FUMIGATION OF CITRUS TREES. 11 



tents are exclusively (parts of two outfits excepted) used in southern 

 California. A tent of this character is easy to construct, easy to 

 repair, and its manipulation has been so perfected by years of use 

 that it is very easily handled in the field by intelligent workmen. 



Sizes. — The standard sizes of sheet tents are 17, 24, 30, 36, 41, 43, 

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

 been employed. The size of this style of tent is properly based on 

 the distance between parallel sides, not on the distance between oppo- 

 site corners. 



Materials used. — The materials now generally used for sheet tents 

 in southern California are 6J-ounce or 7-ounce special drills and 

 8-ounce special army duck, though 10-ounce army duck is sometimes 

 used in very large tents. These cloths are spoken of in ounces, 

 meaning such a weight per yard 30 inches wide. Drills are used as 

 freely as ducks. In some other countries where fumigation is prac- 

 ticed, notably South Africa, even heavier than 10-ounce cloth is 

 sometimes used. This is largely because of its strength and tightness 

 of texture. The tendency in California has been to sacrifice tight- 

 ness in favor of lightness, as the lighter tents are so much more easily 

 manipulated in the field. The main reason for this tendency is prob- 

 ably that the practice has been largely in the hands of contract fumi- 

 gators rather than in the hands of the growers themselves. The con- 

 cerns furnishing the fumigation tents apparently have made no special 

 effort to supply the very tightest goods available on the market, prob- 

 ably because the profits on these goods would be smaller than on 

 the cheaper and more porous cloths. Several of these firms have 

 speciaL goods which they recommend for fumigation use. For the 

 most part these goods are about on a par as regards the requirements 

 for fumigation. Only one grade distinctly superior to the others has 

 been seen; it is used solely by private outfits, and is slightly more 

 expensive than the other grades. The results secured depend di- 

 rectly on the tightness of the cloth; in fact, this consideration of 

 tightness of tenting is one of the most important factors in the entire 

 fumigation procedure. On it depends not only the efficacy of the 

 treatment but also to some extent the cost of the operation. A 

 dosage recommended as securing certain results with tents of a given 

 degree of tightness will not produce the same results with tents of 

 less closely woven material. Even though the initial cost is greater, 

 tightly woven material is the most economical in the long run. 



Xew tenting material. — Considerable attention has been given dur- 

 ing this investigation to the character of cloth used in fumigating 

 tents, and an attempt has been made to secure the most suitable 

 material possible. The leading manufacturers and dealers in cotton 

 ducks and drills in the United States were consulted and samples of 

 their tightest cloths secured. Many of the nearly two hundred sam- 



