302 Report of the Department of Entomology of the 



always a marked odor of hydrocyanic acid gas to the leeward 

 of the trees. Hence conclusions as to what can be accomplished 

 by fumigation with the above gas should not be based upon 

 results obtained with tents. 



COTSrOLUSIONS. 



Excluding conditions whore tents were used, the results of 

 tests for 1900-1901 show that with good covers, debarring acci- 

 dents and infestation from other sources, fumigation with hydro- 

 cyanic acid gas can be depended upon to exterminate the San 

 los6 scale, on medium sized orchard trees, over small areas. 



Under favorable conditions, the scales were all killed by the 

 use of .15 gram per cu. ft. (one-half ounce of potassium cyanide 

 per 100 cu. ft.), but as shown later, at least double this amount 

 should be used unless the operator is certain that all conditions 

 arc favorable. 



AMOUNT AND GRADE OF CHEMICALS TO USE. 



Writers on fumigation with hydrocyanic acid gas have recom- 

 mended the use of various formulas. The principal variations 

 are in the amounts. of acid and water used. Coquillet,^ Marlatt^ 

 and Webster^ have recommended the use of one part by weight 

 of potassium cyanide, one part by volume of sulphuric acid, and 

 two parts by volume of water. Based on the weight of potas- 

 sium cyanide, the charge would be represented by the formula 

 1-1-2. Smith^ recommends a formula of 1-1-3. Johnson^ and 

 Gould^ have recommended the use of l-l|-2^; while Craw'' rec- 

 ommends l-l|-3. The author has recommended the use of 

 1-1^-4. The latter was based on results obtained in all-night 

 treatment of forcing houses and in nursery fumigation, where 



^Insect Life, 2:203 (1890). 



"Year Book U. S. Dept. Agr. 1896:228. 



^Ohio Agr. Exp. Sta. Bui. 103 (1899). 



*Rept. N. J. Agr. Coll. Exp. Sta., 1897:467. 



^Mtl. Exp. Sta. Bui. 57:79 (1S9S). 



•Md. Exp. Sta. Bui. 73:163 (1901). 



* Fourth Kept. St. Board Ilort. Cal. 1894:107. 



