SHEET TENTS. 17 



Distillate sprays have been employed b}' southern California 

 horticulturists for many years, and at one time very extensively in 

 the control of citrus scales. The accumulated experience with these 

 sprays appears to have demonstrated that the results secured are 

 not entirely satisfactory. To-day distillate sprays are used only on 

 a small acreage of citrus groves, having been supplanted by the more 

 satisfactory fumigation with hydrocyanic-acid gas. Nothing illus- 

 trates more distinctly the superiority of fumigation over spraying 

 with distillate oils than the readoption of fumigation by the more 

 successful citrus growers, and the attitude of the officials of the 

 county horticultural commissions of tliis region who, almost to a 

 man, now recommend fumigation for the control of scale-insects. 



A kerosene-water spray has found a limited use during the past 

 year in Riverside and Ventura counties. 



FUMIGATION. 



Fumigation with hydrocyanic-acid gas originated and was first 

 practiced in California by Mr. D. W. Coquillett, of the Bureau of En- 

 tomology, m 1886, in combating citras insect pests. Since that time 

 it has gradually risen in favor as a means of destroying scale enemies 

 of citrus plants until to-day it is in use in almost all the important 

 citiTis-producing countries of the world. The apparatus first used 

 in fumigation was somewhat comphcated and cumbersome, making 

 the operation very expensive.'* As the use of this gas became more 

 widespread a gradual improvement in equipment as well as methods 

 has taken place, so that to-day the process is comparatively simple. 



SHEET TENTS. 



Sheet tents exclusively are now used in southern California. The 

 manipulation of sheet tents and the general procedure in fumigation 

 have been so clearly explained in Bulletin No. 76 of this Bureau 

 that it will not be necessary to devote space to them here. The tents 

 are octagonal in shape, the standard sizes being 17, 24, 30, 36, 41, 43, 

 45, 48, 52, 55, and 64 feet, but larger ones up to 72 or 84 feet have been 

 employed. The size of this style tent is properly based on the dis- 

 tance between the parallel sides, not on the distance between opposite 

 corners. 



The materials especially recommended, and now generally used for 

 fumigation tents in southern California, are 6i-ounce special drill 

 and 8-ounce special army duck, although 10-ounce special army duck 

 is sometunes used in very large tents. The 6^-ounce special drill is 

 made of smgle threads twisted hard and closely woven. It is light, 

 strong, and flexible. The special army duck is made of double 



« See Ann. Rept. U. S. Dept. Agr. for 1887, p. 123, 

 77488— Bui. 79—09—2 



