202 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



NATURAL CONTROL. 



Climatic. The sudden decrease in numbers of insects without any 

 known specific cause, a condition frequently observed, is often attributed 

 to the general and more or less indiscriminate term "weather condi- 

 tions." Grape leaf -hoppers, are known to occur in excessive numbers 

 more or less periodically, and, for lack of definite information, we 

 ascribe the cause to climatic conditions. Whether these conditions 

 have a direct effect in reducing the numbers through exposure, or 

 through sudden increase or decrease of temperature, or through humid- 

 ity, or the indirect effect of these influences in inducing disease, in 

 favoring the development of parasitic and predatory enemies, in 

 reducing fecundity, or in unfavorably affecting the 

 food supply, are points not easily determined. 



We have, however, apparently some evidence on the 

 direct effect of unfavorable weather conditions in 

 reducing the number of grape leaf -hoppers. During 

 March, 1907, there were two or three weeks of almost 

 continuous rain. At the end of this period the 

 hoppers were found dead in large numbers in all the 

 vineyards in the Lodi section. Between seven and 

 eight hundred dead hoppers have been counted under 

 a single bunch of alfilaria. In certain vineyards that 



Fig 15 The larva of were flooded with two or three feet of water the 

 hoppers were nearly all destroyed. This latter, of 



enemy of the hopper course, is an unusual condition, but unfavorable 



in California. . 



weather conditions of winter or early spring no 

 doubt destroy large numbers of these insects. 



Parasitic. The grape leaf-hopper appears to be particularly free 

 from natural enemies, and this is especially true of parasitic enemies. 

 In many hundreds of eggs, nymphs and adults examined and kept in 

 breeding cages during the season we did not find a single specimen 

 parasitized. 



Predatory. Most of the natural enemies of the hopper come under 

 this category of predatory or predaceous enemies, but even this list 

 is not large. The most common of these observed during the season 

 was one of the aphis lions or larvag of a lace-wing fly (Fig. 15). These 

 were present in most of the vineyards, and were frequently seen feed- 

 ing on the nymphs of the hoppers, but their number was not large 

 enough to have any appreciable effect whatever in reducing the abund- 

 ance of the grape leaf-hoppers. Ladybird beetles and their larvae 

 were also seen to feed upon the nymphs, but they did not occur in 



