December, '22] quayle: scale resistant to fumigation 403 



Certain stages of the red scale show more resistance to HCN gas than 

 others. The molting period, particularly the second molt, and the adult 

 or young-producing period, are the two most resistant stages, and of 

 these two, the molting stage is the more resistant. 



Among the variable meteorological conditions, himiidity and wind 

 are important, as affecting the results of fumigation. The ordinary 

 canvas tenting material varies greatly in gas-holding capacity according 

 to the dryness of the cloth, which is dependent upon the amount of 

 htimidity in the atmosphere, and also, varies according to the movement 

 of the atmosphere. By means of a gas-tight tent we have been able to 

 overcome the effect of himiidity on the tent, and have eliminated tent 

 leakage, the most variable factor in fiunigation work. By the use of a 

 gas-tight tent in different localities, further proof of exceptional scale 

 resistance in certain localities has been secured. 



In the case of the red scale at Corona, definite proof of resistance was 

 lacking until comparative tests were made in which the variable factors 

 of time and place were eliminated. At this time, however, after seven 

 years' experience, observation of experimental and commercial work 

 in the field is sufficient to establish the fact of resistance. 



In 1915 our attention was also called to the difficulty of killing the' 

 black scale in the vicinity of Charter Oak, California. 



Because the black scale does not infest the fruit (at least to any 

 considerable extent) it has not been possible to carry out comparative 

 fimiigation tests on it from two or more localities at the same time, and 

 in the same tent, as with the red scale. Infested twigs can be collected 

 from different localities, however, although drying interferes with the 

 results and is difficult to overcome. Our experimental fumigation 

 work, as well as commercial work in the Charter Oak district for the 

 past six years, furnishes ample proof that the black scale is much more 

 difficult to kill with HCN gas there than in most other localities. In 

 general, the black scale is most susceptible to HCN gas when it is small. 

 When it reaches the mature stage fumigation results are much less 

 satisfactory. In the Charter Oak district, however, with dosages 

 greatly in excess of that ordinarily necessary to secure a 100 per cent 

 kill, small scales in considerable ntunbers will come through the fumiga- 

 tion unharmed. In most other localities 25 per cent less gas would in- 

 sure very satisfactory results on similar small scales. 



There are thus two localities where it seems well established that the 

 red scale is very resistant to hydrocyanic acid gas, and one locality where 

 the black scale is specially resistant. In these localities these scales are 



