy dew which occurs during the rainy period in the 

 Sacramento Valley. 



Sunshine. — The Sacramento and northern part of the San Joaquin 

 Valleys have a lower percentage of sunshine than the middle and south- 

 em San Joaquin Valley. 



Temperature. — The foothill slopes up to about 1,200 feet elevation 

 in the Sacramento Valley have a minimum temperature during the 

 winter months of 1° to 5° higher than that over the flatter and lower 

 part of the valley floor. The difference in temperature between the 

 sloping positions and the valley floor ranges from about 5° to 8° in the 

 upper San Joaquin Valley. 



VIII. Barriers 

 In all probability the exterminating factor of the overwintering beet 

 leafhopper in the Sacramento Valley is htmiidity. The rainfall in this 

 valley varies from 19.28 to 27.75 inches. The hot dry stmimers in the 



