76 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA. 



that it will quickly become uniformly distributed throughout the bot- 

 tom of the tent even if at any time it becomes as concentrated here as 

 at the top. The greater burning effect and better killing effect in the 

 top of the tree would tend to substantiate this assumption. Field 

 observations in fumigating large trees show that the gas is of no great 

 strength at the lower part of the tent for several minutes after the 

 charge is set off. With this new cover the gas is broken up and dis- 

 tributed through the bottom of the tent first. By the time it reaches 

 the top it is pretty generally distributed throughout the tent. As 

 the bottom of the tree is among the first to receive the full benefit of 

 the gas, a more complete killing of scale at the bottom of the tent may 

 be expected than with an open generator. 



THE EFFECT OF CLIMATIC CONDITIONS ON SCALE INSECTS. 



Climatic conditions exercise a marked effect on the different insects 

 affecting citrus trees. The purple scale and black scale thrive best in 

 the more moist country adjacent to the ocean. The red scale thrives 

 well in the drier interior regions of southern California as well as near 

 the coast, while the yellow scale is more of a heat-withstanding form 

 than any of the others. This is demonstrated by its prevalence in 

 citrus trees in the hot interior valleys of northern California where the 

 purple scale and to a large extent the black scale appear unable to 

 survive. 



The direct effect of heat on scale insects may be evidenced by data 

 on the black scale collected during the summer of 1907. Com- 

 mencing at Pasadena, which is at the opening or gateway of the San 

 Gabriel Valley, the writer proceeded to Duarte, Pomona, Ontario, 

 Riverside, Orange, and Santa Ana, Cal., respectively. The San 

 Gabriel is one of the interior valleys and Pasadena is situated near 

 that end which opens toward the ocean. As one approaches River- 

 side from Pasadena the climate becomes generally hotter. Orange 

 and Santa Ana are nearer the ocean and much cooler than any of the 

 other places examined. 



It might be mentioned that about a month previous to this special 

 investigation there had been a very hot spell of a few days' duration. 

 At Pasadena examination showed that about one-fourth of the eggs 

 under the old scales were dried up and brown, this condition showing 

 the effects of heat. At Duarte the destruction was somewhat 

 greater. At Pomona and Ontario, which were much hotter even 

 than the two preceding places, more than three-fourths of the eggs 

 and young insects were dead. At Riverside, where the heat was 

 most intense, a very small percentage of healthy eggs or live insects 

 was found. In the cooler sections of Orange and Santa Ana very 

 much less than a fourth of the eggs and young were destroyed, while 

 insects in all stages of development were in evidence. 



