182 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



Many of the hoppers succumbed to long continued wet weather or 

 other unusual conditions in winter. After about two weeks of almost 

 continuous rain we have counted as many as 700 dead hoppers under 

 a single bunch of alfilaria. This mortality may be partly accounted for 

 because of unsuitable conditions for obtaining food, though the direct 

 effect of exposure to such conditions is probably the more important 

 factor. 



Relation of Food to Development and Activity. The activity of the 

 insects is dependent primarily upon food and temperature, and the 

 latter largely influences the former. No matter how much nutritious 

 food may be available, if the temperature is sufficiently low they become 

 dormant and are revived only upon the rise of temperature; but once 

 they become active through the influence of a higher temperature, they 

 require food to maintain their activity. 



Influence of Temperature upon Activity. On the warm days of 

 winter the hoppers are very active and fly up in large numbers before 

 a person as he walks through the vineyard. On the contrary, on a 

 cold or wet day they may only be disturbed by actually moving the 

 object upon which they are resting, and then they will fly but a very 

 short distance, not more than a foot or two generally. It is possible 

 during such days to pick up the leaves, with a dozen or more hoppers 

 resting on the under surface, and place them in a cyanide bottle without 

 disturbing them. They are most active during the warmer portions of 

 the day, from nine or ten o'clock in the morning, when the dew is 

 dried from the leaves, until three or four in the afternoon. 



Experiments carried on in the laboratory to determine the effect of 

 temperature on overwintering adults indicated that a temperature of 

 110 F. was nearly always fatal. These experiments consisted in con- 

 fining the hoppers in a double glass vial and heating gradually with 

 artificial heat. A small homo vial was contained within a larger one, 

 and a thermometer extended through the corks of both viate so that the 

 bulb was contained in the center of the inner vial with the hoppers. 

 Cotton was placed in the bottom of this vial to prevent them from 

 coming in contact with the glass when they would fall down. The 

 whole apparatus prevented so far as possible the unequal heating of 

 the sides of the vial as compared with the air in the interior. 



In some of the experiments, as given in the table below, the tem- 

 perature was first reduced by ice and salt to 30F. or lower, and 

 suddenly raised again to a point at which all the insects were killed. 

 By consulting the table it will be seen that they became dormant at 

 60 to 65F. and revived again at about 70F. At 80 to 90F. the first 



